linux-kernel - Re: Kernel and ADM hardware roulette ( was AMD graphics performance regression in 4.15 and later )

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Date:   Wed, 6 Jun 2018 16:29:10 +0200
From:   Christian König <christian.koenig@....com>
To:     Gabriel C <nix.or.die@...il.com>
Cc:     Jean-Marc Valin <jmvalin@...illa.com>,
        Dave Airlie <airlied@...ux.ie>, alexander.deucher@....com,
        Felix Kuehling <Felix.Kuehling@....com>,
        Laura Abbott <labbott@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        michel.daenzer@....com, dri-devel@...ts.freedesktop.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Kernel and ADM hardware roulette ( was AMD graphics performance
 regression in 4.15 and later )

Am 06.06.2018 um 15:33 schrieb Gabriel C:
> 2018-06-06 14:19 GMT+02:00 Christian König <christian.koenig@....com>:
>> Am 06.06.2018 um 14:08 schrieb Gabriel C:
>>> [SNIP]
>>>
>>> That has nothing TODO with the driver nor the original bug you reported. The
>>> problem is that SME is active and that is currently not supported at all
>>> with a that hardware.
> Ok .. so are we playing now kernel an AMD Hardware roulette on each release ?
>
> SME was like this in kernel 4.16.x here and all worked.
>
> Also if you don't support SME at all now on that Hardware while worked before
> please add proper error handling and proper dmesg messages
> letting the user know.
>
> radeon: xxxx : SME not supported on that Hardware anymore , please
> disable SME...
> radeon: xxxx: Update your GPU < or whatever >
>
> How hard would be that ?

Yes, to be precise that isn't the job of the GFX driver to care about 
such things.

It is a well known and documented limitation of SME that it is in 
general mostly incompatible with GFX (or compute) hardware, and it 
actually doesn't matter which hardware or driver you use.

In other words what happens is that as soon as you use GFX (or compute) 
SME gets disabled transparently.

The problem is that this happens only on the DMA slow path we just 
disabled because of the performance problems.

Going to propose to revert that or at least only use it when SME is 
disabled.

Regards,
Christian.