fidzu : Maemo

This is available on OpenRepos: https://openrepos.net/content/thp/rebounce

Also on TMO: https://talk.maemo.org/showthread.php?t=101160

Also on Twitter: https://twitter.com/thp4/status/1359758278620758019

Source code (copy'n'paste this into a file named "rebounce.c", then run it using your shell):

#if 0

gcc -Os -shared -fPIC -lSDL -o librebounce.so rebounce.c

LD_PRELOAD=$(pwd)/librebounce.so /opt/bounce/bin/bounce

exit 0

#endif

/**

* reBounce -- softfp-to-hardfp LD_PRELOAD hack for Bounce on N9

*

* Bounce was a really nice 2009 tech demo on the N900. This

* makes this tech demo work on an N9 by translating the calls

* that use floating point arguments to the hardfp ABI. It also

* fixes input using SDL1.2 to get sensor values from sensorfw.

*

* Known issues: Audio is muted on startup until mute is toggled.

*

* 2021-02-11 Thomas Perl <m@thp.io>

**/

#define _GNU_SOURCE

#include <stdio.h>

#include <dlfcn.h>

#include <pthread.h>

#include <SDL/SDL.h>

#define SFP __attribute__((pcs("aapcs")))

typedef unsigned int GLuint;

static void *

sensor_thread(void *user_data)

{

SDL_Init(SDL_INIT_JOYSTICK | SDL_INIT_VIDEO);

SDL_Joystick *accelerometer = SDL_JoystickOpen(0);

while (1) {

SDL_JoystickUpdate();

float x = 0.053888f * SDL_JoystickGetAxis(accelerometer, 0);

float y = 0.053888f * SDL_JoystickGetAxis(accelerometer, 1);

float z = 0.053888f * SDL_JoystickGetAxis(accelerometer, 2);

FILE *out = fopen("/dev/shm/bounce.sensor.tmp", "wb");

fprintf(out, "%f %f %f\n", -y, x, z);

fclose(out);

rename("/dev/shm/bounce.sensor.tmp", "/dev/shm/bounce.sensor");

SDL_Delay(10);

}

return NULL;

}

FILE *

fopen(const char *filename, const char *mode)

{

FILE *(*fopen_orig)(const char *, const char *) = dlsym(RTLD_NEXT, "fopen");

if (strcmp(filename, "/sys/class/i2c-adapter/i2c-3/3-001d/coord") == 0) {

static int sensor_inited = 0;

if (!sensor_inited) {

sensor_inited = 1;

pthread_t thread;

pthread_create(&thread, NULL, sensor_thread, NULL);

}

filename = "/dev/shm/bounce.sensor";

}

return fopen_orig(filename, mode);

}

#define f_f(name) float SFP name(float x) { return ((float (*)(float))dlsym(RTLD_NEXT, #name))(x); }

#define d_d(name) double SFP name(double x) { return ((double (*)(double))dlsym(RTLD_NEXT, #name))(x); }

#define f_ff(name) float SFP name(float x, float y) { return ((float (*)(float, float))dlsym(RTLD_NEXT, #name))(x, y); }

#define d_dd(name) double SFP name(double x, double y) { return ((double (*)(double, double))dlsym(RTLD_NEXT, #name))(x, y); }

f_f(sinhf) f_f(coshf) f_f(tanhf) f_f(asinf) f_f(acosf) f_f(atanf) f_f(sinf) f_f(cosf) f_f(tanf) f_f(expf) f_f(logf)

f_f(log10f) f_f(ceilf) f_f(floorf) d_d(log) d_d(sin) f_ff(atan2f) f_ff(fmodf) d_dd(atan2) d_dd(pow) d_dd(fmod)

double SFP

frexp(double value, int *exp)

{

return ((double (*)(double, int *))dlsym(RTLD_NEXT, "frexp"))(value, exp);

}

double SFP

ldexp(double x, int n)

{

return ((double (*)(double, int))dlsym(RTLD_NEXT, "ldexp"))(x, n);

}

double SFP

modf(double value, double *iptr)

{

return ((double (*)(double, double *))dlsym(RTLD_NEXT, "modf"))(value, iptr);

}

void SFP

glClearColor(float r, float g, float b, float a)

{

((void (*)(float, float, float, float))dlsym(RTLD_NEXT, "glClearColor"))(r, g, b, a);

}

void SFP

glUniform4f(GLuint location, float v0, float v1, float v2, float v3)

{

((void (*)(GLuint, float, float, float, float))dlsym(RTLD_NEXT, "glUniform4f"))(location, v0, v1, v2, v3);

}

void SFP

glUniform1f(GLuint location, float v0)

{

((void (*)(GLuint, float))dlsym(RTLD_NEXT, "glUniform1f"))(location, v0);

}

This is the second part of my porting odyssey; for the first part, follow this link.

The good news is that I've done some progress; the bad news is that there are still plenty of issues, and solving them involves deep diving into nearly all components and technologies that make up the core of an Android device, so completing the porting is going to take quite some time. On the other hand, I'm learning a lot of new stuff, and I might be able to share it by writing some documentation. And, who knows, maybe work on some other device port.

Anyway, enough with the introduction! Let's see what progress I've been doing so far.

The new device tree

While asking for help to debug the audio issue I was facing (more about that later), I was also told that the lavender tree, which I was using as a reference, was obsolete. The new one was in gitlab, and was build with a totally different system, described here.

So, I picked the lavender tree and adapted it for violet: I changed the deviceinfo file to point to my kernel tree, use my kernel configuration, and use the same boot command line as before. By the way, here's my current "new" device tree. The build failed, with errors like:

In function 'memcpy',
    inlined from 'proc_ipc_auto_msgmni.part.1' at /home/mardy/projects/port/xiaomi-violet/bd/downloads/android_kernel_xiaomi_violet/ipc/ipc_sysctl.c:82:2:
/home/mardy/projects/port/xiaomi-violet/bd/downloads/android_kernel_xiaomi_violet/include/linux/string.h:340:4: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
    __read_overflow2();
    ^

I then replayed the kernel build using the old system, and noticed that it was using clang as the compiler; so I changed the related flag in the deviceinfo file, and the build went past that point. It failed later, though:

/home/mardy/projects/port/xiaomi-violet/bd/downloads/android_kernel_xiaomi_violet/drivers/staging/qcacld-3.0/core/sap/src/sap_fsm.c:1498:26: error: cast to smaller integer type 'eSapStatus' from 'void *' [-Werror,-Wvoid-pointer-to-enum-cast]
                bss_complete->status = (eSapStatus) context;
                                       ^~~~~~~~~~~~~~~~~~~~

