On Wed, 20 May 2020 16:19:00 +0200 Daniel Vetter <daniel@xxxxxxxx> wrote: > On Wed, May 20, 2020 at 3:20 PM Simon Ser <contact@xxxxxxxxxxx> wrote: > > > > On Wednesday, May 20, 2020 2:55 PM, Daniel Vetter <daniel@xxxxxxxx> 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. Hi, yeah, we can make mmap read/write end result undefined, recycle char minors like pids, and let new open()s and new leases fail. Pretty much everything Daniel and Simon said make sense to me. I'll spin a v2, but maybe next week. What about the drm_ioctl() issue Andrey pointed out? Thanks, pq
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