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Date:   Fri, 29 Sep 2023 12:32:22 +0200
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Subject: Re: [PATCH v8] drm/doc: Document DRM device reset expectations
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To:     =?UTF-8?Q?Andr=c3=a9_Almeida?= <andrealmeid@igalia.com>,
        dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
Cc:     pierre-eric.pelloux-prayer@amd.com,
        Sebastian Wick <sebastian.wick@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        =?UTF-8?B?TWFyZWsgT2zFocOhaw==?= <maraeo@gmail.com>,
        =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel.daenzer@mailbox.org>,
        Simon Ser <contact@emersion.fr>, amaranath.somalapuram@amd.com,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Daniel Stone <daniel@fooishbar.org>,
        Rob Clark <robdclark@gmail.com>, kernel-dev@igalia.com,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        alexander.deucher@amd.com,
        Pekka Paalanen <pekka.paalanen@collabora.com>,
        contactshashanksharma@gmail.com, Dave Airlie <airlied@gmail.com>,
        christian.koenig@amd.com
References: <20230929092509.42042-1-andrealmeid@igalia.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
In-Reply-To: <20230929092509.42042-1-andrealmeid@igalia.com>
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Precedence: bulk

Am 29.09.23 um 11:25 schrieb André Almeida:
> Create a section that specifies how to deal with DRM device resets for
> kernel and userspace drivers.
>
> Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> Acked-by: Sebastian Wick <sebastian.wick@redhat.com>
> Signed-off-by: André Almeida <andrealmeid@igalia.com>

Reviewed-by: Christian König <christian.koenig@amd.com>

I think that is now ready to be pushed. Do you have commit rights to 
drm-misc-next? If not please ping me.

Regards,
Christian.

> ---
> v8 changes:
> - Add acked-by tags
>
> v7: https://lore.kernel.org/dri-devel/20230818200642.276735-1-andrealmeid@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 eef5fd19bc92..632989df3727 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