[PATCH v5 1/1] drm/doc: Document DRM device reset expectations

[Sebastian Wick]

On Tue, Jun 27, 2023 at 3:23 PM André Almeida <andrealmeid at igalia.com> wrote:
>
> Create a section that specifies how to deal with DRM device resets for
> kernel and userspace drivers.
>
> Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>
> Signed-off-by: André Almeida <andrealmeid at igalia.com>
> ---
>
> v4: https://lore.kernel.org/lkml/20230626183347.55118-1-andrealmeid@igalia.com/
>
> Changes:
>  - Grammar fixes (Randy)
>
>  Documentation/gpu/drm-uapi.rst | 68 ++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
>
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
> index 65fb3036a580..3cbffa25ed93 100644
> --- a/Documentation/gpu/drm-uapi.rst
> +++ b/Documentation/gpu/drm-uapi.rst
> @@ -285,6 +285,74 @@ 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
> +sections describes the expectations for DRM and usermode drivers when a
> +device resets and how to propagate the reset status.
> +
> +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 stats for an 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.
> +
> +User Mode Driver
> +----------------
> +
> +The UMD should check before submitting new commands to the KMD if the device has
> +been reset, 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 an application 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 the
> +UMD can implement policies to close the app if it is a repeating offender,
> +likely in a broken loop. This is done to ensure that it does not keep blocking
> +the user interface from being correctly displayed. This should be done even if
> +the app is correct but happens to trigger some bug in the hardware/driver.

I still don't think it's good to let the kernel arbitrarily kill
processes that it thinks are not well-behaved based on some heuristics
and policy.

Can't this be outsourced to user space? Expose the information about
processes causing a device and let e.g. systemd deal with coming up
with a policy and with killing stuff.

> +
> +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. If it is possible to
> +determine that robustness is not in use, the UMD will terminate the app when a
> +reset is detected, giving that the contexts are lost and the app won't be able
> +to figure this out and recreate the contexts.
> +
> +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
> +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
> --
> 2.41.0
>