Re: [PATCH 3/3] drm: Add CRTC_GET_SEQUENCE and CRTC_QUEUE_SEQUENCE ioctls

[Daniel Vetter]

On Thu, Jul 6, 2017 at 12:16 PM, Ville SyrjÃlÃ
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
>> > +   if (!dev->irq_enabled)
>> > +           return -EINVAL;
>> > +
>> > +   crtc = drm_crtc_find(dev, get_seq->crtc_id);
>> > +   if (!crtc)
>> > +           return -ENOENT;
>> > +
>> > +   pipe = drm_crtc_index(crtc);
>> > +
>> > +   get_seq->sequence = drm_vblank_count_and_time(dev, pipe, &now);
>>
>> This can give you and old vblank if the vblank is off (i.e. sw state
>> hasn't be regularly updated). I think we want a new
>> drm_crtc_accurate_vblank_count_and_time variant.
>
> Or better yet just do what Chris did for the old ioctl in commit
> b33b02707ba3 ("drm: Peek at the current counter/timestamp for vblank queries")

Yeah the READ_ONCE(vblank->enabled) is a nice fastpath. But we still
need the accurate one as slowpath in case the vblank irq is off.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch