linux-kernel - Re: [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86

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Message-ID: <ccc775d3-3053-ab79-dac8-4feb28e8e690@daenzer.net>
Date:   Mon, 21 Jan 2019 19:04:22 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will.deacon@....com>,
        David Zhou <David1.Zhou@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, Junwei Zhang <Jerry.Zhang@....com>,
        Huang Rui <ray.huang@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alex Deucher <alexander.deucher@....com>,
        Sean Paul <sean@...rly.run>,
        Christian Koenig <christian.koenig@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent
 devices on non-x86

On 2019-01-21 6:59 p.m., Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@...nzer.net> wrote:
>>
>> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
>>> On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@...radead.org> wrote:
>>>
>>>> Until that happens we should just change the driver ifdefs to default
>>>> the hacks to off and only enable them on setups where we 100%
>>>> positively know that they actually work.  And document that fact
>>>> in big fat comments.
>>>
>>> Well, as I mentioned in my commit log as well, if we default to off
>>> unless CONFIG_X86, we may break working setups on MIPS and Power where
>>> the device is in fact non-cache coherent, and relies on this
>>> 'optimization' to get things working.
>>
>> FWIW, the amdgpu driver doesn't rely on non-snooped transfers for
>> correct basic operation (the scenario Christian brought up is a very
>> specialized use-case), so that shouldn't be an issue.
>>
> 
> The point is that this is only true for x86.
> 
> On other architectures, the use of non-cached mappings on the CPU side
> means that you /do/ rely on non-snooped transfers, since if those
> transfers turn out not to snoop inadvertently, the accesses are
> incoherent with the CPU's view of memory.

The driver generally only uses non-cached mappings if
drm_arch/device_can_wc_memory returns true.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer