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

[Robin Murphy]

On 05/03/18 15:00, Nipun Gupta wrote:
> > -----Original Message-----
> > From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> > Sent: Monday, March 05, 2018 20:23
> > To: Nipun Gupta <nipun.gupta@xxxxxxx>; will.deacon@xxxxxxx;
> > mark.rutland@xxxxxxx; catalin.marinas@xxxxxxx
> > Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; robh+dt@xxxxxxxxxx; hch@xxxxxx;
> > m.szyprowski@xxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; joro@xxxxxxxxxx;
> > Leo Li <leoyang.li@xxxxxxx>; shawnguo@xxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; linuxppc-dev@xxxxxxxxxxxxxxxx; Bharat Bhushan
> > <bharat.bhushan@xxxxxxx>; stuyoder@xxxxxxxxx; Laurentiu Tudor
> > <laurentiu.tudor@xxxxxxx>
> > Subject: Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree binding
> >
> > On 05/03/18 14:29, Nipun Gupta wrote:
> > > 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.