Enabling sample_c optimization for Broadwell GPUs

[André Almeida]

Hi Rodrigo,

Thank you very much for providing that information in a precise manner.

Às 07:16 de 05/05/21, Rodrigo Vivi escreveu:
> 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