[Intel-gfx] [PATCH 10/37] drm: add per-crtc locks

[Daniel Vetter]

On Thu, Dec 13, 2012 at 12:38 PM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>>   And if we _really_ want such semantics, we can always get them by
>>   introducing another pageflip mutex between the mode_config.mutex and
>>   the individual crtc locks. Pageflips crossing more than one crtc
>>   would then need to take that lock first, to lock out concurrent
>>   multi-crtc pageflips.
>
> I'm actually concerned with multi CRTC page flips, not for moving planes
> between CRTCs, but mainly for updating content on genlocked displays
> atomically. We need to avoid deadlocks between multiple CRTC locks. Trying
> to take the CRTC locks in the same order would be a solution, but since
> user space can send the props in any order, it's going to require extra
> of work.

Ordering CRTC locks should also work - modeset_lock_all takes them
always in the same order, and as long as you only take a single crtc
nested within the modeset lock that's still ok (e.g. the load-detect
code). We don't have many CRTCs, so even the dumbest sort will be fast
enough. A bit of work will be required to make lockdep happy. But we
can achieve this by nesting CRTCs within a fake lock encapsulated by
the lock/unlock helper functions.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch