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

-----Original Message-----
From: Robin Murphy [mailto:robin.murphy@arm.com]
Sent: Monday, March 05, 2018 21:07
To: Nipun Gupta <redacted>; will.deacon@arm.com;
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Cc: iommu@lists.linux-foundation.org; robh+dt@kernel.org; hch@lst.de;
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[off-list ref]
Subject: Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree binding

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

quoted

quoted

-----Original Message-----
From: Robin Murphy [mailto:robin.murphy@arm.com]
Sent: Monday, March 05, 2018 20:23
To: Nipun Gupta <redacted>; will.deacon@arm.com;
mark.rutland@arm.com; catalin.marinas@arm.com
Cc: iommu@lists.linux-foundation.org; robh+dt@kernel.org; hch@lst.de;
m.szyprowski@samsung.com; gregkh@linuxfoundation.org;

joro@8bytes.org;

quoted

quoted

Leo Li [off-list ref]; shawnguo@kernel.org; linux-
kernel@vger.kernel.org; devicetree@vger.kernel.org; linux-arm-
kernel@lists.infradead.org; linuxppc-dev@lists.ozlabs.org; Bharat Bhushan
[off-list ref]; stuyoder@gmail.com; Laurentiu Tudor
[off-list ref]
Subject: Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree

binding

quoted

quoted

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

quoted

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

quoted

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

quoted

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.