Re: [PATCH 2/2] drm/i915: Add FIXME for bdw semaphore detection in hancheck

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Mar 14, 2014 at 05:38:55PM -0700, Ben Widawsky wrote:
> On Sat, Mar 15, 2014 at 12:08:56AM +0100, Daniel Vetter wrote:
> > Currently not an issue since we don't emit sempahores, but better
> > not forget about those.
> > 
> > As a little prep work extract the ipehr decoding for cleaner control
> > flow. And apply a bit of polish.
> > 
> > Cc: Ben Widawsky <ben@bwidawsk.net>
> > Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
> 
> I've made a note to add the relevant BDW patch already.

Thanks. I'll pull it in as soon as the previous bugfix patch is reviewed
by Mika/Chris. Btw I've almost written the bdw patch myself (it's just two
lines or so + adjusting the number of dwords we scan back) but then
realized that the #defines I'm adding will needlessly conflict with your
series, so let it be.

> one comment below.
> > ---
> >  drivers/gpu/drm/i915/i915_irq.c | 20 ++++++++++++++++++--
> >  drivers/gpu/drm/i915/i915_reg.h |  3 ++-
> >  2 files changed, 20 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> > index 473372a6c97d..0f3a6d791502 100644
> > --- a/drivers/gpu/drm/i915/i915_irq.c
> > +++ b/drivers/gpu/drm/i915/i915_irq.c
> > @@ -2525,6 +2525,23 @@ ring_idle(struct intel_ring_buffer *ring, u32 seqno)
> >  		i915_seqno_passed(seqno, ring_last_seqno(ring)));
> >  }
> >  
> > +static bool
> > +ipehr_is_semaphore_wait(struct drm_device *dev, u32 ipehr)
> > +{
> > +	if (INTEL_INFO(dev)->gen >= 8) {
> > +		/*
> > +		 * FIXME: gen8 semaphore support - currently we don't emit
> > +		 * semaphores on bdw anyway, but this needs to be addressed when
> > +		 * we merge that code.
> > +		 */
> > +		return false;
> > +	} else {
> 
> If you wanted to be paranoid:
> WARN_ON((ipehr & MI_SEMAPHORE_SYNC_MASK) == MI_SEMAPHORE_SYNC_INVALID)

Good idea but wrong spot. At the end of semaphore_waits_for we use the
SYNC_MASK to compute the ring pointer of the signaller. That's completely
unchecked code (and full of magic stuff). I'll throw another patch on top
to extract this and add a few paranoid checks in the spirit of your
suggestion above.

Cheers, Daniel

> 
> > +		ipehr &= ~MI_SEMAPHORE_SYNC_MASK;
> > +		return ipehr == (MI_SEMAPHORE_MBOX | MI_SEMAPHORE_COMPARE |
> > +				 MI_SEMAPHORE_REGISTER);
> > +	}
> > +}
> > +
> >  static struct intel_ring_buffer *
> >  semaphore_waits_for(struct intel_ring_buffer *ring, u32 *seqno)
> >  {
> > @@ -2533,8 +2550,7 @@ semaphore_waits_for(struct intel_ring_buffer *ring, u32 *seqno)
> >  	int i;
> >  
> >  	ipehr = I915_READ(RING_IPEHR(ring->mmio_base));
> > -	if ((ipehr & ~(0x3 << 16)) !=
> > -	    (MI_SEMAPHORE_MBOX | MI_SEMAPHORE_COMPARE | MI_SEMAPHORE_REGISTER))
> > +	if (!ipehr_is_semaphore_wait(ring->dev, ipehr))
> >  		return NULL;
> >  
> >  	/*
> > diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> > index 146609ab42bb..23267859156f 100644
> > --- a/drivers/gpu/drm/i915/i915_reg.h
> > +++ b/drivers/gpu/drm/i915/i915_reg.h
> > @@ -244,7 +244,8 @@
> >  #define   MI_SEMAPHORE_SYNC_BVE	    (0<<16) /* VECS wait for BCS  (VEBSYNC) */
> >  #define   MI_SEMAPHORE_SYNC_VVE	    (1<<16) /* VECS wait for VCS  (VEVSYNC) */
> >  #define   MI_SEMAPHORE_SYNC_RVE	    (2<<16) /* VECS wait for RCS  (VERSYNC) */
> > -#define   MI_SEMAPHORE_SYNC_INVALID  (3<<16)
> > +#define   MI_SEMAPHORE_SYNC_INVALID (3<<16)
> > +#define   MI_SEMAPHORE_SYNC_MASK    (3<<16)
> >  #define MI_SET_CONTEXT		MI_INSTR(0x18, 0)
> >  #define   MI_MM_SPACE_GTT		(1<<8)
> >  #define   MI_MM_SPACE_PHYSICAL		(0<<8)
> > -- 
> > 1.8.1.4
> > 
> 
> -- 
> Ben Widawsky, Intel Open Source Technology Center

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch