drm/doc: device hot-unplug for userspace

Set up the expectations on how hot-unplugging a DRM device should look like to
userspace.

Written by Daniel Vetter's request and largely based on his comments in IRC and
from https://lists.freedesktop.org/archives/dri-devel/2020-May/265484.html .

A related Wayland protocol change proposal is at
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/35Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Simon Ser <contact@emersion.fr>
Cc: Ben Skeggs <skeggsb@gmail.com>
Cc: Karol Herbst <kherbst@redhat.com>
Acked-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707113805.30936-1-ppaalanen@gmail.com

Documentation/gpu/drm-uapi.rst

+.. Copyright 2020 DisplayLink (UK) Ltd.
+
 ===================
 Userland interfaces
 ===================
@@ -162,6 +164,116 @@ other hand, a driver requires shared state between clients which is
 visible to user-space and accessible beyond open-file boundaries, they
 cannot support render nodes.
 
+Device Hot-Unplug
+=================
+
+.. note::
+   The following is the plan. Implementation is not there yet
+   (2020 May).
+
+Graphics devices (display and/or render) may be connected via USB (e.g.
+display adapters or docking stations) or Thunderbolt (e.g. eGPU). An end
+user is able to hot-unplug this kind of devices while they are being
+used, and expects that the very least the machine does not crash. Any
+damage from hot-unplugging a DRM device needs to be limited as much as
+possible and userspace must be given the chance to handle it if it wants
+to. Ideally, unplugging a DRM device still lets a desktop continue to
+run, but that is going to need explicit support throughout the whole
+graphics stack: from kernel and userspace drivers, through display
+servers, via window system protocols, and in applications and libraries.
+
+Other scenarios that should lead to the same are: unrecoverable GPU
+crash, PCI device disappearing off the bus, or forced unbind of a driver
+from the physical device.
+
+In other words, from userspace perspective everything needs to keep on
+working more or less, until userspace stops using the disappeared DRM
+device and closes it completely. Userspace will learn of the device
+disappearance from the device removed uevent, ioctls returning ENODEV
+(or driver-specific ioctls returning driver-specific things), or open()
+returning ENXIO.
+
+Only after userspace has closed all relevant DRM device and dmabuf file
+descriptors and removed all mmaps, the DRM driver can tear down its
+instance for the device that no longer exists. If the same physical
+device somehow comes back in the mean time, it shall be a new DRM
+device.
+
+Similar to PIDs, chardev minor numbers are not recycled immediately. A
+new DRM device always picks the next free minor number compared to the
+previous one allocated, and wraps around when minor numbers are
+exhausted.
+
+The goal raises at least the following requirements for the kernel and
+drivers.
+
+Requirements for KMS UAPI
+-------------------------
+
+- KMS connectors must change their status to disconnected.
+
+- Legacy modesets and pageflips, and atomic commits, both real and
+  TEST_ONLY, and any other ioctls either fail with ENODEV or fake
+  success.
+
+- Pending non-blocking KMS operations deliver the DRM events userspace
+  is expecting. This applies also to ioctls that faked success.
+
+- open() on a device node whose underlying device has disappeared will
+  fail with ENXIO.
+
+- Attempting to create a DRM lease on a disappeared DRM device will
+  fail with ENODEV. Existing DRM leases remain and work as listed
+  above.
+
+Requirements for Render and Cross-Device UAPI
+---------------------------------------------
+
+- All GPU jobs that can no longer run must have their fences
+  force-signalled to avoid inflicting hangs on userspace.
+  The associated error code is ENODEV.
+
+- Some userspace APIs already define what should happen when the device
+  disappears (OpenGL, GL ES: `GL_KHR_robustness`_; `Vulkan`_:
+  VK_ERROR_DEVICE_LOST; etc.). DRM drivers are free to implement this
+  behaviour the way they see best, e.g. returning failures in
+  driver-specific ioctls and handling those in userspace drivers, or
+  rely on uevents, and so on.
+
+- dmabuf which point to memory that has disappeared will either fail to
+  import with ENODEV or continue to be successfully imported if it would
+  have succeeded before the disappearance. See also about memory maps
+  below for already imported dmabufs.
+
+- Attempting to import a dmabuf to a disappeared device will either fail
+  with ENODEV or succeed if it would have succeeded without the
+  disappearance.
+
+- open() on a device node whose underlying device has disappeared will
+  fail with ENXIO.
+
+.. _GL_KHR_robustness: https://www.khronos.org/registry/OpenGL/extensions/KHR/KHR_robustness.txt
+.. _Vulkan: https://www.khronos.org/vulkan/
+
+Requirements for Memory Maps
+----------------------------
+
+Memory maps have further requirements that apply to both existing maps
+and maps created after the device has disappeared. If the underlying
+memory disappears, the map is created or modified such that reads and
+writes will still complete successfully but the result is undefined.
+This applies to both userspace mmap()'d memory and memory pointed to by
+dmabuf which might be mapped to other devices (cross-device dmabuf
+imports).
+
+Raising SIGBUS is not an option, because userspace cannot realistically
+handle it. Signal handlers are global, which makes them extremely
+difficult to use correctly from libraries like those that Mesa produces.
+Signal handlers are not composable, you can't have different handlers
+for GPU1 and GPU2 from different vendors, and a third handler for
+mmapped regular files. Threads cause additional pain with signal
+handling as well.
+
 .. _drm_driver_ioctl:
 
 IOCTL Support on Device Nodes
@@ -199,7 +311,7 @@ EPERM/EACCES:
         difference between EACCES and EPERM.
 
 ENODEV:
-        The device is not (yet) present or fully initialized.
+        The device is not present anymore or is not yet fully initialized.
 
 EOPNOTSUPP:
         Feature (like PRIME, modesetting, GEM) is not supported by the driver.