Re: [PATCH 10/10] drm/i915: Use MI_BATCH_BUFFER_START on 830/845

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

On Tue, Dec 15, 2015 at 11:22:29AM +0000, Chris Wilson wrote:
> On Tue, Dec 15, 2015 at 01:05:56PM +0200, Ville Syrjälä wrote:
> > On Tue, Dec 15, 2015 at 10:24:13AM +0000, Chris Wilson wrote:
> > > The other question, does this obsolete our work around? Can I be that
> > > optimisitic?
> > 
> > The CS TLB one? I think I tried it at some point, and things till
> > failed. But stuff fails even with the w/a (like I said piglit will
> > hang the GPU eventually), so I can't be sure that I actually tested
> > the CS TLB fail. I think I need to retest at some point.
> > 
> > As far as the docs go, I only remember it mentioning some TLB fail
> > affecting the blitter. I guess the CS TLB fail isn't actually
> > documented anywhere?
> 
> It's hard to be sure since the issue is only mentioned obliquely in
> bspec. I strongly suspect there is only one set of TLB on the device, so
> I think it is the same. But I never did figure out what flush they
> meant, as all the chipset or MI level flushes never seemed to do anything
> to improve the situation.

Programming Environment 1.4.9.4 claims that there are several TLBs. But
not sure if that really holds for all devices.

It also has the following table:

GTT TLBs
TLB            | Normal Invalidation Mechanism  | Invalidated by Page Table Enable bit
               |                                | of PGTBL_CTL register?
...
Command Stream | Through a Page Table PTE write | Yes

Which might hint that PGTBL_CTL might be the way to force invalidate
them. But IIRC you may have once said that you already tried it. In any
case, even if it would work we'd need to make sure no GTT access is
happening when toggling the bit (assuming we'd have to toggle it).

-- 
Ville Syrjälä
Intel OTC
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