Hi, On 8/11/23 11:55, 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@xxxxxxxxxx> > > --- > > 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@xxxxxxxxxx/ > --- > 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..6bfaf7d4c074 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 > +ioclt to the KMD to check the reset status, and this can be checked more often ioctl > +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 a > +userspace that doesn't support robust interfaces (like an non-robust like a > +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 -- ~Randy