I ended up editing the Kbuild file in the module source directory, and removed the -Werror from there (as well in another place that failed a bit later). This made the build proceed until the end, where it failed because the device tree file was not found:

+ cp -v /home/mardy/projects/port/xiaomi-violet/bd/downloads/KERNEL_OBJ/arch/arm64/boot/dts/qcom/sm8150-mtp-overlay.dtbo /home/mardy/projects/port/xiaomi-violet/bd/tmp/partitions/dtbo.img
cp: cannot stat '/home/mardy/projects/port/xiaomi-violet/bd/downloads/KERNEL_OBJ/arch/arm64/boot/dts/qcom/sm8150-mtp-overlay.dtbo': No such file or directory

I quickly realized that this was due to an error of mine: the Xiaomi violet is a sm6150, not a sm8150 as mentioned in my deviceinfo file! But what overlay file should I use then, since there isn't a sm6150-mtp-overlay.dts in my source tree? Having a loot at other deviceinfo files, I saw that the deviceinfo_kernel_dtb_overlay line is not always there, so I tried commenting it out, and it helped.

The next step was getting flashable images, of course. While the device is not supported by the UBports system-image server, we can use use some scripts to create a fake OTA (Over The Air update) and generate flashable images starting from it. The steps can be read in the devel-flashable target of the .gitlab-ci.yml file (if this step is not present, we should try to find another device repository which has it. They are the following:

./build/prepare-fake-ota.sh out/device_violet.tar.xz ota
./build/system-image-from-ota.sh ota/ubuntu_command out

Once these commands have completed their execution, these commands will push the images to the device:

fastboot flash boot out/boot.img; fastboot flash system out/system.img

There's also fastboot flash recovery out/recovery.img, but I left it out since I was not interested in the recovery image at this stage. And unless you are ready to submit your port for inclusion into the "supported devices" list, I'd recommend not flashing the UT recovery image since TWRP is more powerful and will likely help you in recover your device from a broken image.

Kernel command line

It's important that the kernel command line contains the systempart parameter. In my case, the first boot failed because the command line was longer than 512 bytes, and this parameter was getting truncated. So one thing to be careful about is the length of the kernel command line.

This was fixed by removing some unnecessary kernel parameters from the deviceinfo file.

Missing thumbnails in the Gallery app, content-hub not working

Another issue I noticed is that photo thumbnails were all black in the Gallery app, and the content-hub also was not working. I noticed a relevant commit in the lavender kernel tree and found a launchpad bug which mentioned the issues I was seeing. In the Telegram channel I was told that patch is a forward port of a commit from kernel 3.4 that was present in all of the cores devices, and that it was indeed needed to have the ContentHub working.

The patch did not apply cleanly on top of my kernel tree, but luckily it was just an offset issue: adjusting the patch was easy, and indeed after applying it thumbnails started appearing in the Gallery and Imaginario could import photos again via the ContentHub.

Time and date

Time was always reset to a far away date after a reboot. This is a common issue on Qualcomm devices, and can be fixed by disabling the time service in the Android container.

https://github.com/Halium/android_device_halium_halium_arm64/pull/3

For some reason, at a certain point my override stopped working (or, more likely, the override never worked, but I happened to have fixed the issue directly modifying the vendor init file). I had to copy /android/vendor/etc/init/hw/init.qcom.rc into /usr/share/halium-overrides/, modify it and bind mount the modified file in order to get it working.

This actually seems to match my understanding of the Android init documentation, because according to the path priorities a configuration file stored under /system/ will never be able to override one stored under /vendor.

Fixing the audio configuration

Audio was not working at all. The sound indicator icon was not shown, only the raw (unstranslated) "indicator-sound" text was shown in the panel. pulseaudio was not running. Trying to run it manually (as the phablet user, since pulseaudio is run in the user session) led to this:

phablet@ubuntu-phablet:~$ pulseaudio -n -vvv -F /etc/pulse/touch-android9.pa
I: [pulseaudio] main.c: setrlimit(RLIMIT_NICE, (31, 31)) failed: Operation not permitted
I: [pulseaudio] main.c: setrlimit(RLIMIT_RTPRIO, (9, 9)) failed: Operation not permitted
...
D: [pulseaudio] cli-command.c: Parsing script '/etc/pulse/touch-android9.pa'
D: [pulseaudio] database-tdb.c: Opened TDB database '/home/phablet/.config/pulse/ubuntu-phablet-stream-volumes.tdb'
I: [pulseaudio] module-stream-restore.c: Successfully opened database file '/home/phablet/.config/pulse/ubuntu-phablet-stream-volumes'.
...
D: [pulseaudio] module.c: Checking for existence of '/usr/lib/pulse-8.0/modules/module-droid-card-28.so': success
I: [pulseaudio] module-droid-card.c: Create new droid-card
D: [pulseaudio] droid-util.c: No configuration provided for opening module with id primary
I: [pulseaudio] config-parser-xml.c: Failed to open file (/odm/etc/audio_policy_configuration.xml): No such file or directory
D: [pulseaudio] droid-config.c: Failed to parse configuration from /odm/etc/audio_policy_configuration.xml
D: [pulseaudio] config-parser-xml.c: Read /vendor/etc/audio/audio_policy_configuration.xml ...
D: [pulseaudio] config-parser-xml.c: New module: "primary"
W: [pulseaudio] config-parser-xml.c: [/vendor/etc/audio/audio_policy_configuration.xml:78] Could not find element attribute "samplingRates"
E: [pulseaudio] config-parser-xml.c: [/vendor/etc/audio/audio_policy_configuration.xml:78] Failed to parse element <profile>
E: [pulseaudio] config-parser-xml.c: parsing aborted at line 78
D: [pulseaudio] droid-config.c: Failed to parse configuration from /vendor/etc/audio/audio_policy_configuration.xml
D: [pulseaudio] config-parser-xml.c: Read /vendor/etc/audio_policy_configuration.xml ...
D: [pulseaudio] config-parser-xml.c: New module: "primary"
W: [pulseaudio] config-parser-xml.c: [/vendor/etc/audio_policy_configuration.xml:78] Could not find element attribute "samplingRates"
E: [pulseaudio] config-parser-xml.c: [/vendor/etc/audio_policy_configuration.xml:78] Failed to parse element <profile>
E: [pulseaudio] config-parser-xml.c: parsing aborted at line 78
D: [pulseaudio] droid-config.c: Failed to parse configuration from /vendor/etc/audio_policy_configuration.xml
I: [pulseaudio] config-parser-legacy.c: Failed to open config file (/vendor/etc/audio_policy.conf): No such file or directory
D: [pulseaudio] droid-config.c: Failed to parse configuration from /vendor/etc/audio_policy.conf
I: [pulseaudio] config-parser-xml.c: Failed to open file (/system/etc/audio_policy_configuration.xml): No such file or directory
D: [pulseaudio] droid-config.c: Failed to parse configuration from /system/etc/audio_policy_configuration.xml
I: [pulseaudio] config-parser-legacy.c: Failed to open config file (/system/etc/audio_policy.conf): No such file or directory
D: [pulseaudio] droid-config.c: Failed to parse configuration from /system/etc/audio_policy.conf
E: [pulseaudio] droid-config.c: Failed to parse any configuration.
...

