linux-kernel - Re: [PATCH 3/3] drm: Add CRTC_GET_SEQUENCE and CRTC_QUEUE

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Date:   Thu, 6 Jul 2017 13:04:18 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Keith Packard <keithp@...thp.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Airlie <airlied@...hat.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 3/3] drm: Add CRTC_GET_SEQUENCE and CRTC_QUEUE_SEQUENCE ioctls

On Thu, Jul 6, 2017 at 12:16 PM, Ville Syrjälä
<ville.syrjala@...ux.intel.com> 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