On Sat, Mar 11, 2017 at 11:53:04AM +0100, Daniel Vetter wrote: > On Fri, Mar 10, 2017 at 01:13:12PM -0500, Sean Paul wrote: > > On Fri, Mar 10, 2017 at 05:52:34PM +0100, Daniel Vetter wrote: > > > On Fri, Mar 10, 2017 at 11:01:54AM -0500, Sean Paul wrote: > > > > On Fri, Mar 10, 2017 at 02:12:14PM +1000, Dave Airlie wrote: > > > > > Talk to Jonas (jadahl) on irc, he had 3 mutters running and on hotplug > > > > > all 3 of them were diving into the connector getting code. Now I think > > > > > we can avoid this in userspace by not probing when not owning the VT, > > > > > but it's still messy behaviour. > > > > > > > > > > It looks like one thread does a getconnector, this fills out the EDID > > > > > blob with a specific blob id, > > > > > then second thread does getconnector, which replaces the EDID blob > > > > > with another one > > > > > the first thread calls get property blob, but the blob id it is > > > > > looking for is destroyed. > > > > > > > > > > Ideas? > > > > > > > > I think you could probably fix this by introducing a new > > > > drm_property_overwrite_global_blob() function. If the size of the current blob is > > > > the same as the new blob, you could just overwrite the data and preserve the id > > > > and references. You could then replace the drm_property_replace_global_blob() call in > > > > drm_mode_connector_update_edid_property() to call the new function. Of course, > > > > you would need to fall back on *replace_global_blob() if size differed, but it > > > > would probably solve the issue above. > > > > > > blobs are imutable (not that userspace can use that much since we can > > > reuse ids), > > > > Pedantic distinction: blobs themselves aren't inherantly immutable. However, the > > EDID property is defined as immutable. I suppose there could conceivably be some > > userspace somewhere that relies on this. Bummer. > > It's more tricky: The property is labelled as IMMUTABLE, but that's a lie, > it essentially only means that userspace isn't allowed to change it. The > kernel is. > > But blobs themselves are immutable, this is the same semantics as with > framebuffer objects, and we carried that over when we started to allow > userspace to create blobs (for atomic). The only benefit is for drivers, > as long as you hold a life reference you can just compare pointers and if > they're equal, you know nothing changed. Ah, right. I had conveniently forgotten this detail. > Due to id reuse, userspace > unfortunately can't do this (and yes we've had clever userspace who got > this wrong). > > > > I think if we do this we should compare the contents of the > > > blob. > > > > > > > That said, I'm not convinced it should be kernel's responsibility to really > > > > solve this problem. How much effort should we put into mitigating the effects of > > > > a racey and wasteful userspace? > > > > > > It's a tradeoff between "why should the kernel care" and "every compositor > > > will get this wrong". Maybe something like libweston will fix this > > > eventually .... > > > > So the question is whether it's worth it to penalize well-behaved compositors > > with superfluous memcmp on EDID fetches to make up for the ill-behaved ones. I > > don't think it is, but perhaps there are other benefits to preserving the single > > blob for the lifetime of the connection. > > If you can profile that memcmp in a 25ms edid read, I'll buy this argument Sure. It just sucks to add extra code/complexity to cover such a basic faux pas. > :-) I think the better argument against is that there's still cases where > userspace must recover from similar issues (getconnector racing with a hpd > which can also update the edid), but those are even rarer. It might be > good to keep the easier-to-hit races open, to force better error handling > in userspace. Ok, we're saying the same thing, you just have better reasoning :-) Sean > -Daniel > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > https://lists.freedesktop.org/mailman/listinfo/dri-devel _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