Indeed, line 78 of /vendor/etc/audio_policy_configuration.xml had an error, where a property was spelt like simplingRate instead of samplingRate. However, the "vendor" partition is read-only, so I couldn't change that file directly. Another option could have been creating a fixed copy of the file and place it with /system/etc/audio_policy_configuration.xml, but the "system" is also read-only (there are ways to modify these partitions, of course, but I couldn't find a clean way to do it from the device tree scripts. So I went for the bind-mount approach: I would ship the fixed file in some other directory of the file-system, and then modify the /etc/init/mount-android.conf file (this is the job that upstart executes before starting the Android LXC container) to bind-mount the file onto /vendor/etc/audio_policy_configuration.xml.

This worked, but my joy was short-lived: audio was coming up only once every 5 boots or so. I will not list here all the things I tried, as they were plenty of them; and more than once I went to sleep happy and convinced of having fixed the issue for good, until the next day the device booted without audio. It was clearly a timing issue occurring in the early boot, because one thing I clearly noticed very early on is that in those cases when the audio was booting, the following lines appeared in the kernel log:

[    7.130057] wcd937x_codec wcd937x-codec: msm_cdc_dt_parse_vreg_info: cdc-vdd-ldo-rxtx: vol=[1800000 1800000]uV, curr=[25000]uA, ond 0
[    7.130068] wcd937x_codec wcd937x-codec: msm_cdc_dt_parse_vreg_info: cdc-vddpx-1: vol=[1800000 1800000]uV, curr=[10000]uA, ond 0
[    7.130076] wcd937x_codec wcd937x-codec: msm_cdc_dt_parse_vreg_info: cdc-vdd-mic-bias: vol=[3296000 3296000]uV, curr=[25000]uA, ond 0
[    7.130084] wcd937x_codec wcd937x-codec: msm_cdc_dt_parse_vreg_info: cdc-vdd-buck: vol=[1800000 1800000]uV, curr=[650000]uA, ond 1
[    7.137759] wcd937x_codec wcd937x-codec: bound wcd937x-slave.1170224 (ops cleanup_module [wcd937x_slave_dlkm])
[    7.138065] wcd937x_codec wcd937x-codec: bound wcd937x-slave.1170223 (ops cleanup_module [wcd937x_slave_dlkm])

I started adding some printk to the kernel driver, and modified it slightly to register itself with the module_driver() macro instead of the simpler, but logless, module_platform_driver(). This showed that the driver was always loaded at about 7 seconds, but the platform_driver_register() method only called the driver's bind method (wcd937x_bind()) in those boots where audio was working.

After more debugging into platform_driver_register(), I stumbled upon the platform_match() function, added some debugging message in there to print the device name and the driver name, and observed how in those boots where audio was failing this function was called to find a driver for the wcd937x_codec device before the wcd937x_codec driver (provided by the wcd937x_dlmk module) was available. So, I tried adding wcd937x_dlmk to /etc/modules-load.d/modules.conf and this caused the driver to be loaded at about 3 seconds, and apparently fixed the audio issue. At least, till the time of writing this, I never had my phone boot without audio anymore.

Not all is fine with the audio, unfortunately: the mic records a background noise along with the actual sounds, and the recording volume is quite low. This also affects the call quality. On the other hand, the noise disappears when recording happens via the earphones. But I've yet to investigate this; I hope to give you some updates in part three.

In case you have a sense of deja-vu when reading this post, it's because indeed this is not the first time I try porting a device to Ubuntu Touch. The previous attempt, however, was with another phone model (and manufacturer), and did not have a happy ending. This time it went better, although the real ending is still far away; but at least I have something to celebrate.

The phone

I made myself a Christmas present and bought a Xiaomi Redmi Note 7 Pro, a dual-SIM phone from 2019 with 6GB of RAM and 128GB of flash storage. To be totally honest, I bought it by mistake: the phone I really wanted to buy is the Redmi Note 7 (without the "Pro"), because it's a modern phone that is working reasonable well with Ubuntu Touch. The online shop where I bought it from let me choose some options, including the RAM size, so I chose the maximum available (6GB) without being aware that this would mean that I would be buying the "Pro" device - the shop did not alter the item name, so I couldn't really know. Unfortunately, the two versions are two rather different beasts, powered by different SoC; both are produced by Qualcomm, so they are not that different, but it's enough to make the installation of Ubuntu Touch impossible.

But between the choice of retuning the phone to the shop and begin a new porting adventure, I stood firm and went for the latter. Hopefully I won't regret it (if everything goes bad, I can still use it with LineageOS, which runs perfectly on it).

Moreover, there already exist a port of Ubuntu Touch for this phone, which actually works reasonably well (I tried it briefly, and many things were working), but the author claims to be a novice and indeed did not follow the best git practices when working on the source code, so it's hard to understand what was changed and why. But if you are looking for a quick way to get Ubuntu Touch working on this phone, you are welcome to have a look at this Telegram channel.

What follows are the raw notes of my attempts. They are here so that search engines can index the error messages and the various logs, and hopefully help someone hitting similar errors on other devices to find his way out.

Getting Halium and the device source code

Installed the dependencies like in the first step. repo was not found in the Ubuntu 20.04 archives, but I had it installed anyway due to my work on a Yocto device.

Since my device has Android 9 on it, I went for Halium 9:

repo init -u git://github.com/Halium/android.git -b halium-9.0 --depth=1
repo sync -c -j 16

The official LineageOS repository for the Xiaomi Redmi Note 7 Pro is android_device_xiaomi_violet, but the page with the official build has been taken down (some DMCA violation, if you believe the forums) and no development has been happening since last year. A more active forum thread uses another repository which seems to be receiving more frequent updates, so I chose to base my port on that.

I actually tested that LineageOS image on my phone, and verified that all the hardware was working properly.

