[RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

[Koenig, Christian]

Am 14.01.19 um 20:13 schrieb Will Deacon:
> On Mon, Jan 14, 2019 at 07:07:54PM +0000, Koenig, Christian wrote:
>> Am 14.01.19 um 18:32 schrieb Ard Biesheuvel:
>>              - The reason remapping the CPU side as cacheable does work (which I
>>              did test) is because the GPU's uncacheable accesses (which I assume
>>              are made using the NoSnoop PCIe transaction attribute) are actually
>>              emitted as cacheable in some cases.
>>                 . On my AMD Seattle, with or without SMMU (which is stage 2 only), I
>>              must use cacheable accesses from the CPU side or things are broken.
>>              This might be a h/w flaw, though.
>>                 . On systems with stage 1+2 SMMUs, the driver uses stage 1
>>              translations which always override the memory attributes to cacheable
>>              for DMA coherent devices. This is what is affecting the Cavium
>>              ThunderX2 (although it appears the attributes emitted by the RC may be
>>              incorrect as well.)
>>
>>              The latter issue is a shortcoming in the SMMU driver that we have to
>>              fix, i.e., it should take care not to modify the incoming attributes
>>              of DMA coherent PCIe devices for NoSnoop to be able to work.
>>
>>              So in summary, the mismatch appears to be between the CPU accessing
>>              the vmap region with non-cacheable attributes and the GPU accessing
>>              the same memory with cacheable attributes, resulting in a loss of
>>              coherency and lots of visible corruption.
>>
>>          Actually it is the other way around. The CPU thinks some data is in the
>>          cache and the GPU only updates the system memory version because the
>>          snoop flag is not set.
>>
>>
>>      That doesn't seem to be what is happening. As far as we can tell from
>>      our experiments, all inbound transactions are always cacheable, and so
>>      the only way to make things work is to ensure that the CPU uses the
>>      same attributes.
>>
>>
>> Ok that doesn't make any sense. If inbound transactions are cacheable or not is
>> irrelevant when the CPU always uses uncached accesses.
>>
>> See on the PCIe side you have the snoop bit in the read/write transactions
>> which tells the root hub if the device wants to snoop caches or not.
>>
>> When the CPU accesses some memory as cached then devices need to snoop the
>> cache for coherent accesses.
>>
>> When the CPU accesses some memory as uncached then devices can disable snooping
>> to improve performance, but when they don't do this it is mandated by the spec
>> that this still works.
> Which spec?

The PCIe spec. The snoop bit (or rather the NoSnoop) in the transaction 
is perfectly optional IIRC.

> The Arm architecture (and others including Power afaiu) doesn't
> guarantee coherency when memory is accessed using mismatched cacheability
> attributes.

Well what exactly goes wrong on ARM?

As far as I know Power doesn't really supports un-cached memory at all, 
except for a very very old and odd configuration with AGP.

I mean in theory I agree that devices should use matching cacheability 
attributes, but in practice I know of quite a bunch of devices/engines 
which fails to do this correctly.

Regards,
Christian.

>
> Will