Messages in this thread | | | Subject | Re: [PATCH 1/6] Docs: dt: add fsl-mc iommu-parent device-tree binding | From | Robin Murphy <> | Date | Mon, 5 Mar 2018 15:37:29 +0000 |
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On 05/03/18 15:00, Nipun Gupta wrote: > > >> -----Original Message----- >> From: Robin Murphy [mailto:robin.murphy@arm.com] >> Sent: Monday, March 05, 2018 20:23 >> To: Nipun Gupta <nipun.gupta@nxp.com>; 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; >> Leo Li <leoyang.li@nxp.com>; 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 >> <bharat.bhushan@nxp.com>; stuyoder@gmail.com; Laurentiu Tudor >> <laurentiu.tudor@nxp.com> >> 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.
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