[Intel-gfx] [PATCH v2 3/3] drm/i915: Prevent MI_DISPLAY_FLIP straddling two cachelines on IVB

[Daniel Vetter]

On Tue, Feb 11, 2014 at 05:50:05PM +0200, Ville Syrjälä wrote:
> On Tue, Feb 11, 2014 at 02:14:28PM +0000, Chris Wilson wrote:
> > On Tue, Feb 11, 2014 at 03:55:50PM +0200, ville.syrjala at linux.intel.com wrote:
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > According to BSpec the entire MI_DISPLAY_FLIP packet must be contained
> > > in a single cacheline. Make sure that happens.
> > > 
> > > v2: Use intel_ring_begin_cacheline_safe()
> > 
> > Ugh, no. Let's not make intel_ring_begin() any more complicated and just
> > introduce a function to align the current head in the ringbuffer to a
> > cacheline. Especially with such an interface that is hard to get right.
> 
> This doesn't make intel_ring_begin() itself more complicated, but I
> guess you meant that the new special version is too complicated for
> your taste?
> 
> So I guess you want somehting like this:
> 
> int ring_align()
> {
> 	nops = (64 - (tail & 63)) / 4;
> 
> 	ret = ring_begin(nops);
> 	if (ret)
> 		return ret;
> 	while (nops--)
> 		ring_emit(MI_NOOP);
> 	ring_advance()
> 
> 	return 0;
> }
> 
> int queue_flip()
> {
> 	ret = ring_align();
> 	if (ret)
> 		return ret;
> 
> 	ret = ring_begin(len);
> 	if (ret)
> 		return ret;
> 	if (RCS)
> 		emit LRI DERRMR;
> 	emit MI_DISPLAY_FLIP;
> 	ring_advance()
> 
> 	return 0;
> }
> 
> So we end up relying on the fact that the entire
> LRI+MI_DISPLAY_FLIP sequence will fit within one cacheline. Although if
> that would be problem I suppose we could always emit the LRI before
> aligning the tail.
> 
> If no one is concerned about the useless MI_NOOPs we'll be emitting for
> most flips, I guess it's a good enough solution.

Yeah Chris&me discussed this quickly on irc and I concur with him that an
intel_ring_cacheline_align(ring) helper is sufficient and much less crazy
;-)

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