AMD graphics performance regression in 4.15 and later

[Gabriel C]

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 ?