[PATCH] drm/doc: device hot-unplug for userspace

[Pekka Paalanen]

From: Pekka Paalanen <pekka.paalanen at collabora.com>

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 .

Signed-off-by: Pekka Paalanen <pekka.paalanen at collabora.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Sean Paul <sean at poorly.run>

---

Disclaimer: I am a userspace developer writing for other userspace developers.
I took some liberties in defining what should happen without knowing what is
actually possible or what existing drivers already implement.
---
 Documentation/gpu/drm-uapi.rst | 75 ++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 56fec6ed1ad8..80db4abd2cbd 100644
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -1,3 +1,5 @@
+.. Copyright 2020 DisplayLink (UK) Ltd.
+
 ===================
 Userland interfaces
 ===================
@@ -162,6 +164,79 @@ 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 13).
+
+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 to continue
+running, 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 or in some cases specific
+ioctls returning EIO.
+
+This goal raises at least the following requirements for the kernel and
+drivers:
+
+- The kernel must not hang, crash or oops, no matter what userspace was
+  in the middle of doing when the device disappeared.
+
+- All GPU jobs that can no longer run must have their fences
+  force-signalled to avoid inflicting hangs to userspace.
+
+- KMS connectors must change their status to disconnected.
+
+- Legacy modesets and pageflips fake success.
+
+- Atomic commits, both real and TEST_ONLY, fake success.
+
+- Pending non-blocking KMS operations deliver the DRM events userspace
+  is expecting.
+
+- If underlying memory disappears, the mmaps are replaced with harmless
+  zero pages where access does not raise SIGBUS. Reads return zeros,
+  writes are ignored.
+
+- dmabuf which point to memory that has disappeared are rewritten to
+  point to harmless zero pages, similar to mmaps. Imports still succeed
+  both ways: an existing device importing a dmabuf pointing to
+  disappeared memory, and a disappeared device importing any dmabuf.
+
+- Render ioctls return EIO which is then handled in userspace drivers,
+  e.g. Mesa, to have the device disappearance handled in the way
+  specified for each API (OpenGL, GL ES: GL_KHR_robustness;
+  Vulkan: VK_ERROR_DEVICE_LOST; etc.)
+
+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 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.
+
+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.
+
 .. _drm_driver_ioctl:
 
 IOCTL Support on Device Nodes
-- 
2.20.1