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

From: Harry Wentland <harry.wentland-5C7GfCeVMHo@public.gmane.org>
To: Dave Airlie <airlied-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: "Michel Dänzer" <michel.daenzer-5C7GfCeVMHo@public.gmane.org>,
	"amd-gfx mailing list"
	<amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org>,
	"Christian König"
	<deathsimple-ANTagKRnAhcb1SvskN2V4Q@public.gmane.org>,
	"Dmytro Laktyushkin"
	<Dmytro.Laktyushkin-5C7GfCeVMHo@public.gmane.org>,
	"Alex Deucher" <Alexander.Deucher-5C7GfCeVMHo@public.gmane.org>,
	"Tony Cheng" <Tony.Cheng-5C7GfCeVMHo@public.gmane.org>,
	"Christian König" <christian.koenig-5C7GfCeVMHo@public.gmane.org>
Subject: Re: [PATCH] drm/amd/display: Limit DCN to x86 arch
Date: Fri, 19 May 2017 16:28:44 -0400	[thread overview]
Message-ID: <4e0c342b-d18e-5f7a-bd30-9becd0e97e3c@amd.com> (raw)
In-Reply-To: <CAPM=9tzcA8t-ji6qNQhycB0cezz2vH6HMixCgvKQ78cY1Y8fsA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

On 2017-05-19 04:18 PM, Dave Airlie wrote:
> On 20 May 2017 at 05:36, Harry Wentland <harry.wentland@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.
> 
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