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

Michel Dänzer
2019-01-21T18:24:05Z

On 2019-01-21 7:20 p.m., Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 19:04, Michel Dänzer <michel@daenzer.net> wrote:
>>
>> 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.
>>
>
> Indeed. And so we should take care to only return 'true' from that
> function if it is guaranteed that non-cached CPU mappings are coherent
> with the mappings used by the GPU, either because that is always the
> case (like on x86), or because we know that the platform in question
> implements NoSnoop correctly throughout the interconnect.
>
> What seems to be complicating matters is that in some cases, the
> device is non-cache coherent to begin with, so regardless of whether
> the NoSnoop attribute is used or not, those accesses will not snoop in
> the caches and be coherent with the non-cached mappings used by the
> CPU. So if we restrict this optimization [on non-X86] to platforms
> that are known to implement NoSnoop correctly, we may break platforms
> that are implicitly NoSnoop all the time.

Since the driver generally doesn't rely on non-snooped accesses for
correctness, that couldn't "break" anything that hasn't always been broken.

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