[PATCH v3] drm/doc: device hot-unplug for userspace

Wed Jun 10 15:40:48 UTC 2020

On Mon, Jun 01, 2020 at 05:32:03PM +0300, Pekka Paalanen wrote:
> 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 .
> 
> A related Wayland protocol change proposal is at
> https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/35
> 
> 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>
> Cc: Simon Ser <contact at emersion.fr>
> 
> ---
> 
> v3:
> - update ENODEV doc (Daniel)
> - clarify existing vs. new mmaps (Andrey)
> - split into KMS and render/cross sections (Andrey, Daniel)
> - open() returns ENXIO (open(2) man page)
> - ioctls may return ENODEV (Andrey, Daniel)
> - new wayland-protocols MR
> 
> v2:
> - mmap reads/writes undefined (Daniel)
> - make render ioctl behaviour driver-specific (Daniel)
> - restructure the mmap paragraphs (Daniel)
> - chardev minor notes (Simon)
> - open behaviour (Daniel)
> - DRM leasing behaviour (Daniel)
> - added links
> 
> 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 | 114 ++++++++++++++++++++++++++++++++-
>  1 file changed, 113 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
> index 56fec6ed1ad8..db56c681b648 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 +164,116 @@ 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 to continue

I think it's "continue to run" not "to continue running". But not native
speaker either.

> +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, 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 to userspace.

I think "on userspace" not "to 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 disappeared, 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.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Only gap I could spot is additional other fd that float around, like
dma-buf, sync_file, drm_syncobj and stuff like that. I guess we can figure
these out as we go, which shouldn't be a big problem since this entire
document is very much work in progres.

Thanks for pushing this forward.
-Daniel

> +
>  .. _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 anymore present or is not yet fully initialized.
>  
>  EOPNOTSUPP:
>          Feature (like PRIME, modesetting, GEM) is not supported by the driver.
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch