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

[Simon Ser]

On Wednesday, May 20, 2020 2:55 PM, Daniel Vetter <daniel at ffwll.ch> wrote:

> Maybe we should add an explicit note that there's no guarantee about the
> new chardev minor this new device will get, it could both inherit the
> existing one (you can't open the old one anymore anyway) or get a new one?
>
> Or does userspace want a guarantee, i.e. as long as there's still a handle
> open kernel guarantees to not recycle the chardev minor (which is what we
> currently do). In that case better to add that to your list of guarantees
> above.

The are race conditions to consider too, e.g.

- Compositor sends /dev/dri/card0 to a client
- card0 goes away
- Another device takes card0
- Client receives /dev/dri/card0 and then starts using it, but it's the
  wrong device

At first glance these seem like edge-cases that almost never happen.
However I've seen these happen in practice with connectors, especially
with docks.

One solution would be to number minor numbers like PIDs: don't recycle
card0 before we've reached the upper minor number limit.