[PATCH v7] drm/doc: Document DRM device reset expectations

[Sebastian Wick]

On Fri, Aug 18, 2023 at 05:06:42PM -0300, André Almeida wrote:
> Create a section that specifies how to deal with DRM device resets for
> kernel and userspace drivers.
> 
> Signed-off-by: André Almeida <andrealmeid at igalia.com>
> 
> ---
> 
> v7 changes:
>  - s/application/graphical API contex/ in the robustness part (Michel)
>  - Grammar fixes (Randy)
> 
> v6: https://lore.kernel.org/lkml/20230815185710.159779-1-andrealmeid@igalia.com/
> 
> v6 changes:
>  - Due to substantial changes in the content, dropped Pekka's Acked-by
>  - Grammar fixes (Randy)
>  - Add paragraph about disabling device resets
>  - Add note about integrating reset tracking in drm/sched
>  - Add note that KMD should return failure for contexts affected by
>    resets and UMD should check for this
>  - Add note about lack of consensus around what to do about non-robust
>    apps
> 
> v5: https://lore.kernel.org/dri-devel/20230627132323.115440-1-andrealmeid@igalia.com/
> ---
>  Documentation/gpu/drm-uapi.rst | 77 ++++++++++++++++++++++++++++++++++
>  1 file changed, 77 insertions(+)
> 
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
> index 65fb3036a580..3694bdb977f5 100644
> --- a/Documentation/gpu/drm-uapi.rst
> +++ b/Documentation/gpu/drm-uapi.rst
> @@ -285,6 +285,83 @@ for GPU1 and GPU2 from different vendors, and a third handler for
>  mmapped regular files. Threads cause additional pain with signal
>  handling as well.
>  
> +Device reset
> +============
> +
> +The GPU stack is really complex and is prone to errors, from hardware bugs,
> +faulty applications and everything in between the many layers. Some errors
> +require resetting the device in order to make the device usable again. This
> +section describes the expectations for DRM and usermode drivers when a
> +device resets and how to propagate the reset status.
> +
> +Device resets can not be disabled without tainting the kernel, which can lead to
> +hanging the entire kernel through shrinkers/mmu_notifiers. Userspace role in
> +device resets is to propagate the message to the application and apply any
> +special policy for blocking guilty applications, if any. Corollary is that
> +debugging a hung GPU context require hardware support to be able to preempt such
> +a GPU context while it's stopped.
> +
> +Kernel Mode Driver
> +------------------
> +
> +The KMD is responsible for checking if the device needs a reset, and to perform
> +it as needed. Usually a hang is detected when a job gets stuck executing. KMD
> +should keep track of resets, because userspace can query any time about the
> +reset status for a specific context. This is needed to propagate to the rest of
> +the stack that a reset has happened. Currently, this is implemented by each
> +driver separately, with no common DRM interface. Ideally this should be properly
> +integrated at DRM scheduler to provide a common ground for all drivers. After a
> +reset, KMD should reject new command submissions for affected contexts.
> +
> +User Mode Driver
> +----------------
> +
> +After command submission, UMD should check if the submission was accepted or
> +rejected. After a reset, KMD should reject submissions, and UMD can issue an
> +ioctl to the KMD to check the reset status, and this can be checked more often
> +if the UMD requires it. After detecting a reset, UMD will then proceed to report
> +it to the application using the appropriate API error code, as explained in the
> +section below about robustness.
> +
> +Robustness
> +----------
> +
> +The only way to try to keep a graphical API context working after a reset is if
> +it complies with the robustness aspects of the graphical API that it is using.
> +
> +Graphical APIs provide ways to applications to deal with device resets. However,
> +there is no guarantee that the app will use such features correctly, and a
> +userspace that doesn't support robust interfaces (like a non-robust
> +OpenGL context or API without any robustness support like libva) leave the
> +robustness handling entirely to the userspace driver. There is no strong
> +community consensus on what the userspace driver should do in that case,
> +since all reasonable approaches have some clear downsides.
> +
> +OpenGL
> +~~~~~~
> +
> +Apps using OpenGL should use the available robust interfaces, like the
> +extension ``GL_ARB_robustness`` (or ``GL_EXT_robustness`` for OpenGL ES). This
> +interface tells if a reset has happened, and if so, all the context state is
> +considered lost and the app proceeds by creating new ones. There's no consensus
> +on what to do to if robustness is not in use.
> +
> +Vulkan
> +~~~~~~
> +
> +Apps using Vulkan should check for ``VK_ERROR_DEVICE_LOST`` for submissions.
> +This error code means, among other things, that a device reset has happened and
> +it needs to recreate the contexts to keep going.
> +
> +Reporting causes of resets
> +--------------------------
> +
> +Apart from propagating the reset through the stack so apps can recover, it's
> +really useful for driver developers to learn more about what caused the reset in
> +the first place. DRM devices should make use of devcoredump to store relevant
> +information about the reset, so this information can be added to user bug
> +reports.
> +
>  .. _drm_driver_ioctl:
>  
>  IOCTL Support on Device Nodes

Acked-by: Sebastian Wick <sebastian.wick at redhat.com>

> -- 
> 2.41.0
>