Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[hch@xxxxxx]

On Thu, Oct 14, 2021 at 08:13:03AM +0000, Tian, Kevin wrote:
> Based on above information my interpretation is that existing 
> DMA API manages coherency per device and It's not designed for
> devices which are coherent in nature but also set PCI no-snoop
> for selective traffic. Then the new DMA_ATTR_NO_SNOOP, once
> set in dma_map, allows the driver to follow non-coherent
> semantics even when the device itself is considered coherent.
> 
> Does it capture the whole story correct? 

Yes.

> > > What I don't really understand is why ARM, with an IOMMU that supports
> > > PTE WB, has devices where dev_is_dma_coherent() == false ?
> > 
> > Because no IOMMU in the world can help that fact that a periphal on the
> > SOC is not part of the cache coherency protocol.
> 
> but since DMA goes through IOMMU then isn't IOMMU the one who
> should decide the final cache coherency? What would be the case
> if the IOMMU sets WB while the peripheral doesn't want it?

No.  And IOMMU deal with address translation, it can't paper over
a fact that there is no coherency possible.