linux-kernel - Re: [PATCH 0/3] Apple M1 DART IOMMU driver

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Date:   Sat, 27 Mar 2021 16:33:29 +0100
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Robin Murphy" <robin.murphy@....com>,
        "Rob Herring" <robh@...nel.org>,
        "Mark Kettenis" <mark.kettenis@...all.nl>
Cc:     iommu@...ts.linux-foundation.org, joro@...tes.org,
        "Will Deacon" <will@...nel.org>, "Arnd Bergmann" <arnd@...nel.org>,
        "Hector Martin" <marcan@...can.st>,
        "Marc Zyngier" <maz@...nel.org>, mohamed.mediouni@...amail.com,
        stan@...ellium.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 0/3] Apple M1 DART IOMMU driver

Hi Robin,


On Thu, Mar 25, 2021, at 12:50, Robin Murphy wrote:
> On 2021-03-25 07:53, Sven Peter wrote:
> > The iommu binding documentation [1] mentions that
> > 
> >      The device tree node of the IOMMU device's parent bus must contain a valid
> >      "dma-ranges" property that describes how the physical address space of the
> >      IOMMU maps to memory. An empty "dma-ranges" property means that there is a
> >      1:1 mapping from IOMMU to memory.
> > 
> > which, if I understand this correctly, means that the 'dma-ranges' for the
> > parent bus of the iommu should be empty since the DART hardware can see the
> > full physical address space with a 1:1 mapping.
> > 
> > 
> > The documentation also mentions that
> > 
> >       When an "iommus" property is specified in a device tree node, the IOMMU
> >       will be used for address translation. If a "dma-ranges" property exists
> >       in the device's parent node it will be ignored.
> > 
> > which means that specifying a 'dma-ranges' in the parent bus of any devices
> > that use the iommu will just be ignored.
> 
> I think that's just wrong and wants updating (or at least clarifying). 
> The high-level view now is that we use "dma-ranges" to describe 
> limitations imposed by a bridge or interconnect segment, and that can 
> certainly happen upstream of an IOMMU. As it happens, I've just recently 
> sent a patch for precisely that case[1].
> 
> I guess what it might have been trying to say is that "dma-ranges" 
> *does* become irrelevant in terms of constraining what physical memory 
> is usable for DMA, but that shouldn't imply that its meaning doesn't 
> just shift to a different purpose.

Should I add a patch to clarify this paragraph for v2 or submit a separate
one-off patch? I'm not entirely sure about the process here but I could add
a Suggested-by: to the commit if that's fine with you.



Best,

Sven