linux-kernel - Re: [RFC/PATCH] drm/rockchip: don't wait for vblank if fb hasn't changed

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Message-ID: <20160113143425.363ce07a.john@metanate.com>
Date:	Wed, 13 Jan 2016 14:34:25 +0000
From:	John Keeping <john@...anate.com>
To:	Daniel Vetter <daniel@...ll.ch>
Cc:	Mark Yao <mark.yao@...k-chips.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-rockchip@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC/PATCH] drm/rockchip: don't wait for vblank if fb hasn't
 changed

On Wed, 13 Jan 2016 15:23:20 +0100, Daniel Vetter wrote:

> On Wed, Jan 13, 2016 at 12:53:34PM +0000, John Keeping wrote:
> > As commented in drm_atomic_helper_wait_for_vblanks(), userspace
> > relies on cursor ioctls being unsynced.  Converting the rockchip
> > driver to atomic has significantly impacted cursor performance by
> > making every cursor update wait for vblank.
> > 
> > By skipping the vblank sync when the framebuffer has not changed
> > (as is done in drm_atomic_helper_wait_for_vblanks()) we can avoid
> > this for the common case of moving the cursor and only need to
> > delay the cursor ioctl when the cursor icon changes.
> > 
> > I originally inserted a check on legacy_cursor_update as well, but
> > that caused a storm of iommu page faults.  I didn't investigate the
> > cause of those since this change gives enough of a performance
> > improvement for my use case.
> > 
> > This is RFC because of that and because the framebuffer_changed()
> > function is copied from drm_atomic_helper.c as a quick way to test
> > the result.
> > 
> > Signed-off-by: John Keeping <john@...anate.com>
> > ---
> >  drivers/gpu/drm/rockchip/rockchip_drm_fb.c | 27
> > +++++++++++++++++++++++++-- 1 file changed, 25 insertions(+), 2
> > deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c index f784488..8fd9821
> > 100644 --- a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > +++ b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > @@ -177,8 +177,28 @@ static void
> > rockchip_crtc_wait_for_update(struct drm_crtc *crtc)
> > crtc_funcs->wait_for_update(crtc); }
> >  
> > +static bool framebuffer_changed(struct drm_device *dev,
> > +				struct drm_atomic_state *old_state,
> > +				struct drm_crtc *crtc)
> > +{
> > +	struct drm_plane *plane;
> > +	struct drm_plane_state *old_plane_state;
> > +	int i;
> > +
> > +	for_each_plane_in_state(old_state, plane, old_plane_state,
> > i) {
> > +		if (plane->state->crtc != crtc &&
> > +		    old_plane_state->crtc != crtc)
> > +			continue;
> > +
> > +		if (plane->state->fb != old_plane_state->fb)
> > +			return true;
> > +	}
> > +
> > +	return false;
> > +}  
> 
> Please don't hand-roll logic that affects semantics like this. Instead
> please use drm_atomic_helper_wait_for_vblanks(), which should do this
> correctly for you.
> 
> If that's not the case then we need to improve the generic helper, or
> figure out what's different with rockhip.

According to commit 63ebb9f (drm/rockchip: Convert to support atomic
API) it's because rockchip doesn't have a hardware vblank counter.

I'm not entirely clear on why this prevents the use of
drm_atomic_helper_wait_for_vblanks().

> > +
> >  static void
> > -rockchip_atomic_wait_for_complete(struct drm_atomic_state
> > *old_state) +rockchip_atomic_wait_for_complete(struct drm_device
> > *dev, struct drm_atomic_state *old_state) {
> >  	struct drm_crtc_state *old_crtc_state;
> >  	struct drm_crtc *crtc;
> > @@ -194,6 +214,9 @@ rockchip_atomic_wait_for_complete(struct
> > drm_atomic_state *old_state) if (!crtc->state->active)
> >  			continue;
> >  
> > +		if (!framebuffer_changed(dev, old_state, crtc))
> > +			continue;
> > +
> >  		ret = drm_crtc_vblank_get(crtc);
> >  		if (ret != 0)
> >  			continue;
> > @@ -241,7 +264,7 @@ rockchip_atomic_commit_complete(struct
> > rockchip_atomic_commit *commit) 
> >  	drm_atomic_helper_commit_planes(dev, state, true);
> >  
> > -	rockchip_atomic_wait_for_complete(state);
> > +	rockchip_atomic_wait_for_complete(dev, state);
> >  
> >  	drm_atomic_helper_cleanup_planes(dev, state);
> >  
> > -- 
> > 2.7.0.rc3.140.g520a093
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@...ts.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/dri-devel  
> 

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