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

[John Keeping]

On Wed, 13 Jan 2016 16:40:05 +0100, Daniel Vetter wrote:

> On Wed, Jan 13, 2016 at 02:34:25PM +0000, John Keeping wrote:
> > 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@xxxxxxxxxxxx>
> > > > ---
> > > >  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().  
> 
> Hm, that commit isn't terribly helpful. If that's really needed then imo I
> think we should extract a "drm_atomic_helper_plane_needs_vblank_wait()"
> helper that's used by both. But since rockchip does vblank_get/put calls
> I'd hope vblanks actually work correctly. And then the helper should work
> too.

I tried switching the call to rockchip_crtc_wait_for_update() to
drm_atomic_helper_wait_for_vblanks() and it works fine until I switch
the buffer associated with a cursor, at which point I get iommu page
faults, presumably because the GEM buffer is unreferenced too early.

AFAICT the buffer will be released via drm_atomic_state_free()
unconditionally, but I suspect I'm missing something since that would
mean every driver would hit a similar problem.