Initially, I created forks of the relevant repository under my own gitlab account, but then I though (especially looking at the Note 7 port) that creating a group just for this port would make people's life easier, because they wouldn't need to navigate through my 1000 personal projects to find what is relevant for this port. So, I created these forks:

• gitlab.com/ubuntu-touch-xiaomi-violet/android_device_xiaomi_violet
• gitlab.com/ubuntu-touch-xiaomi-violet/android_kernel_xiaomi_violet
• gitlab.com/ubuntu-touch-xiaomi-violet/android_vendor_xiaomi_violet

Next, created the halium/devices/manifests/xiaomi_violet.xml file with this content:

<?xml version="1.0" encoding="UTF-8"?>
<manifest>
    <!-- Remotes -->
    <remote name="ubuntu-touch-xiaomi-violet"
            fetch="https://gitlab.com/ubuntu-touch-xiaomi-violet"
            revision="halium-9.0"/>

    <!-- Device Tree -->
    <project path="device/xiaomi/violet"
             name="android_device_xiaomi_violet"
             remote="ubuntu-touch-xiaomi-violet" />

    <!-- Kernel -->
    <project path="kernel/xiaomi/violet"
             name="android_kernel_xiaomi_violet"
             remote="ubuntu-touch-xiaomi-violet" />

    <!-- Proprietary/Vendor blobs -->
    <project path="vendor/xiaomi/violet"
             name="android_vendor_xiaomi_violet"
             remote="ubuntu-touch-xiaomi-violet" />
</manifest>

Fetching all the sources mentioned in the manifest:

./halium/devices/setup violet

Full output:

*****************************************
I: Configuring for device xiaomi_violet
*****************************************
Fetching projects: 100% (393/393), done.
hardware/qcom/audio-caf/apq8084: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8916: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8952: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8960: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8974: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8994: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8996: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/msm8998: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/sdm845: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio-caf/sm8150: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/audio/default: Shared project LineageOS/android_hardware_qcom_audio found, disabling pruning.
hardware/qcom/display: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/apq8084: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8916: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8952: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8960: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8974: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8994: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8996: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/msm8998: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/sdm845: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/display-caf/sm8150: Shared project LineageOS/android_hardware_qcom_display found, disabling pruning.
hardware/qcom/media: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/apq8084: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8916: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8952: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8960: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8974: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8994: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8996: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/msm8998: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/sdm845: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/qcom/media-caf/sm8150: Shared project LineageOS/android_hardware_qcom_media found, disabling pruning.
hardware/ril: Shared project LineageOS/android_hardware_ril found, disabling pruning.
hardware/ril-caf: Shared project LineageOS/android_hardware_ril found, disabling pruning.
hardware/qcom/wlan: Shared project LineageOS/android_hardware_qcom_wlan found, disabling pruning.
hardware/qcom/wlan-caf: Shared project LineageOS/android_hardware_qcom_wlan found, disabling pruning.
hardware/qcom/bt: Shared project LineageOS/android_hardware_qcom_bt found, disabling pruning.
hardware/qcom/bt-caf: Shared project LineageOS/android_hardware_qcom_bt found, disabling pruning.
Updating files: 100% (67031/67031), done.nel/testsUpdating files:  36% (24663/67031)
lineage/scripts/: discarding 1 commits
Updating files: 100% (1406/1406), done.ineageOS/android_vendor_qcom_opensource_thermal-engineUpdating files:  47% (666/1406)
Checking out projects: 100% (392/392), done.
*******************************************
I: Refreshing device vendor repository: device/xiaomi/violet
I: Processing proprietary blob file: device/xiaomi/violet/./proprietary-files.txt
I: Processing fstab file: device/xiaomi/violet/./rootdir/etc/fstab.qcom
I: Removing components relying on SettingsLib from: device/xiaomi/violet
*******************************************

Starting the build

Setting up the environment:

$ source build/envsetup.sh
including device/xiaomi/violet/vendorsetup.sh
including vendor/lineage/vendorsetup.sh
$

Running the breakfast command:

$ breakfast violet
including vendor/lineage/vendorsetup.sh
Trying dependencies-only mode on a non-existing device tree?

============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=9
LINEAGE_VERSION=16.0-20210109-UNOFFICIAL-violet
TARGET_PRODUCT=lineage_violet
TARGET_BUILD_VARIANT=userdebug
TARGET_BUILD_TYPE=release
TARGET_ARCH=arm64
TARGET_ARCH_VARIANT=armv8-a
TARGET_CPU_VARIANT=kryo300
TARGET_2ND_ARCH=arm
TARGET_2ND_ARCH_VARIANT=armv8-a
TARGET_2ND_CPU_VARIANT=cortex-a75
HOST_ARCH=x86_64
HOST_2ND_ARCH=x86
HOST_OS=linux
HOST_OS_EXTRA=Linux-5.4.0-58-generic-x86_64-Ubuntu-20.04.1-LTS
HOST_CROSS_OS=windows
HOST_CROSS_ARCH=x86
HOST_CROSS_2ND_ARCH=x86_64
HOST_BUILD_TYPE=release
BUILD_ID=PQ3A.190801.002
OUT_DIR=/home/mardy/projects/port/halium/out
PRODUCT_SOONG_NAMESPACES= hardware/qcom/audio-caf/sm8150 hardware/qcom/display-caf/sm8150 hardware/qcom/media-caf/sm8150
============================================

Building the kernel

The configuration needs to be adapted for Halium. Locating the kernel config:

$ grep "TARGET_KERNEL_CONFIG" device/xiaomi/violet/BoardConfig.mk 
TARGET_KERNEL_CONFIG := vendor/violet-perf_defconfig
$

Getting the Mer checker tool and running it:

git clone https://github.com/mer-hybris/mer-kernel-check
cd mer-kernel-check
./mer_verify_kernel_config ../kernel/xiaomi/violet/arch/arm64/configs/vendor/violet-perf_defconfig

Prints lots of warnings, starting with:

WARNING: kernel version missing, some reports maybe misleading

To help the tool, we need to let him know the kernel version. It can be seen at the beginning of the kernel Makefile, located in ../kernel/xiaomi/violet/Makefile; in my case it was

VERSION = 4
PATCHLEVEL = 14
SUBLEVEL = 83
EXTRAVERSION =

So I edited the configuration file ../kernel/xiaomi/violet/arch/arm64/configs/vendor/violet-perf_defconfig and added this line at the beginning:

# Version 4.14.83

then ran the checker tool again. This time the output was a long list of kernel options that needed to be fixed, but as I went asking for some explanation in the Telegram channel for Ubuntu Touch porters, I was told that I could/should skip this test and instead use the check-kernel-config tool from Halium. I downloaded it, made it executable (chmod +x check-kernel-config) and ran it:

./check-kernel-config kernel/xiaomi/violet/arch/arm64/configs/vendor/violet-perf_defconfig
[...lots of red and green lines...]
Config file checked, found 288 errors that I did not fix.

