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

[Nipun Gupta]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> Sent: Monday, March 05, 2018 21:07
> 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 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.

Makes sense. I was going in a little bit wrong direction. Thanks for correcting.
I will send v2 patchset with iommu-map property.

Regards,
Nipun

> 
> > 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.