[Intel-gfx] Enabling sample_c optimization for Broadwell GPUs

[Rodrigo Vivi]

Hi Andre,

I'm not familiar with the sample c message optimization.
Probably Ken can comment.

However I could check the internal spec here and I saw this bit
only exists with this meaning in Haswell.

For all the other platforms, including Broadwell it got re-purposed with
a different meaning and a programming note:
"This bit should be programmed to zero (0h) at all times."

Also, I could not find any workaround documented anywhere recommending
this bit to be set.

So, I would not recommend to use it in any product, even downstream.
Regardless the state of sample c message optimization in later platforms.

Thanks,
Rodrigo.

On Tue, May 04, 2021 at 08:07:14PM -0300, André Almeida wrote:
> Hi there,
> 
> While browsing an old downstream kernel, I found a patch[0] that enables
> sample_c optimizations at Broadwell GPUs. The message from the upstream
> commit that enables it for Haswell[1] (and presumably where the code at[0]
> was copied from) states that "[..] later platforms remove this bit, and
> apparently always enable the optimization".
> 
> Could you confirm that Broadwell and following architectures enable this
> optimization by default (and thus, patch[0] is a no-op), or should I
> upstream it?
> 
> Thanks,
> 	André
> 
> [0] https://github.com/ValveSoftware/steamos_kernel/commit/198990f13e1d9429864c177d9441a6559771c5e2
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=944115934436b1ff6cf773a9e9123858ea9ef3da