I ran it again with the -w option, and it reportedly fixed 287 errors. Weird, does that mean that an error was still left? I ran the tool again (without -w), and it reported 2 errors. Ran it once more in write mode, and if fixed them. So, one might need to run it twice.

Next, added the line

BOARD_KERNEL_CMDLINE += console=tty0

in device/xiaomi/violet/BoardConfig.mk. One more thing that needs to be done before starting the build is fixing the mount points, so I opened device/xiaomi/violet/rootdir/etc/fstab.qcom and changed the type of the userdata partition from f2fs to ext4. There were no lines with the context option, so that was the only change I needed to do.

At this point, while browsing throught the documentation, I found a link to this page which contains some notes on Halium 9 porting, which are substantially different from the official porting guide in halium.org (which I was already told in Telegram to be obsolete in several points).

So, following the instructions from this new link I ran

hybris-patches/apply-patches.sh --mb

which completed successfully.

Then, continuing following the points from this page, I edited device/xiaomi/violet/lineage_violet.mk, commented out the lines

$(call inherit-product, $(SRC_TARGET_DIR)/product/full_base_telephony.mk)
# ...
$(call inherit-product, vendor/lineage/config/common_full_phone.mk)

and replaced the first one with a similar line pointing to halium.mk. For the record, a find revealed that the SRC_TARGET_DIR variable in my case was build/make/target/. It contained also the halium.mk file, which was created by the hybris patches before. As for removing the Java dependencies, I cound't find any modules similar to those listed in this commit in any of the makefiles in my source tree, so I just started the build:

source build/envsetup.sh && breakfast violet
make halium-boot

This failed the first time with the compiler being killed (out of memory, most likely), but it succeeded on the second run. There was a suspicious warning, though:

drivers/input/touchscreen/Kconfig:1290:warning: multi-line strings not supported

And indeed my kernel/xiaomi/violet/drivers/input/touchscreen/Kconfig had this line:

source "drivers/input/touchscreen/ft8719_touch_f7b/Kconfig

(notice the unterminated string quote). I don't know if this had any impact on the build, but just to be on the safe side I added the missing quote and rebuilt.

Building the system image

Running

make systemimage

failed pretty soon:

ninja: error: '/home/mardy/projects/port/halium/out/soong/host/linux-x86/framework/turbine.jar', needed by '/home/mardy/projects/port/halium/out/soong/.intermediates/libcore/core-oj/android_common/turbine/core-oj.jar', missing and no known rule to make it

The error is due to all the Java-related stuff that I should have disabled but couldn't find. So, I tried to have a look at the changes made on another xiaomi device (lavender, the Redmi note 7, which might not be that different, I thought) and started editing device/xiaomi/violet/device.mk and removing a couple of Android packages. Eventually the build proceeded, just to stop at a python error:

  File "build/make/tools/check_radio_versions.py", line 56
    print "*** Error opening \"%s.sha1\"; can't verify %s" % (fn, key)
          ^
SyntaxError: invalid syntax

Yes, it's the python3 vs python2 issue, since in my system python is python version 3. In order to fix it, I created a virtual environment:

virtualenv --python 2.7 ../python27  # adjust the path to your prefs
source ../python27/bin/activate

Remember that the second line must be run every time you'll need to setup the Halium build environment (that is, every time you run breakfast).

The build then proceeded for several minutes, until it failed due to some unresolved symbols:

prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/aarch64-linux-android/bin/ld.gold: error: /home/mardy/projects/port/halium/out/target/product/violet/obj/STATIC_LIBRARIES/lib_driver_cmd_qcwcn_intermediates/lib_driver_cmd_qcwcn.a: member at 7694 is not an ELF object
external/wpa_supplicant_8/hostapd/src/drivers/driver_nl80211.c:7936: error: undefined reference to 'wpa_driver_set_p2p_ps'
/home/mardy/projects/port/halium/out/target/product/violet/obj/EXECUTABLES/hostapd_intermediates/src/drivers/driver_nl80211.o:driver_nl80211.c:wpa_driver_nl80211_ops: error: undefined reference to 'wpa_driver_set_ap_wps_p2p_ie'
/home/mardy/projects/port/halium/out/target/product/violet/obj/EXECUTABLES/hostapd_intermediates/src/drivers/driver_nl80211.o:driver_nl80211.c:wpa_driver_nl80211_ops: error: undefined reference to 'wpa_driver_get_p2p_noa'
/home/mardy/projects/port/halium/out/target/product/violet/obj/EXECUTABLES/hostapd_intermediates/src/drivers/driver_nl80211.o:driver_nl80211.c:wpa_driver_nl80211_ops: error: undefined reference to 'wpa_driver_set_p2p_noa'
/home/mardy/projects/port/halium/out/target/product/violet/obj/EXECUTABLES/hostapd_intermediates/src/drivers/driver_nl80211.o:driver_nl80211.c:wpa_driver_nl80211_ops: error: undefined reference to 'wpa_driver_nl80211_driver_cmd'

As people told me in the Telegram channel, this driver is not used since "our wpa_supplicant talks to kernel directly". OK, so I simply disabled the driver, copying again from the lavender Halium changes: commented out the BOARD_WLAN_DEVICE := qcwcn line (and all lines referring this variable) from device/xiaomi/violet/BoardConfig.mk and ran make systemimage again. This time, surprisingly, it all worked.

Try it out

I first tried to flash the kernel only. I rebooted into fastboot, and on my PC ran this:

cout
fastboot flash boot halium-boot.img

I then rebooted my phone, but after showing the boot logo for a few seconds it would jump to the fastboot screen. Indeed, flashing the previous kernel would restore the normal boot, so there had to be something wrong with my own kernel.

While looking at what the problem could be, I noticed that in the lavender port the author did not modify the lineageos kernel config file in place, but instead created a new one and changed the BoardConfig.mk file to point to his new copy. Since it sounded like a good idea, I did the same and created kernel/xiaomi/violet/arch/arm64/configs/vendor/violet_halium_defconfig for the Halium changes. And then the line in the board config file became

TARGET_KERNEL_CONFIG := vendor/violet_halium_defconfig

I then continued investigating the boot issue, and I was told that it might have been due to an Android option, skip_initramfs, which is set by the bootloader and causes our Halium boot to fail. The fix is to just disable this option in the kernel, by editing init/initramfs.c and change the skip_initramfs_param function to always set the do_skip_initramfs variable to 0, rather than to 1. After doing this, the boot proceeded to show the Ubuntu splash screen with the five dots being lit, but it didn't proceed from there.

