Re: [PATCHv4 5/7] iommu/tegra: smmu: Support "mmu-masters" binding

On 11/13/2013 11:41 PM, Hiroshi Doyu wrote:

Stephen Warren [off-list ref] wrote @ Wed, 13 Nov 2013 18:58:23 +0100:

quoted

quoted

 smmu: iommu@xxxxxx {
       #iommu-cells = <3>;
       ^^^^^^^^^^^^^^^^^^
   };

   host1x {
           compatible = "nvidia,tegra30-host1x", "simple-bus";
           iommu = <&smmu 0x??????? 0x??????? "asid">;
	   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^#######
           gr3d {
                   compatible = "nvidia,tegra30-gr3d";
                   iommu = <&smmu 0x??????? 0x???????>;
           }

I think that this "asid" part can be set 0 in tegra??.dtsi and the
actual value can be overwritten in tegra??-<boardname>.dts file.

The one issue here is that we can only override entire properties, so
it's not possible for a board file to *just* replace the ASID, it'd have
to duplicate the entire property, just to change the one value.

Is the ASID mapping really likely to be board-specific though? To my
naive thinking, it seems that the chip design (e.g. number of
peripherals, number of available ASIDs) would tend to imply the
device->ASID mapping, since it would have been considered as part of
chip design. Hence, wouldn't soc.dtsi typically specify the expected
ASID mapping, and boards rarely if ever override it?

If the ASID mapping really is likely to vary per board, perhaps it makes
sense to put it into a separate property somehow so it's easier to override?

  Older Tegra like T30: swgroups > asid(==4)
  Newer Tegra         : swgroups < asid

In that case, I'd vote for hard-coding the mapping in the driver in all
cases. For older Tegra, we'll have to hard-code some static mapping just
like you've already done in the driver. For newer Tegra, we would just
assign a new AS for each swgroup as you say. If we ever need to tweak
this, we can invent a new DT property to affect the default. That makes
the DT content quite a bit simpler for now:-)