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

[Christian König]

Am 19.05.2017 um 22:28 schrieb 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.
>

Ok, that makes at least a bit more sense. On APUs we obviously know 
exactly what CPU we have.

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

Yeah, there are actually quite a number of people. That's one of the 
reasons why we still have a bunch of "#ifdef __BIG_ENDIAN" in the amdgpu 
source.

> 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?

Yeah, especially setting "-msse" is rather questionable. As far as I 
know on 64bit systems it is the default, but on 32bit systems that could 
silently break some assumptions.

Additional to that as far as I know "-msse" is just for optimization and 
that isn't performance critical code, so why exactly do we need it?

Christian.

>
> 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.
>>
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