fidzu : freedesktop

This week, I reviewed the last available version of the Linux KMS Color API. Specifically, I explored the proposed API by Harry Wentland and Alex Hung (AMD), their implementation for the AMD display driver and tracked the parallel efforts of Uma Shankar and Chaitanya Kumar Borah (Intel) in bringing this plane color management to life. With this API in place, compositors will be able to provide better HDR support and advanced color management for Linux users.

To get a hands-on feel for the API's potential, I developed a fork of drm_info compatible with the new color properties. This allowed me to visualize the display hardware color management capabilities being exposed. If you're curious and want to peek behind the curtain, you can find my exploratory work on the drm_info/kms_color branch. The README there will guide you through the simple compilation and installation process.

Note: You will need to update libdrm to match the proposed API. You can find an updated version in my personal repository here. To avoid potential conflicts with your official libdrm installation, you can compile and install it in a local directory. Then, use the following command: export LD_LIBRARY_PATH="/usr/local/lib/"

In this post, I invite you to familiarize yourself with the new API that is about to be released. You can start doing as I did below: just deploy a custom kernel with the necessary patches and visualize the interface with the help of drm_info. Or, better yet, if you are a userspace developer, you can start developing user cases by experimenting with it.

The more eyes the better.

KMS Color API on AMD

The great news is that AMD's driver implementation for plane color operations is being developed right alongside their Linux KMS Color API proposal, so it's easy to apply to your kernel branch and check it out. You can find details of their progress in the AMD's series.

I just needed to compile a custom kernel with this series applied, intentionally leaving out the AMD_PRIVATE_COLOR flag. The AMD_PRIVATE_COLOR flag guards driver-specific color plane properties, which experimentally expose hardware capabilities while we don't have the generic KMS plane color management interface available.

If you don't know or don't remember the details of AMD driver specific color properties, you can learn more about this work in my blog posts [1] [2] [3]. As driver-specific color properties and KMS colorops are redundant, the driver only advertises one of them, as you can see in AMD workaround patch 24.

So, with the custom kernel image ready, I installed it on a system powered by AMD DCN3 hardware (i.e. my Steam Deck). Using my custom drm_info, I could clearly see the Plane Color Pipeline with eight color operations as below:

└───"COLOR_PIPELINE" (atomic): enum {Bypass, Color Pipeline 258} = Bypass
    ├───Bypass
    └───Color Pipeline 258
        ├───Color Operation 258
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 1D Curve
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   └───"CURVE_1D_TYPE" (atomic): enum {sRGB EOTF, PQ 125 EOTF, BT.2020 Inverse OETF} = sRGB EOTF
        ├───Color Operation 263
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = Multiplier
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   └───"MULTIPLIER" (atomic): range [0, UINT64_MAX] = 0
        ├───Color Operation 268
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 3x4 Matrix
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   └───"DATA" (atomic): blob = 0
        ├───Color Operation 273
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 1D Curve
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   └───"CURVE_1D_TYPE" (atomic): enum {sRGB Inverse EOTF, PQ 125 Inverse EOTF, BT.2020 OETF} = sRGB Inverse EOTF
        ├───Color Operation 278
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 1D LUT
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   ├───"SIZE" (atomic, immutable): range [0, UINT32_MAX] = 4096
        │   ├───"LUT1D_INTERPOLATION" (immutable): enum {Linear} = Linear
        │   └───"DATA" (atomic): blob = 0
        ├───Color Operation 285
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 3D LUT
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   ├───"SIZE" (atomic, immutable): range [0, UINT32_MAX] = 17
        │   ├───"LUT3D_INTERPOLATION" (immutable): enum {Tetrahedral} = Tetrahedral
        │   └───"DATA" (atomic): blob = 0
        ├───Color Operation 292
        │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 1D Curve
        │   ├───"BYPASS" (atomic): range [0, 1] = 1
        │   └───"CURVE_1D_TYPE" (atomic): enum {sRGB EOTF, PQ 125 EOTF, BT.2020 Inverse OETF} = sRGB EOTF
        └───Color Operation 297
            ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, Multiplier, 3D LUT} = 1D LUT
            ├───"BYPASS" (atomic): range [0, 1] = 1
            ├───"SIZE" (atomic, immutable): range [0, UINT32_MAX] = 4096
            ├───"LUT1D_INTERPOLATION" (immutable): enum {Linear} = Linear
            └───"DATA" (atomic): blob = 0

Note that Gamescope is currently using AMD driver-specific color properties implemented by me, Autumn Ashton and Harry Wentland. It doesn't use this KMS Color API, and therefore COLOR_PIPELINE is set to Bypass. Once the API is accepted upstream, all users of the driver-specific API (including Gamescope) should switch to the KMS generic API, as this will be the official plane color management interface of the Linux kernel.

KMS Color API on Intel

On the Intel side, the driver implementation available upstream was built upon an earlier iteration of the API. This meant I had to apply a few tweaks to bring it in line with the latest specifications. You can explore their latest work here. For a more simplified handling, combining the V9 of the Linux Color API, Intel's contributions, and my necessary adjustments, check out my dedicated branch.

