Re: [PATCH] drm/i915: Eliminate race from gen2/3 page flip interrupt handling

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

On Mon, Feb 18, 2013 at 01:07:53PM +0100, Paul Menzel wrote:
> Am Montag, den 18.02.2013, 13:57 +0200 schrieb ville.syrjala@linux.intel.com:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > If the interrupt handler were to process a previous vblank interrupt and
> > the following flip pending interrupt at the same time, the page flip
> > would be complete too soon.
> 
> »would complete« or »would be complete*d*«

The second on is what I meant. Will fix.

> 
> > To eliminate this race check the live pending flip status from the ISR
> > register before finishing the page flip.
> 
> Could this be tested somehow? Could a test case be written for this?

It shouldn't be too difficult to force it from within the kernel. Just
turn off interrupts before vblank start, wait until vblank start is
passed, execute the MI_DISPLAY_FLIP, and finally turn the interrupts
back on again.

Given the timing constraints I'm not sure we can come up with anything
that would realiably hit it otherwise.

> 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_irq.c | 21 +++++++++++++++------
> >  1 file changed, 15 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> > index 9fde49a..3de570c 100644
> > --- a/drivers/gpu/drm/i915/i915_irq.c
> > +++ b/drivers/gpu/drm/i915/i915_irq.c
> > @@ -2284,8 +2284,11 @@ static irqreturn_t i8xx_irq_handler(int irq, void *arg)
> >  		    drm_handle_vblank(dev, 0)) {
> >  			if (iir & I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT) {
> >  				intel_prepare_page_flip(dev, 0);
> > -				intel_finish_page_flip(dev, 0);
> > -				flip_mask &= ~I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT;
> > +
> > +				if ((I915_READ16(ISR) & I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT) == 0) {
> > +					intel_finish_page_flip(dev, 0);
> > +					flip_mask &= ~I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT;
> > +				}
> >  			}
> >  		}
> >  
> > @@ -2293,8 +2296,11 @@ static irqreturn_t i8xx_irq_handler(int irq, void *arg)
> >  		    drm_handle_vblank(dev, 1)) {
> >  			if (iir & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) {
> >  				intel_prepare_page_flip(dev, 1);
> > -				intel_finish_page_flip(dev, 1);
> > -				flip_mask &= ~I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
> > +
> > +				if ((I915_READ16(ISR) & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) == 0) {
> > +					intel_finish_page_flip(dev, 1);
> > +					flip_mask &= ~I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
> > +				}
> >  			}
> >  		}
> >  
> > @@ -2491,8 +2497,11 @@ static irqreturn_t i915_irq_handler(int irq, void *arg)
> >  			    drm_handle_vblank(dev, pipe)) {
> >  				if (iir & flip[plane]) {
> >  					intel_prepare_page_flip(dev, plane);
> > -					intel_finish_page_flip(dev, pipe);
> > -					flip_mask &= ~flip[plane];
> > +
> > +					if ((I915_READ(ISR) & flip[plane]) == 0) {
> 
> Why not `I915_READ16`?

It matches the rest if i915_irq_handler(). I suppose the interrupt
registers were 16 bit on gen2 and 32 bit on gen3+.

> 
> > +						intel_finish_page_flip(dev, pipe);
> > +						flip_mask &= ~flip[plane];
> > +					}
> >  				}
> >  			}
> 
> 
> Thanks,
> 
> Paul



-- 
Ville Syrjälä
Intel OTC