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

[Daniel Vetter]

On Wed, May 20, 2020 at 3:20 PM Simon Ser <contact at emersion.fr> wrote:
>
> 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

Oh reminds me, what should we do about open() - that one will fail,
the chardev is going away after all, not failing kinda doesn't make
sense. And more tricky, about creating new leases?

I think reasonable semantics here would be that both of these "create
a new open drm fd" operations can fail as soon as the device is
unplugged. Userspace needs to be able to deal with that.
-Daniel

>
> 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.



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch