[Intel-gfx] [PATCH v5 09/35] drm/i915: Force MMIO flips when scheduler enabled

[Ville Syrjälä]

On Fri, Feb 19, 2016 at 11:28:05AM -0800, Jesse Barnes wrote:
> On 02/18/2016 06:26 AM, John.C.Harrison at Intel.com wrote:
> > From: John Harrison <John.C.Harrison at Intel.com>
> > 
> > MMIO flips are the preferred mechanism now but more importantly, pipe
> > based flips cause issues for the scheduler. Specifically, submitting
> > work to the rings around the side of the scheduler could cause that
> > work to be lost if the scheduler generates a pre-emption event on that
> > ring.
> > 
> > For: VIZ-1587
> > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index 6e12ed7..731d20a 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -46,6 +46,7 @@
> >  #include <linux/dma_remapping.h>
> >  #include <linux/reservation.h>
> >  #include <linux/dma-buf.h>
> > +#include "i915_scheduler.h"
> >  
> >  /* Primary plane formats for gen <= 3 */
> >  static const uint32_t i8xx_primary_formats[] = {
> > @@ -11330,6 +11331,8 @@ static bool use_mmio_flip(struct intel_engine_cs *ring,
> >  		return true;
> >  	else if (i915.enable_execlists)
> >  		return true;
> > +	else if (i915_scheduler_is_enabled(ring->dev))
> > +		return true;
> >  	else if (obj->base.dma_buf &&
> >  		 !reservation_object_test_signaled_rcu(obj->base.dma_buf->resv,
> >  						       false))
> > 
> 
> Why can't we use mmio flips unconditionally?  Maarten or Ville?

We do when execlists are used, which is always on gen9+. So I guess I'm
missing the point of this patch. For gen5+ we could also do it trivially.

For older platforms it'd require a bit of work since we'd need to
complete the flips from the vblank interrupt. Well, we actually do
that already even with CS flips on those platforms, but we do look
at the flip pending interrupt to figure out if CS already issued
the flip or not. So that part would need changing.

I also think we should switch to using the vblank interrupt for this
stuff on all platforms, mainly since the flip done interrupt is somewhat
broken on at least BDW (no idea if it got fixed in SKL or later), and
doing things in more than one way certainly won't decrease our bug count.

-- 
Ville Syrjälä
Intel OTC