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

[Will Deacon <will.deacon@arm.com>]

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

Will

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