AMD graphics performance regression in 4.15 and later

[Gabriel C]

2018-04-12 0:20 GMT+02:00 Gabriel C <nix.or.die at gmail.com>:
> 2018-04-11 20:35 GMT+02:00 Jean-Marc Valin <jmvalin at mozilla.com>:
>> On 04/11/2018 05:37 AM, Christian König wrote:
>>>> With your patches my EPYC box is unusable with  4.15++ kernels.
>>>> The whole Desktop is acting weird.  This one is using
>>>> an Cape Verde PRO [Radeon HD 7750/8740 / R7 250E] GPU.
>>>>
>>>> Box is  2 * EPYC 7281 with 128 GB ECC RAM
>>>>
>>>> Also a 14C Xeon box with a HD7700 is broken same way.
>>>
>>> The hardware is irrelevant for this. We need to know what software stack
>>> you use on top of it.
>>
>> Well, the hardware appears to be part of the issue too. I don't think
>> it's a coincidence that Gabriel has the problem on 2xEPYC, I have it on
>> 2xXeon and the previous reported had it on a Core 2 Quad that internally
>> has two dies.
>>
>> I've not yet tested your disable CONFIG_SWIOTLB fix yet -- might try it
>> over the weekend and report what happens.
>>
>
> To get that right .. is only a matter of disabling SWIOTLB *code*
> while CONFIG_SWIOTLB is still set ?

Ok I tested that on 4.16.1 and yes it does work. However I didn't like the
#if 0 method and so compile an kernel twice just to compare an test.

I created an small patch and added swiotlb option for amdgpu and radeon
so I can boot and compare / test with and without SWIOTLB code.

( not meant for upstream )

http://ftp.frugalware.org/pub/other/people/crazy/0001-Make-it-possible-to-disable-SWIOTLB-code-on-admgpu-a.patch

With SWIOTLB code off all works fine , while hell breaks when turning on.

Maybe similar options should be added upstream until code is more
stable in 4.17/4.18

Regards