Re: [RFC PATCH v3 1/4] drm/doc: Document DRM device reset expectations

["André Almeida" <andrealmeid@igalia.com>]

Em 21/06/2023 04:58, Pekka Paalanen escreveu:
> On Tue, 20 Jun 2023 21:57:16 -0300
> André Almeida <andrealmeid@igalia.com> wrote:
> 
>> Create a section that specifies how to deal with DRM device resets for
>> kernel and userspace drivers.
>>
>> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> 
> Hi André,
> 
> nice to see this! I ended up giving lots of grammar comments, but I'm
> not a native speaker. Generally it looks good to me.

Thank you for your feedback :)

> 
>> ---
>>   Documentation/gpu/drm-uapi.rst | 65 ++++++++++++++++++++++++++++++++++
>>   1 file changed, 65 insertions(+)
>>
>> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
>> index 65fb3036a580..da4f8a694d8d 100644
>> --- a/Documentation/gpu/drm-uapi.rst
>> +++ b/Documentation/gpu/drm-uapi.rst
>> @@ -285,6 +285,71 @@ 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. To recover
>> +from this kind of state, sometimes is needed to reset the device. This section
> 
> It seems unclear what "this kind of state" refers to, so maybe just write "errors"?
> 
> Maybe:
> 
> 	Some errors require resetting the device in order to make the
> 	device usable again.
> 
> I presume that recovery does not mean that the failed job could recover.
> 
>> +describes what's 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 hung is detected when a job gets stuck executing. KMD
> 
> s/hung/hang/ ?
> 
>> +then update it's internal reset tracking to be ready when userspace asks the
> 
> updates its
> 
> "update reset tracking"... do you mean that KMD records information
> about the reset in case userspace asks for it later?

Yes, kernel drivers do annotate whenever a reset happens, so it can 
report to userspace when it asks about resets.

For instance, this is the amdgpu implementation of 
AMDGPU_CTX_OP_QUERY_STATE2:

https://elixir.bootlin.com/linux/v6.3.8/source/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c#L548 


You can see there stored information about resets.

> 
>> +kernel about reset information. Drivers should implement the DRM_IOCTL_GET_RESET
>> +for that.
> 
> At this point, I'm not sure what "reset tracking" or "reset
> information" entails. Could something be said about those?
>  >> +
>> +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 it requires to. The
>> +DRM_IOCTL_GET_RESET is the default interface for those kind of checks. After
>> +detecting a reset, UMD will then proceed to report it to the application using
>> +the appropriated API error code, as explained in the bellow section about
> 
> s/bellow/below/
> 
>> +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 is using.
> 
> that it is using.
> 
>> +
>> +Graphical APIs provide ways to application to deal with device resets. However,
> 
> provide ways for applications to deal with
> 
>> +there's no guarantee that the app will be correctly using such features, and UMD
>> +can implement policies to close the app if it's a repeating offender, likely in
>> +a broken loop. This is done to ensure that it doesn't keeps blocking the user
> 
> does not keep
> 
> I think contractions are usually avoided in documents, but I'm not
> bothering to flag them all.
> 
>> +interface to be correctly displayed.
> 
> interface from being correctly displayed.
> 
>> +
>> +OpenGL
>> +~~~~~~
>> +
>> +Apps using OpenGL can rely on ``GL_ARB_robustness`` to be robust. This extension
>> +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 robustness isn't in use, 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.
> 
> What about GL ES? Is GL_ARB_robustness implemented or even defined there?
> 

I found this: 
https://registry.khronos.org/OpenGL/extensions/EXT/EXT_robustness.txt

"Since this is intended to be a version of ARB_robustness for OpenGL ES, 
it should be named accordingly."

I can add this to this paragraph.

> What about EGL returning errors like EGL_CONTEXT_LOST, would handling that not
> be enough from the app? The documented expectation is: "The application
> must destroy all contexts and reinitialise OpenGL ES state and objects
> to continue rendering."

I couldn't find the spec for EGL_CONTEXT_LOST, but I found for 
GL_CONTEXT_LOST, which I assume is similar.

GL_CONTEXT_LOST is only returned in some specific commands (that might 
cause a polling application to block indefinitely), so I don't think 
it's enough, given that the we can't guarantee that the application will 
call such commands after a reset, thus not being able to notice a reset.

https://registry.khronos.org/OpenGL-Refpages/gl4/html/glGetGraphicsResetStatus.xhtml

> 
>> +
>> +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 resets causes
>> +-----------------------
>> +
>> +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
> 
> What about VRAM contents? If userspace holds a dmabuf handle, can a GPU
> reset wipe that buffer? How would that be communicated?
> 

Yes, it can.

> The dmabuf may have originated in another process.
> 

Indeed, I think we might need to add an error code for dmabuf calls so 
the buffer user knows that it's invalid now because a reset has happened 
in the other device. I will need to read more dmabuf code to make sure 
how this would be possible.

> 
> Thanks,
> pq