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:

quoted

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