Re: [PATCH 2/6] drm/i915: Remove BXT incoherent seqno write workaround

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

On Mon, Jan 23, 2017 at 01:52:02PM +0000, Chris Wilson wrote:
> On Mon, Jan 23, 2017 at 03:43:27PM +0200, Ville Syrjälä wrote:
> > On Mon, Jan 23, 2017 at 01:05:57PM +0000, Chris Wilson wrote:
> > > This w/a was only used for preproduction hw, which is no longer in use.
> > > Remove the workaround to simplify the code.
> > > 
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_lrc.c | 17 -----------------
> > >  1 file changed, 17 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > > index 8ffa4961aa40..202ce1e6e499 100644
> > > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > > @@ -1638,21 +1638,6 @@ static int gen8_emit_flush_render(struct drm_i915_gem_request *request,
> > >  	return 0;
> > >  }
> > >  
> > > -static void bxt_a_seqno_barrier(struct intel_engine_cs *engine)
> > > -{
> > > -	/*
> > > -	 * On BXT A steppings there is a HW coherency issue whereby the
> > > -	 * MI_STORE_DATA_IMM storing the completed request's seqno
> > > -	 * occasionally doesn't invalidate the CPU cache. Work around this by
> > > -	 * clflushing the corresponding cacheline whenever the caller wants
> > > -	 * the coherency to be guaranteed. Note that this cacheline is known
> > > -	 * to be clean at this point, since we only write it in
> > > -	 * bxt_a_set_seqno(), where we also do a clflush after the write. So
> > > -	 * this clflush in practice becomes an invalidate operation.
> > > -	 */
> > > -	intel_flush_status_page(engine, I915_GEM_HWS_INDEX);
> > > -}
> > 
> > Ahem. Don't we have hardware in the pipeline which is going to need
> > this stuff?
> 
> I sincerely hope we don't have such broken hw again. Less functional
> than gen2. Anyway, restoring it, or whatever is actually required, is
> trivial. Deciding whether that interrupt signaling doesn't work in
> future and what we need to do instead is a whole new ballgame.

Hmm. Looks like the plans have changed since I last looked at that
specific case and apparently snooping is back. Which is nice.
Consider my objections lifted.

-- 
Ville Syrjälä
Intel OTC
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