Re: [PATCH] drm/i915: Enable eLLC caching of display buffers for SKL+

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

On Fri, Apr 26, 2019 at 04:01:02PM +0100, Chris Wilson wrote:
> Quoting Ville Syrjälä (2019-04-26 15:54:54)
> > On Wed, Apr 17, 2019 at 08:15:43PM +0300, Ville Syrjälä wrote:
> > > On Wed, Apr 17, 2019 at 08:09:07AM +0100, Chris Wilson wrote:
> > > > Quoting Ville Syrjala (2019-04-15 15:16:41)
> > > > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > > > 
> > > > > Since SKL the eLLC has been sitting on the far side of the system
> > > > > agent, meaning the display engine can utilize it. Let's enable that.
> > > > > 
> > > > > I chose WB for the caching mode, because my numbers are indicating
> > > > > that WT might actually be WB and WC might actually be UC. I'm not
> > > > > 100% sure that is indeed the case but at least my simple rendercopy
> > > > > based benchmark didn't see any difference in performance.
> > > > > 
> > > > > Also if I configure things to do LLCeLLC+WT I still get cache dirt
> > > > > on my screen, suggesting that is in fact operating in WB mode
> > > > > anyway. This is also the reason I had to fix the MOCS target cache
> > > > > to really say PTE rather than LLC+eLLC.
> > > > 
> > > > We also need to check with hybrid setups that supply buffers via prime,
> > > > and we may need to end up marking those as explicitly uncached.
> > > 
> > > I think all memory access should be able to snoop the eLLC. But yeah,
> > > this should be confirmed on actual hardware. Anyone have a prime setup
> > > handy?
> > 
> > It occurred to me that finding a machine for this might be a little
> > difficult as most gt3e/gt4e chips are only available in laptops/nucs/etc.
> > IIRC there are some Xeons that would qualify, but I suppose those are
> > somewhat rare. Not sure if there are any other desktop parts that have
> > ellc.
> 
> For now at least. Would an ePCI be a fun mix of coherency problems? Not
> that they are any more common.

Hmm. I keep forgetting what century we're in. Some kind of external
thunderbolt enclosure might do the trick. Never actually seen one but
I suppose it shouldn't be an impossible task to procure some.

> 
> Did you finish up the rendercopy tests? I think I saw that you were
> working on something that looked like it could be used for verifying
> rendering into the frontbuffer (or at least leaving cache dirty prior to
> flips)?

I just fixed up the mocs setup in rendercopy. I didn't write a specific
testase yet.

-- 
Ville Syrjälä
Intel
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