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    SubjectRe: [PATCH v2] devicetree: Add generic IOMMU device tree bindings
    On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
    > On 6/24/2014 2:18 AM, Will Deacon wrote:
    > > On Sat, Jun 21, 2014 at 12:16:25AM +0100, Olav Haugan wrote:
    > >> We have multiple-master SMMUs and each master emits a variable number of
    > >> StreamIDs. However, we have to apply a mask (the ARM SMMU spec allows
    > >> for this) to the StreamIDs due to limited number of StreamID 2 Context
    > >> Bank entries in the SMMU. If my understanding is correct we would
    > >> represent this in the DT like this:
    > >>
    > >> iommu {
    > >> #address-cells = <2>;
    > >> #size-cells = <0>;
    > >> };
    > >>
    > >> master@a {
    > >> ...
    > >> iommus = <&iommu StreamID0 MASK0>,
    > >> <&iommu StreamID1 MASK1>,
    > >> <&iommu StreamID2 MASK2>;
    > >> };
    > >
    > > Stupid question, but why not simply describe the masked IDs? What use does
    > > the `raw' ID have to Linux?
    >
    > We do describe the masked StreamID (SID) but we need to specify the mask
    > that the SMMU should apply to the incoming SIDs, right?
    >
    > We have a bus master that emits 43 unique SIDs. However, we have only 40
    > SMMU_SMRn registers in the SMMU. So we need to mask out some of the
    > incoming SID bits so that the 43 SIDs can match one of 40 entries in the
    > SMR.

    Hmm, so you're talking about stream matching, right? That doesn't belong in
    the device-tree. I appreciate that the current driver does a terrible job at
    allocating the SMRs (it's bloody difficult!), but we should try to improve
    the dynamic behaviour instead of moving configuration of the SMMU out into
    device-tree, where it's inflexible at best.

    There have been patches previously posted by Andreas Herrmann helping here.
    I'd be glad to see them revived.

    Will


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