On Wed, May 24, 2017 at 07:01:38PM +0100, Jean-Philippe Brucker wrote:
> Address Translation Service (ATS) is an extension to PCIe allowing
> endpoints to manage their own IOTLB, called Address Translation Cache
> (ATC). Instead of having every memory transaction processed by the IOMMU,
> the endpoint can first send an Address Translation Requests for an IOVA,
> obtain the corresponding Physical Address from the IOMMU and store it in
> its ATC. Subsequent transactions to this memory region can be performed on
> the PA, in which case they are marked 'translated' and (partially) bypass
> the IOMMU.
> Since the extension uses fields that were previously reserved in the
> PCIe Translation Layer Packet, it seems ill-advised to enabled it on a
> system that doesn't fully support ATS.
> To "old" root complexes that simply ignored the new AT field, an Address
> Translation Request will look exactly like a Memory Read Request, so the
> root bridge will forward a memory read to the IOMMU instead of a
> translation request. If the access succeeds, the RC will send a Read
> Completion, which looks like a Translation Completion, back to the
> endpoint. As a result the endpoint might end up storing the content of
> memory instead of a physical address in its ATC. In reality, it's more
> likely that the size fields will be invalid and either end will detect the
> error, but in any case, it is undesirable.
> Add a way for firmware to tell the OS that ATS is supported by the PCI
> root complex.

Can't firmware have already enabled ATS? Often for things like this, not 
present means "use firmware setting".

> Signed-off-by: Jean-Philippe Brucker <jean-philippe.bruc...@arm.com>
> ---
>  Documentation/devicetree/bindings/pci/pci-iommu.txt | 8 ++++++++
>  1 file changed, 8 insertions(+)
> diff --git a/Documentation/devicetree/bindings/pci/pci-iommu.txt 
> b/Documentation/devicetree/bindings/pci/pci-iommu.txt
> index 0def586fdcdf..f21a68ec471a 100644
> --- a/Documentation/devicetree/bindings/pci/pci-iommu.txt
> +++ b/Documentation/devicetree/bindings/pci/pci-iommu.txt
> @@ -44,6 +44,14 @@ Optional properties
>  - iommu-map-mask: A mask to be applied to each Requester ID prior to being
>    mapped to an IOMMU specifier per the iommu-map property.
> +- ats-supported: if present, the root complex supports the Address
> +  Translation Service (ATS). It is able to interpret the AT field in PCIe
> +  Transaction Layer Packets, and forward Translation Completions or
> +  Invalidation Requests to endpoints.

Why can't this be based on the compatible strings?

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