[Intel-gfx] Does display engine read contents from LLC of scanout buffer?

[Ville Syrjälä]

On Wed, Dec 07, 2016 at 11:30:37AM +0000, Weng, Chuanbo wrote:
> Hi Ville,
> 	Thanks for your useful info.
> 	How about L3 cache? In my scenario, Beignet will use Read Write Data Port to write data (typed write) to bo.
> So the cache path is L3 -> LLC -> memory. So only leave LLC cache ability the same as PTEs is not enough.
> 	In order to make data access efficient, I set L3 to WB. So is there a way to flush L3 cache to memory?

I don't recall. But actually caching in L3 is dangerous anyway since
IIRC L3 evictions can land in LLC despite the LLC cacheability being set
to UC/WT. Or at least there was a note stating that in bspec at some
point. So to be totally correct you should not use L3 either. Well, I
suppose you could use L3 as a RO cache, but sounds like you want to write
as well.

> 
> Thanks,
> Chuanbo Weng
> 
> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com] 
> Sent: Friday, November 25, 2016 3:07 AM
> To: Weng, Chuanbo <chuanbo.weng at intel.com>
> Cc: intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] Does display engine read contents from LLC of scanout buffer?
> 
> On Thu, Nov 24, 2016 at 06:20:22PM +0000, Weng, Chuanbo wrote:
> > Hi all,
> >                I have a question of display (forgive me if it's too simple because I'm not familiar with display):
> > 
> >                My scenario is as below:
> >                               gbm_bo_create to create gbm_bo
> >                               Get its handle
> >                               drmModeAddFB to specify this bo as scanout buffer
> >                               do rendering to this bo by OpenCL(Beignet)
> >                               drmModePageFlip to do page flip
> > 
> >                Does display engine directly read contents from scanout 
> > buffer or read contents from LLC of this scanout buffer?
> 
> The display engine sits between the LLC and main memory effectively, so the answer is that it always reads directly from memory. When you make a bo a scanout buffer the kernel will flush the caches and reconfigure the PTEs to UC/WC so that subsequent rendering will hit memory directly.
> And that also means you should never override potential scanout buffers to WB via MOCS, and instead you should leave the choice up to the PTEs.
> 
> --
> Ville Syrjälä
> Intel OTC

-- 
Ville Syrjälä
Intel OTC