Enabling sample_c optimization for Broadwell GPUs

Enabling sample_c optimization for Broadwell GPUs

[André Almeida]

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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Re: 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
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Re: 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
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Re: Enabling sample_c optimization for Broadwell GPUs

[Kenneth Graunke]

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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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