[PATCH] drm/amd/display: Limit DCN to x86 arch

[Harry Wentland]

On 2017-05-19 04:18 PM, Dave Airlie wrote:
> On 20 May 2017 at 05:36, Harry Wentland <harry.wentland at amd.com> wrote:
>> On 2017-05-19 11:02 AM, Christian König wrote:
>>>
>>> Am 19.05.2017 um 16:01 schrieb Harry Wentland:
>>>>
>>>> DCN bw calcs currently rely on the following gcc options:
>>>>     -mhard-float -msse -mpreferred-stack-boundary=4
>>>
>>>
>>> Mhm, price question: Why does DCN rely on the gcc options?
>>>
>>
>> Tony and Dmytro can probably provide more info here but my understanding is
>> that DCN bandwidth calcs requires floating point support. This code comes
>> pretty much straight from hardware teams with a guarantee that the output is
>> good.
>>
>> If we were to rewrite bandwidth calculations that guarantee would basically
>> fly out the window, which means when there's a bandwidth bug we cannot
>> easily get HW support unless we can prove that our calculations yield the
>> exact same results in all cases as HWs formula. Covering all scenarios that
>> bandwidth calcs covers would be quite an extensive undertaking and I'm sure
>> we'd miss important cases.
> 
> Is this only going to happen for X86 APUs? Using floating point in the
> kernel requires
> a lot of care to be taken, are we doing it properly?
> 

This case would be only for AMD X86 APUs, although I wouldn't be 
surprised if we'd see something similar for discrete ASICs in the future.

Are you aware of anyone using our GPUs on non-X86 architectures? If so, 
I never heard of it.

I realize this is raising a lot of concern. I was concerned myself when 
I first saw this. Beside calling kernel_fpu_begin() and kernel_fpu_end() 
are there other things to watch out for?

Harry

> Really rewriting the calcs in fixed point is the best option, maybe push back on
> the hardware team to have a fixed point version created.
> 
> Dave.
>