[PATCH v4 1/1] drm/doc: Document DRM device reset expectations

[André Almeida]

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>
---
 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..25a11b9b98fa 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
+section describes what is 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 it requires to. After
+detecting a reset, UMD will then proceed to report it to the application using
+the appropriated API error code, as explained in the below section 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 application to deal with device resets. However,
+there is no guarantee that the app will be correctly using such features, and
+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 keeps 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.
+
+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 is possible to
+determine that robustness is not 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.
+
+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
-- 
2.41.0