Setting up the udev rules

Even in this state, the device was detected by my host PC and these lines appeared in the system log:

kernel: usb 1-3: new high-speed USB device number 26 using xhci_hcd
kernel: usb 1-3: New USB device found, idVendor=0fce, idProduct=7169, bcdDevice= 4.14
kernel: usb 1-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
kernel: usb 1-3: Product: Unknown
kernel: usb 1-3: Manufacturer: GNU/Linux Device
kernel: usb 1-3: SerialNumber: GNU/Linux Device on usb0 10.15.19.82

Indeed, I guess the reason I could do this without even flashing my systemimage is because I had first flashed another UT system image from another porter. I'm not sure if I'd had the same results with my own image. Anyway, the USB networking was there, so I connected and ran the following commands to generate the udev rules:

ssh phablet@10.15.19.82
# used 0000 as password
sudo -i
# same password again
cd /home/phablet
DEVICE=violet
cat /var/lib/lxc/android/rootfs/ueventd*.rc /vendor/ueventd*.rc \
        | grep ^/dev \
        | sed -e 's/^\/dev\///' \
        | awk '{printf "ACTION==\"add\", KERNEL==\"%s\", OWNER=\"%s\", GROUP=\"%s\", MODE=\"%s\"\n",$1,$3,$4,$2}' \
        | sed -e 's/\r//' \
        >70-$DEVICE.rules

I then copied (with scp) this file to my host PC, and I moved it to device/xiaomi/violet/ubuntu/70-violet.rules (I had to create the ubuntu directory first). Then I edited the device/xiaomi/violet/device.mk file and added these lines at the end:

### Ubuntu Touch ###
PRODUCT_COPY_FILES += \
    $(LOCAL_PATH)/ubuntu/70-violet.rules:system/halium/lib/udev/rules.d/70-android.rules
### End Ubuntu Touch ###

It was now time to try my own system image. I rebooted my device into TWRP, I cloned the halium-install repository into my halium build dir, downloaded a rootfs for Halium9, and ran

./halium-install/halium-install -p ut \
    ~/Downloads/ubuntu-touch-android9-arm64.tar.gz \
    out/target/product/violet/system.img

The first time this failed because the simg2img tool was not installed. The second time it proceeded to create the image, asked me for a password for the phablet user (gave "0000") and pushed the image onto the device, into the /data/ partition. I then rebooted.

My first Ubuntu Touch image

Upon reboot, the usual splash screen appeared, followed by several seconds of black screen. It definitely didn't look right, but at least it proved that something had been flashed. After some more seconds, to my big surprise, the Ubuntu boot screen appeared, just with a smaller logo than how it used to be before, which also confimed that my system image was being used - in fact, I did not adjust the GRID_UNIT_PX variable before. Since this was an easy fix, I chose to focus on that, rather than fix the boot issues (indeed, my device did not move on from the Ubuntu boot screen). SSH was working.

I took the scaling.conf file from the lavender changes, put it in device/xiaomi/violet/ubuntu/ and added this line in the PRODUCT_COPY_FILES in device.mk:

$(LOCAL_PATH)/ubuntu/scaling.conf:system/halium/etc/ubuntu-touch-session.d/android.conf

I initially used system/ubuntu/etc/... as the target destination for the config file, like in the lavender commit, but this didn't work out for me. Then I changed the path to start with system/halium/, like it's mentioned in the ubports documentation, but it apparently had no effect either.

After a couple of days spent trying to understand why my files were not appearing under /etc, it turned out that the device was not using my system image at all: with halium-install I had my image installed in /userdata/system.img, while the correct path for Halium 9 devices is /userdata/android-rootfs.img. I was told that the option -s of halium-install would do the trick. Instead of re-running the script, I took the shortcut of renaming /userdata/system.img to /userdata/android-rootfs.img and after rebooting I could see that the Ubuntu logo was displayed at the correct size. And indeed my system image was being used.

So I started to debug why unity8 didn't start. The logs terminated with this line:

terminate called after throwing an instance of 'std::runtime_error'
  what():  org.freedesktop.DBus.Error.NoReply: Message recipient disconnected from message bus without replying
initctl: Event failed

It means, that unity8 did not handle correctly the situation where another D-Bus service crashed while handing a call from unity8. The problem now was how to figure out which service it was. I ran dbus-monitor and restarted unity8 (initctl start unity8), then examined the dbus logs; I saw the point where unity8 got disconnected from the bus, but before that point I didn't find any failed D-Bus calls. So it had to be the system bus. I did exactly the same steps, just this time after running dbus-monitor --system as root, I found the place where unity8 got disconnected, and found this D-bus error shortly before that:

method call time=1605676421.968667 sender=:1.306 -> destination=com.ubuntu.biometryd.Service serial=3 path=/default_device; interface=com.ubuntu.biometryd.Device; member=Identifier
method return time=1605676421.970222 sender=:1.283 -> destination=:1.306 serial=4 reply_serial=3
   object path "/default_device/identifier"
...
method call time=1605676421.974218 sender=:1.283 -> destination=org.freedesktop.DBus serial=6 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetConnectionAppArmorSecurityContext
   string ":1.306"
error time=1605676421.974278 sender=org.freedesktop.DBus -> destination=:1.283 error_name=org.freedesktop.DBus.Error.AppArmorSecurityContextUnknown reply_serial=6
   string "Could not determine security context for ':1.306'"
...
signal time=1605676421.989074 sender=org.freedesktop.DBus -> destination=:1.283 serial=6 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameLost
   string "com.ubuntu.biometryd.Service"
...
error time=1605676421.989302 sender=org.freedesktop.DBus -> destination=:1.306 error_name=org.freedesktop.DBus.Error.NoReply reply_serial=4
   string "Message recipient disconnected from message bus without replying"

So, it looks like unity8 (:1.306) made a request to biometryd, who asked the D-Bus daemon what was the AppArmor label of the caller, but since I didn't backport the D-Bus mediation patches for AppArmor, the label could not be resolved. biometryd decided to crash instead of properly handling the error, and so did unity8.

So I backported the AppArmor patches. I took them from the Canonical kernel repo, but they did not apply cleanly because they are meant to be applied on top of a pristine 4.14 branch, whereas the Android kernel had already some AppArmor security fixes backported from later releases. So I set and quickly inspected the contents of each patch, and found out that a couple of them had already been applied, and the last patch had to be applied as the first (yeah, it's hard to explain, but it all depends on other patches that Android has backported from newer kernels). Anyway, after rebuilding the kernel and reflashing it, my phone could finally boot to unity8!

Conclusion (of the first episode)

The actual porting, as I've been told, starts here. What has been documented here are only the very first steps of the bring-up; what awaits me now is to make all the hardware subsystems work properly, and this, according to people more experienced in porting, is the harder part.

