SMR masking and PCI

[Robin Murphy]

Hi Stuart,

On 27/10/16 18:10, Stuart Yoder wrote:
> Hi Robin,
> 
> A question about how the SMR masking defined in the arm,smmu binding
> relates to the PCI iommu-map.
> 
> The #iommu-cells property defines the number of cells an "IOMMU specifier"
> takes and 2 is specified to be:
> 
>    SMMUs with stream matching support and complex masters
>    may use a value of 2, where the second cell represents
>    an SMR mask to combine with the ID in the first cell.
> 
> An iommu-map entry is defined as:
> 
>    (rid-base,iommu,iommu-base,length)
> 
> What seems to be currently missing in the iommu-map support is
> the possibility the case where #iommu-cells=<2>.

Indeed. The bindings have so far rather implicitly assumed the case of
#{msi,iommu}-cells = 1, and the code has followed suit.

> In this case iommu-base which is an IOMMU specifier should
> occupy 2 cells.  For example on an ls2085a we would want:
> 
> 	iommu-map = <0x0   0x6 0x7 0x3ff 0x1
> 		       0x100 0x6 0x8 0x3ff 0x1>;
> 
> ...to mask our stream IDs to 10 bits.
> 
> This should work in theory and comply with the bindings, no?

In theory, but now consider:

	iommu-map = <0x0 0x6 0x7 0x3ff 0x2>;

faced with ID 1. The input base is 0, the output base is the 2-cell
value 0x7000003ff, so the final ID value should be 0x700000400, right?

> of_pci_map_rid() seems to have a hardcoded assumption that
> each field in the map is 4 bytes.

It does. I guess we should explicitly check that #{msi,iommu}-cells = 1
on the target node, and maybe clarify in the binding that that cell
should represent a plain linear ID value (although that's pretty obvious
from context IMO).

> (Also, I guess that msi-map is not affected by this since it
> is not related to the IOMMU...but we do have common code
> handling both maps.)

I'd say it's definitely affected, since #msi-cells can equally be more
than 1, and encodes an equally opaque value.

It seems pretty reasonable to me that we could extend the binding to
accommodate #cells > 1 iff length == 1. Mark?

That said, is there a concrete need for this, i.e. do you actually have
one device with a single requester ID, which maps to multiple output IDs
(which differ only in the upper bits) in a non-predictable manner?

Robin.

> 
> Thanks,
> Stuart
>