[Intel-gfx] Enabling sample_c optimization for Broadwell GPUs

[Kenneth Graunke]

Hello,

Yes, that bit only exists on Haswell.  On Haswell, sample_c operations
were processed at 1 pixel/clock unless you set that bit, in which case
they get processed at 4 pixels/clock.  The downside is that it breaks
some obscure media feature that apparently no one used.

Broadwell and later alway process sample_c operations at the fast speed,
and there is no bit to control it.  I would say that downstream patch is
incorrect.

--Ken

On Tuesday, May 4, 2021 4:07:14 PM PDT 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
> 

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