So far, very few things work, to the point that it's faster to me to list the things that do work; it's safe to assume that all what is not listed here is not working:

• Mir, with graphics and input: Unity8 starts and is usable
• Camera: can take photos; video recording start but an error appears when the stop button is pressed
• Flashlight
• Fingerprint reader: surprisingly, this worked out of the box
• Screen brightness (though it can be changed only manually)

For all the rest, please keep an eye on this blog: I'll write, when I make some progress!

I need to print this out and put it above my bed:

Thiago Macieira added a comment - 13 Jul '17 16:06

oh, you were ranting. Doesn't mean you're wrong.

Thanks for prioritizing this Thiago.

Yes, this is yet another post in the internet talking about using exceptions versus error returns. The topic has been flaming up at my workplace for quite some time now, and I felt that writing a blog post about it during the week-end would help me focus my thoughts and give me time to explain my point with the due care. In case you didn't know, I'm against using exceptions for error handling (maybe having spent many years working with Qt has had an effect on this); that does not mean that I never write code using exceptions: I certainly do my good share of try ... catch when dealing with third-party code (including the STL), but you won't find a throw in my programs.

I'm not going to write here all the reasons why I refrain myself from implementing error handling using exceptions; I'd rather like to focus on the one I consider to be the major one, and which I rarely see being given the due weight in the debate.

And please note that this post is about C++ only; it may be that exception handling in other languages is designed in such a way that all my concerns are addressed (either by the language itself, or by common error handling policies).

Code safety

I was about to title this "Code readability", but this is more about code verifiability, that is making sure that the code is correct and, ultimately, safe. As we all know, code is written once but read many times, and even if it's code you've written yourself, chances are that in a few weeks time you'll have forgotten several details about it; error cases and error handling are one typical thing that doesn't stick in our memory for long.

When I look at a small piece of code, such as the one that can fit into my screen, or which I can read from a merge request diff, I want to be able to ascertain that the code I'm looking at is correct. Let's look at some examples.

A throw-free project

assert(track != nullptr);

Car car;
car.setMaximumSpeed(90);
car.setName("Herbie");

if (!car.executeLap(track)) {
    log("Car failed to complete track");
    return false;
}

Path *path = car.getPath();
if (!path) {
    log("GPX path could not be retrieved");
    return false;
}

double temperature = car.engineTemperature();
double boundingRectArea = path->boundingRectArea();

I just made this up, so please bear with me if it doesn't make any sense. What I want to show is that code like the above has very few fault risks, if found in a project which bans throwing errors as exceptions: if we exclude out-of-memory errors, that are generally not handled to let the application crash (though you can always catch them if you like), the reader can easily verify that this code is safe. Coding style policies and naming conventions can guarantee that setMaximumSpeed() and setName() won't have a return value that needs to be checked, and all other method calls either return an error that our code is properly handling, or return some value. Of course, by just looking at this piece of code we cannot know if the engineTemperature() method has some other overloaded sibling which accepts passing a reference to a boolean and which could be used to detect an error; so, it may be that our code could be improved in that respect, if we had a look at the header files for the Car class - but this does deny the fact that a simple glance at this snippet tells us exactly what errors are handled and what could be going wrong.

Let's look at this code instead:

assert(track != nullptr);

Car car;
car.setMaximumSpeed(90);
car.setName("Herbie");

car.executeLap(track);
Path *path = car.getPath();

double temperature = car.engineTemperature();
double boundingRectArea = path->boundingRectArea();

If we continue on the assumption that we are working on a project which bans throwing exceptions, we can immediately say that this code is not safe: we don't know if the car successfully executed a lap on the track, and our process will crash if boundingRectArea() is invoked on a null object.

Enter the exception

In a project where exceptions are actively used, the code from the second snippet is not obviously wrong anymore: maybe executeLap() cannot throw any exceptions, or, if does, the caller of this snippet is catching the exception? In order to figure out whether this code is correct, I need to see the declaration of the executeLap() method, and hope that there's a nice noexcept in there; if there isn't, I have to look at its implementation, and recursively descend through all the methods it calls - at which point the safest attitude is just to assume that it can throw. But that's only half of the story, because once I accept the fact that executeLap() can throw, I need to check whether the exception is properly handled: I have to check the implementation of all the callers of my method, and if I don't find a catch there, I'll have to recursively walk up the tree of their callers.

And indeed even the first snippet, which looked so harmless when exception throwing was banned, suddenly becomes not obviously correct anymore: what if executeLap() or getPath() also throw an exception? You might say that it would be quite a silly thing to do, and I'd certainly agree; but it may be that indeed they don't throw any exceptions in their implementation, but some of the methods they call does.

A compromise: catch early, catch often

The obvious solution to the above issue is having a policy of handling exceptions right away, and explicitly rethrowing them (or even better, rethrow a different, more appropriate exception) up the stack:

assert(track != nullptr);

Car car;
car.setMaximumSpeed(90);
car.setName("Herbie");

try {
    car.executeLap(track);
    Path *path = car.getPath();

    double temperature = car.engineTemperature();
    double boundingRectArea = path->boundingRectArea();
} catch (std::runtime_error &e) {
    log("Car failed to complete track");
    throw;
}

What I can tell from the above snippet is that the code is handling errors, and this is somehow a relief. I'm sure some of you would suggest using a more specific catch clause, but for the sake of this example let's assume that this one is fine.

(Quick note: the above example does not catch std::exception, because that would also catch the std::bad_alloc exception which is typically thrown in out-of-memory situations; my advice is not handle it at all, unless you know what you are doing)

In real life, though, you might find that try-ing on a rather large block of operations is not enough: suppose that the Car methods all emit the same exception type, and that you need to handle them differently depending on when they occur. Then you'd need to split up the try into smaller blocks, and at that point your code won't look any cleaner than the equivalent code which uses ifs on return values. Of course if you own the Car class you could modify it to throw different exceptions, in order to keep more operations inside the try block and have specific catches at the end.

The big catch (pun intended)

Even once you've refactored your methods to get the best out of exceptions (where "best" is highly subjective, but let's assume that it just means that you are happy with your exception-throwing code), there's something that still bothers me, and that's exactly the same thing that proponents of exceptions use as a "pro" in their argumentations: the business logic of your code gets separated from the error handling. You get a nice block of pure logic, not cluttered with error checking, and a catch section (which I call "the big catch") where error cases are handled.

