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

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

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@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@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.

-- 
Ville Syrjälä
Intel OTC