Re: [PATCH] drm/atomic: Call ww_acquire_done after check phase is complete

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 06-08-15 om 00:25 schreef Daniel Vetter:
> On Wed, Aug 5, 2015 at 8:13 PM, Maarten Lankhorst
> <maarten.lankhorst@linux.intel.com> wrote:
>> Op 05-08-15 om 17:03 schreef Daniel Vetter:
>>> On Wed, Aug 5, 2015 at 4:57 PM, Maarten Lankhorst
>>> <maarten.lankhorst@linux.intel.com> wrote:
>>>> Op 05-08-15 om 15:08 schreef Daniel Vetter:
>>>>> We want to make sure that no one tries to acquire more locks and
>>>>> states, and ww mutexes provide debug facilities for that. So use them.
>>>>>
>>>>> Cc: Rob Clark <robdclark@gmail.com>
>>>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>>> ---
>>>>>  drivers/gpu/drm/drm_atomic.c | 2 ++
>>>>>  1 file changed, 2 insertions(+)
>>>> I like the idea, played with the thought myself, but I think it might need to be slightly less strict for transitional drivers.
>>> What would blow up? This should only be called fairly late in the
>>> transition when most of the atomic handling is correctly done. And
>>> i915 is probably the most extreme example of a conversion, so if it
>>> works out for us I think everyone else should be fine too.
>> Might blow up with transitional helpers, though not 100% sure if it would.
> Transitional helpers don't use the top-level atomic_commit/check entry
> points and hence don't use this function here at all.
>
>> Also if atomic_check returns -EDEADLK you would still say acquire_done, that will definitely blow up in legitimate cases..
>>
>> If it doesn't blow up transitional helpers and you fix the -EDEADLK, acked-by. :-)
> Yeah that needs to be fixed ;-)
After some sleep I think only when ret == 0 we should call acquire_done.
>>> Generally drivers only started to do fancy stuff with get_*_state once
>>> converted to atomic to start exploiting it, not before the transition
>>> is completed. i915 is different since we have a lot of our own modeset
>> Should we electrify drm_atomic_get_{*}_state too, to force everyone to use the get_existing_state versions?
> And I think this is the killer - we unconditionally take the locks
> again, taking advantage of -EALREADY. But with this patch that will
> blow and we need to patch up all the existing code to use the
> get_existing_state functions. That will be a bigger series I guess ...
Depends, this only happens when the object can not be found in the state the locks are taken for planes and crtc's.
But it seems for connectors it happens unconditionally.
> I'll make a note about this and defer this for now. But the
> get_*_state vs. get_existing_*_state thing will actually make these
> two functions more distinctive, so I think this patch here will be
> really useful. But needs driver fixes and kerneldoc updates too.
I think this is fine; grabbing existing state for crtc's and planes will work and grabbing new state during atomic_commit is a bug anyway.
The only driver that uses drm_atomic_get_connector_state is i915, which uses it for the setup phase.

~Maarten
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