Re: [PATCH 6/6] drm/i915: Really wait for pending flips in intel_pipe_set_base()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Feb 13, 2013 at 04:49:35PM +0100, Daniel Vetter wrote:
> On Tue, Jan 29, 2013 at 06:13:38PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Since obj->pending_flips was never set, intel_pipe_set_base() never
> > actually waited for pending page flips to complete.
> > 
> > We really do want to wait for the pending flips, because otherwise the
> > mmio surface base address update could overtake the flip, and you
> > could end up with an old frame on the screen once the flip really
> > completes.
> > 
> > Just call intel_crtc_wait_pending_flips_locked() instead of
> > intel_finish_fb() from intel_pipe_set_base() to achieve the
> > desired result.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index a2e04f7..7e047c1 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -2330,8 +2330,7 @@ intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
> >  		return ret;
> >  	}
> >  
> > -	if (crtc->fb)
> > -		intel_finish_fb(crtc->fb);
> > +	intel_crtc_wait_for_pending_flips_locked(crtc);
> 
> Ah, I see now why you grab dev->struct_mutex and need to kick waiters from
> the pending flip queue. I think grabbing the mutex twice isn't a major
> offense, since both the crtc disable code and set_base are slowpaths used
> rarely. So what about simply calling wait_for_pending_flips before
> grabbing the mutex in intel_pipe_set_base? We could then also inline
> finish_fb into it's only callsite ...

I didn't want to slow down intel_pipe_set_base() too much. If we wait
for pending flips before pinning the new fb, we can never achieve any
parallelism there. But if no-one cares about that, we can reorder.

-- 
Ville Syrjälä
Intel OTC