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

[Koenig, Christian]

Am 24.01.19 um 12:26 schrieb Ard Biesheuvel:
> On Thu, 24 Jan 2019 at 12:23, Koenig, Christian
> <Christian.Koenig@xxxxxxx> wrote:
>> Am 24.01.19 um 10:59 schrieb Ard Biesheuvel:
>>> [SNIP]
>>> This is *exactly* my point the whole time.
>>>
>>> The current code has
>>>
>>> static inline bool drm_arch_can_wc_memory(void)
>>> {
>>> #if defined(CONFIG_PPC) && !defined(CONFIG_NOT_COHERENT_CACHE)
>>>      return false;
>>>
>>> which means the optimization is disabled *unless the system is
>>> non-cache coherent*
>>>
>>> So if you have reports that the optimization works on some PowerPC, it
>>> must be non-cache coherent PowerPC, because that is the only place
>>> where it is enabled in the first place.
>>>
>>>> The only problematic here actually seems to be ARM, so you should
>>>> probably just add an "#ifdef .._ARM return false;".
>>>>
>>> ARM/arm64 does not have a Kconfig symbol like
>>> CONFIG_NOT_COHERENT_CACHE, so we can only disable it everywhere. If
>>> there are non-coherent ARM systems that are currently working in the
>>> same way as those non-coherent PowerPC systems, we will break them by
>>> doing this.
>> Summing the things I've read so far for ARM up I actually think it
>> depends on a runtime configuration and not on compile time one.
>>
>> So the whole idea of providing the device to the drm_*_can_wc_memory()
>> function isn't so far fetched.
>>
> Thank you.
>
>> But for now I do prefer working and slightly slower system over broken
>> one, so I think we should just disable this on ARM for now.
>>
> Again, this is not about non-cache coherent being slower without the
> optimization, it is about non-cache coherent likely not working *at
> all* unless the optimization is enabled.

As Michel tried to explain this CAN'T happen. The optimization is a 
purely optional request from userspace.

> Otherwise, the driver will vmap() DMA pages with cacheable attributes,
> while the non-cache coherent device uses uncached attributes, breaking
> coherency.

Again this is mandated by the userspace APIs anyway. E.g. we can't 
vmap() pages in any other way or our userspace APIs would break.

Regards,
Christian.