I really don't see how that makes the code any more readable or safe: sure, the logic is not intertwined with error handling and might help focus on the expected flow of the operations (though, really, I do not think that normal brains have a problem skipping over if blocks), but that's hardly what I'm interested in when I want to check that the code is correct. Most of program errors and bugs lie in handling the edge cases and the abnormal situations, the seldomly taken code paths, and that's where I need to focus my attention.

try {
    operationA();
    if (value > B.maxValue()) {
        operationB();
    } else {
        operationC();
    }
    operationD();
} catch (ExceptionI &e) {
    ...
} catch (ExceptionII &e) {
    ...
} catch (ExceptionIII &e) {
    ...
} catch (std::runtime_error &e) {
    ...
}

When I see code like this one, I need to mentally build a mapping of "operationX() → possible exceptions" (which, unless exception naming is making this obvious, requires me to look at the implementation of the operationX() functions), and then mentally reconstruct the possible code paths in case operationX() fails, for each line of the try block.

Not seeing the errors right there, right away makes the correctness verification harder, which in turns means that the code becomes less safe. It will make you focus on the best case scenario, while ignoring all those annoying edge cases - too bad that 90% of the bugs are there.

Reading through the ISO C++ propaganda FAQ

I've been given a link to the C++ FAQ about exceptions, and unfortunately I read it. While there isn't much to argue on the technical side of it, it also carries some misleading statements, which might be true in absolute terms but don't let you see the big picture by not mentioning all that you need to know (which is the fundamental technique behind propaganda). An example is when they mention that eliminating ifs makes for more robust code, without mentioning that the same applies to all code branches, including exceptions.

Another argument that bothered me when I read it is the one about error propagation; this is the example they make:

void f1()
{
    try {
        // ...
        f2();
        // ...
    } catch (some_exception& e) {
        // ...code that handles the error...
    }
}
void f2() { ...; f3(); ...; }
void f3() { ...; f4(); ...; }
void f4() { ...; f5(); ...; }
void f5() { ...; f6(); ...; }
void f6() { ...; f7(); ...; }
void f7() { ...; f8(); ...; }
void f8() { ...; f9(); ...; }
void f9() { ...; f10(); ...; }
void f10()
{
    // ...
    if ( /*...some error condition...*/ )
        throw some_exception();
    // ...
}

The claim is that this code is more readable than the one with explicit error handling, because all the f2(), f3, …, f9() functions don't have to handle the error occurring in f10(). It is indeed a convincing argument, when presented in these terms, but is this really how our code looks like? In real life, you'll hardly have a chain of 1-liner functions, all defined next to each other in the same file. The moment that you realize that each one of these fn() functions might be twenty or thirty lines long, and that they might be scattered over different files, and be called not just by fn-1() but by any other function in the codebase, the picture does not look so rosy anymore: we get back to my main point of pain, that is that looking at the code of, say, f5(), I will not be able to tell if the errors thrown by it, or by any of the methods invoked by it, are properly handled.

Exceptions in APIs

A side note about projects using exceptions. I'm not really bothered when a library I need to use is throwing exceptions: having to write

try {
    Foo::fetch("http://example.com/resource.txt");
} catch (Foo::Exception &) {
    return false;
}

is not less readable or less safe than the code I'd write if Foo::fetch() returned an error code. I still do have a little complaint, because the library author has given himself the right to decide that a failure in his library should be considered a critical fault, whereas it may be that in my program it is an expected failure and using exceptions imposes a penalty which could have been avoided. But I digress.

As long as the library documents which exceptions are thrown, it is used by many people (which hopefully means that it has few bugs) and it is a library that I don't need to contribute to, wrapping some of its methods in try blocks is something I can live with.

One situation where I actually wish that libraries threw an exception is in out-of-memory situations; in that case, of course, I'd expect them to throw nothing else than std::bad_alloc, which is the exception emitted by the standard library in such situations. That allows the caller to decide whether to ignore the exception and have the process terminated (which is what I usually do, at least in desktop applications) or try their luck and handle the failure - the latter is not easy, but it can certainly be done.

This is one case where error returns can be problematic, because it's likely that your code would look something like

if (!Foo::open(fileName)) {   // suppose that this returns Error::OutOfMemory
    log("Failed to open " << fileName);
    return false;
}

and in this case there's actually a risk that your code is going to trigger an out-of-memory error in logging the message; this shouldn't be a concern in most cases, but I can imagine some situations where one might want to know which was the exact operation that first incurred in the out-of-memory failure.

So, I'm actually fine with new throwing. As for my code, my throw statement is actually spelt as return.

Mappero Geotagger has now moved from its previous page from this site to a new, separate website built with the awesome Nikola static website generator.

The main reason for this change is that I didn't have an online space where to host the application binaries, and I wanted to experiment with a different selling method. Now, downloads are (poorly) hidden behind a payment page, whereas in multiple places of the website I also mention that I can provide the application for free to whomever asks for it. While it might seem weird at first, I do honestly believe that this will not stop people from buying it: first of all, many people just think it's fair to pay for a software applications, and secondly, for some people writing an e-mail and establishing a personal contact with a stranger is actually harder than paying a small amount of money. And in all sincerity, the majority of the income I've had so far for Mappero Geotagger came from donations, rather than purchases; so, not much to lose here.

QBS and MXE

Anyway, since this is primarily a technical blog, I want to share my experiences with cross-building from Linux to Windows. As you might remember, some time ago I switched the build system of Mappero from qmake to QBS, and I haven't regretted it at all. I've managed to build the application in Linux (of course), macOS, as a Debian package on the Ubuntu PPA builders, on Windows with AppVeyor and, last but not least, on Linux for Windows using the mingw setup provided by the MXE project.

QBS worked surprisingly well also in this case, though I had to fight with a small bug on the toolchain detection, which is hopefully going to be fixed soon. For the few of you who are interested in achieving something similar, here's the steps I ran to configure QBS for mingw:

    MXE_BASE=<path-to-mxe>
    MXE_TARGET=x86_64-w64-mingw32.shared # 32 bit or static targets are also available

    MXE_PROFILE="mxe"
    QT_PROFILE="${MXE_PROFILE}-qt"
    qbs setup-toolchains "${MXE_BASE}/usr/bin/${MXE_TARGET}-g++" $MXE_PROFILE
    qbs config profiles.$MXE_PROFILE.cpp.toolchainPrefix "${MXE_TARGET}-" # temporary workaround
    qbs setup-qt "$MXE_BASE/usr/$MXE_TARGET/qt5/bin/qmake" ${QT_PROFILE}
    qbs config profiles.${QT_PROFILE}.baseProfile $MXE_PROFILE

Sorry for using that many environment variables ☺. After qbs is configured, it's just a matter of running

    qbs profile:$QT_PROFILE

to build the application. You will get a nice window binary and, once you collect all the needed library dependencies, you'll be able to run it on Windows. Or WINE ☺.

As part of this effort, I also had to build libraw, so I didn't miss the occasion to contribute its recipe to MXE. I'm also trying to get a change accepted, that would make MXE support the dynamic OpenGL selection available since Qt 5.4.