linux-kernel - Re: [PATCH v3 1/4] drm/vblank: warn on sending stale event

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Message-ID: <20190625200245.GD12905@phenom.ffwll.local>
Date:   Tue, 25 Jun 2019 22:02:45 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Robert Beckett <bob.beckett@...labora.com>,
        dri-devel@...ts.freedesktop.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/4] drm/vblank: warn on sending stale event

On Tue, Jun 25, 2019 at 10:00:42PM +0200, Daniel Vetter wrote:
> On Tue, Jun 25, 2019 at 06:59:12PM +0100, Robert Beckett wrote:
> > Warn when about to send stale vblank info and add advice to
> > documentation on how to avoid.
> > 
> > Signed-off-by: Robert Beckett <bob.beckett@...labora.com>
> > ---
> >  drivers/gpu/drm/drm_vblank.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> > index 603ab105125d..7dabb2bdb733 100644
> > --- a/drivers/gpu/drm/drm_vblank.c
> > +++ b/drivers/gpu/drm/drm_vblank.c
> > @@ -918,6 +918,19 @@ EXPORT_SYMBOL(drm_crtc_arm_vblank_event);
> >   *
> >   * See drm_crtc_arm_vblank_event() for a helper which can be used in certain
> >   * situation, especially to send out events for atomic commit operations.
> > + *
> > + * Care should be taken to avoid stale timestamps. If:
> > + *   - your driver has vblank support (i.e. dev->num_crtcs > 0)
> > + *   - the vblank irq is off (i.e. no one called drm_crtc_vblank_get)
> 
> drm_crtc_vblank_get() so it becomes a neat hyperlink.
> 
> > + *   - from the vblank code's pov the pipe is still running (i.e. not
> > + *     in-between a drm_crtc_vblank_off()/on() pair)
> 
> Not sure the above will lead to great markup, maybe spell out the
> drm_crtc_vblank_on() and maybe make it a bit clearer like "i.e. not after
> the call to drm_crtc_vblank_off() but before the next call to
> drm_crtc_vblank_on()" so it's clear which said of this pair we're talking
> about.
> 
> > + * If all of these conditions hold then drm_crtc_send_vblank_event is
> 
> style nit: the enumeration is one sentence (and and oxford comman missing
> but whatever), but you don't continue it here. Also, does the enumeration
> look pretty in the htmldocs output?
> 
> > + * going to give you a garbage timestamp and and sequence number (the last
> > + * recorded before the irq was disabled). If you call drm_crtc_vblank_get/put
> > + * around it, or after vblank_off, then either of those will have rolled things
> > + * forward for you.
> 
> Again pls fix the markup so all the function reference stick out and work.

One more style nit: s/you/drivers/, so maybe:

"Drivers must either hold a vblank reference acquired through
drm_crtc_vblank_get() or the vblank must have been shut off by calling
drm_crtc_vblank_off()." Those functions then have in turn links and hints
what you also need to do, like not forget to call drm_crtc_vblank_put().
> 
> > + * So, drivers should call drm_crtc_vblank_off() before this function in their
> > + * crtc atomic_disable handlers.
> 
> Imo this sentence here is needed but a bit confusing, and we have it
> documented already in the atomic_disable hook.
> 
> Other option would be to list all the places where a driver might want to
> call this and how they could get it wrong, which imo doesn't make sense.
> 
> With the nits addressed:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
> 
> >   */
> >  void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
> >  				struct drm_pending_vblank_event *e)
> > @@ -925,8 +938,12 @@ void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
> >  	struct drm_device *dev = crtc->dev;
> >  	u64 seq;
> >  	unsigned int pipe = drm_crtc_index(crtc);
> > +	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
> >  	ktime_t now;
> >  
> > +	WARN_ONCE(dev->num_crtcs > 0 && !vblank->enabled && !vblank->inmodeset,
> > +		  "sending stale vblank info\n");
> > +
> >  	if (dev->num_crtcs > 0) {
> >  		seq = drm_vblank_count_and_time(dev, pipe, &now);
> >  	} else {
> > -- 
> > 2.18.0
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch