Re: [PATCH v2 1/2] of/pci: add of_pci_dma_configure() update dma configuration

[Murali Karicheri <m-karicheri2@ti.com>]

On 01/03/2015 04:37 PM, Arnd Bergmann wrote:
> On Friday 02 January 2015 17:33:53 Murali Karicheri wrote:
>>
>> I have no experience with IOMMU and may not offer much help here as I
>> originally wrote above. Will Deacon has added this API and probably able
>> to offer some help in this discussion.
>>
>> Will Deacon,
>>
>> Any comment?
>
> It's complicated :(
>
>> Looking at the iommu documentation and of_iommu.c, I get a feeling that
>> this API is not really used at present as there are no callers of
>> of_iommu_set_ops() and I assume this is a WIP.
>
> Right, but we have patches for some iommu drivers based on this API,
> and we should migrate all of them eventually.
>
>> I believe the way it is
>> expected to work is to have the iommu driver of the master IOMMU devices
>> call of_iommu_set_ops(). The device node of this master IOMMU device is
>> specified as a phandle in the OF node of the device (various bus devices
>> such as platform, PCI etc). This allow to retrieve the iommu ops though
>> the of_iommu_configure() API and use it in arch_setup_dma_ops(). So my
>> gut feeling is that for PCI devices, as there are no DT node, the root
>> bus node may specify iommus phandle to IOMMU master OF nodes.
>
> Yes, but we also need to pass a PCI device specific identifier along
> with the root bus node, because some iommu drivers take the PCI
> bus/device/function number into account for creating per-function
> i/o page tables.
>
>> W.r.t your comment "We may want to address the comment in
>> of_iommu_configure about parent nodes. We should be sure these changes
>> work with how we would do searching parent nodes",
>>
>> I believe, the parent node search itself should work the same way in the
>> case of PCI as with platform bus case. PCI's case, we are providing the
>> OF node of the root bus host bridge. Why should this be any different in
>> terms of search?
>>
>> I see a potential issue with dma-ranges as described in the notes below.
>> As noted below the usage of dma-range for iommu is to be determined. For
>> keystone, the of_iommu_configure() always return false as we don't use
>> the iommu. But don't know if this has any consequences for other
>> platforms. Or I got your questions wrong. Any help here from others on
>> the list?
>>
>> ========================================================================
>> One possible extension to the above is to use an "iommus" property along
>> with a "dma-ranges" property in a bus device node (such as PCI host
>> bridges). This can be useful to describe how children on the bus relate
>> to the IOMMU if they are not explicitly listed in the device tree (e.g.
>> PCI devices). However, the requirements of that use-case haven't been
>> fully determined yet. Implementing this is therefore not recommended
>> without further discussion and extension of this binding.
>> =========================================================================
>
> This probably won't ever apply to PCI devices, so let's ignore it for now.
> For the moment (and for PCI), we should assume that we either configure
> an iommu directly or we use dma-ranges if no iommu is in use.
>
Thanks Arnd,

I will post v3 of the patch with what is agreed before in my response 
and I understand there is no additional change required based on this 
particular discussion about iommu. Right?

Murali
> 	Arnd


-- 
Murali Karicheri
Linux Kernel, Texas Instruments