I then compiled a kernel from this integrated branch and deployed it on a system featuring Intel TigerLake GT2 graphics. Running my custom drm_info revealed a Plane Color Pipeline with three color operations as follows:

├───"COLOR_PIPELINE" (atomic): enum {Bypass, Color Pipeline 480} = Bypass
│   ├───Bypass
│   └───Color Pipeline 480
│       ├───Color Operation 480
│       │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, 1D LUT Mult Seg, 3x3 Matrix, Multiplier, 3D LUT} = 1D LUT Mult Seg
│       │   ├───"BYPASS" (atomic): range [0, 1] = 1
│       │   ├───"HW_CAPS" (atomic, immutable): blob = 484
│       │   └───"DATA" (atomic): blob = 0
│       ├───Color Operation 487
│       │   ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, 1D LUT Mult Seg, 3x3 Matrix, Multiplier, 3D LUT} = 3x3 Matrix
│       │   ├───"BYPASS" (atomic): range [0, 1] = 1
│       │   └───"DATA" (atomic): blob = 0
│       └───Color Operation 492
│           ├───"TYPE" (immutable): enum {1D Curve, 1D LUT, 3x4 Matrix, 1D LUT Mult Seg, 3x3 Matrix, Multiplier, 3D LUT} = 1D LUT Mult Seg
│           ├───"BYPASS" (atomic): range [0, 1] = 1
│           ├───"HW_CAPS" (atomic, immutable): blob = 496
│           └───"DATA" (atomic): blob = 0

Observe that Intel's approach introduces additional properties like "HW_CAPS" at the color operation level, along with two new color operation types: 1D LUT with Multiple Segments and 3x3 Matrix. It's important to remember that this implementation is based on an earlier stage of the KMS Color API and is awaiting review.

A Shout-Out to Those Who Made This Happen

I'm impressed by the solid implementation and clear direction of the V9 of the KMS Color API. It aligns with the many insightful discussions we've had over the past years. A huge thank you to Harry Wentland and Alex Hung for their dedication in bringing this to fruition!

Beyond their efforts, I deeply appreciate Uma and Chaitanya's commitment to updating Intel's driver implementation to align with the freshest version of the KMS Color API. The collaborative spirit of the AMD and Intel developers in sharing their color pipeline work upstream is invaluable. We're now gaining a much clearer picture of the color capabilities embedded in modern display hardware, all thanks to their hard work, comprehensive documentation, and engaging discussions.

Finally, thanks all the userspace developers, color science experts, and kernel developers from various vendors who actively participate in the upstream discussions, meetings, workshops, each iteration of this API and the crucial code review process. I'm happy to be part of the final stages of this long kernel journey, but I know that when it comes to colors, one step is completed for new challenges to be unlocked.

Looking forward to meeting you in this year Linux Display Next hackfest, organized by AMD in Toronto, to further discuss HDR, advanced color management, and other display trends.

19 May 2025 9:05pm GMT

Hi!

Today wlroots 0.19.0 has finally been released! Among the newly supported protocols, color-management-v1 lays the first stone of HDR support (backend and renderer bits are still being reviewed) and ext-image-copy-capture-v1 enhances the previous screen capture protocol with better performance. Explicit synchronization is now fully supported, and display-only devices such as gud or DisplayLink can now be used with wlroots. See the release notes for more details! I hope I'll be able to go back to some feature work and reviews now that the release is out of the way.

In other graphics news, I've finished my review of the core DRM patches for the new KMS color pipeline. Other kernel folks have reviewed the patches, we're just waiting on a user-space implementation now (which various compositor folks are working on). I've started a discussion about libliftoff support.

In addition to wlroots, this month I've also released a new version of my mobile IRC client, Goguma 0.8.0. delthas has sent a patch to synchronize pinned and muted conversations across devices via soju. Thanks to pounce, Goguma now supports message reactions (not included in the release):

My extended-isupport IRCv3 specification has been accepted. It allows servers to advertise metadata such as the maximum nickname length or IRC network name early (before the user provides a nickname and authentication details), which is useful for building nice connection UIs. I've posted another proposal for IRC network icons.

go-smtp 0.22.0 has been released with an improved DATA command API, RRVS support (Require Recipient Valid Since), and custom hello after reset or STARTTLS. I've also spent quite a bit of time reaching out to companies for XDC 2025 sponsorships.

See you next month!

14 May 2025 10:00pm GMT

It has been almost a year since my last update on the Rockchip NPU, and though I'm a bit sad that I haven't had more time to work on it, I'm happy that I found some time earlier this year for this.

Quoting from my last update on the Rockchip NPU driver:

The kernel driver is able to fully use the three cores in the NPU, giving us the possibility of running 4 simultaneous object detection inferences such as the one below on a stream, at almost 30 frames per second.

All feedback has been incorporated in a new revision of the kernel driver and it was submitted to the Linux kernel mailing list.

Though I'm very happy with the direction the kernel driver is taking, I would have liked to make faster progress on it. I have spent the time since the first revision on making the Etnaviv NPU driver ready to be deployed in production (will be blogging about this soon), and also had to take some non-upstream work to pay my bills.

Next I plan to cleanup the userspace driver so it's ready for review, and then I will go for a third revision of the kernel driver.

12 May 2025 5:30am GMT