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

Michel Dänzer
2019-01-21T18:04:30Z

On 2019-01-21 6:59 p.m., Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@daenzer.net> wrote:
>>
>> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
>>> On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@infradead.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