Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree binding

On 05/03/18 15:00, Nipun Gupta wrote:

quoted

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Subject: Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree binding

On 05/03/18 14:29, Nipun Gupta wrote:

quoted

The existing IOMMU bindings cannot be used to specify the relationship
between fsl-mc devices and IOMMUs. This patch adds a binding for
mapping fsl-mc devices to IOMMUs, using a new iommu-parent property.

Given that allowing "msi-parent" for #msi-cells > 1 is merely a
backward-compatibility bodge full of hard-coded assumptions, why would
we want to knowingly introduce a similarly unpleasant equivalent for
IOMMUs? What's wrong with "iommu-map"?

Hi Robin,

With 'msi-parent' the property is fixed up to have msi-map. In this case there is
no fixup required and simple 'iommu-parent' property can be used, with MC bus
itself providing the stream-id's (in the code execution via FW).

We can also use the iommu-map property similar to PCI, which will require u-boot
fixup. But then it leads to little bit complications of u-boot - kernel compatibility.

What needs fixing up? With a stream-map-mask in place to ignore the upper Stream ID bits, you just need:

	iommu-map = <0 &smmu 0 0x80>;

to say that the lower bits of the ICID value map directly to the lower bits of the Stream ID value - that's the same fixed property of the hardware that you're wanting to assume in iommu-parent.

If you suggest we can re-use the iommu-map property. What is your opinion?

I think it makes a lot more sense to directly use the property which already exists, than to introduce a new one to merely assume one hard-coded value of the existing one. Extending msi-parent to msi-map was a case of "oops, it turns out we need more flexibility here"; for the case of iommu-map I can't imagine any justification for saying "oops, we need less flexibility here" (saving 9 whole bytes in the DT really is irrelevant).

Robin.