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

[Ville Syrjälä]

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