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

[Daniel Vetter]

On Fri, May 22, 2020 at 12:54:32PM +0300, Pekka Paalanen wrote:
> On Wed, 20 May 2020 16:19:00 +0200
> Daniel Vetter <daniel at ffwll.ch> wrote:
> 
> > 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.
> 
> 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?

Dropped some thoughts there, tbh dunno, need some more discussions?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch