Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

[Rob Clark]

On Sun, Jul 13, 2014 at 5:43 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Sat, Jul 12, 2014 at 01:57:31PM +0100, Rob Clark wrote:
>> On Sat, Jul 12, 2014 at 8:22 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> > On Saturday 12 July 2014, Rob Clark wrote:
>> >> >> Was there actually a good reason for having the device link to the
>> >> >> iommu rather than the other way around?  How much would people hate it
>> >> >> if I just ignore the generic bindings and use something that works for
>> >> >> me instead.  I mean, it isn't exactly like there is going to be .dts
>> >> >> re-use across different SoC's..  and at least with current IOMMU API
>> >> >> some sort of of_get_named_iommu() API doesn't really make sense.
>> >> >
>> >> > The thing is, if you end up ignoring the generic binding then we have two
>> >> > IOMMUs using the same (ARM SMMU) binding and it begs the question as to
>> >> > which is the more generic! I know we're keen to get this merged, but merging
>> >> > something that people won't use and calling it generic doesn't seem ideal
>> >> > either. We do, however, desperately need a generic binding.
>> >>
>> >> yeah, ignoring the generic binding is not my first choice.  I'd rather
>> >> have something that works well for everyone.  But I wasn't really sure
>> >> if the current proposal was arbitrary, or if there are some
>> >> conflicting requirements between different platforms.
>> >
>> > The common case that needs to be simple is attaching one (master) device
>> > to an IOMMU using the shared global context for the purposes of implementing
>> > the dma-mapping API.
>>
>> well, I don't disagree that IOMMU API has some problems.  It is too
>> tied to the bus type, which doesn't really seem to make sense for
>> platform devices.  (Unless we start having multiple platform busses?)
>>
>> But at least given the current IOMMU API I'm not really sure how it
>> makes a difference which way the link goes.  But if there has already
>> been some discussion about how you want to handle the tie in with
>> dma-mapping, if you could point me at that then maybe your point will
>> make more sense to me.
>
> If you look at the proposed binding in isolation, I think it *is* cleaner
> than the ARM SMMU binding (I've acked it...) and I believe it's more
> consistent with the way we describe linkages elsewhere.
>
> However, the issue you're raising is that it's more difficult to make use of
> in a Linux IOMMU driver. The reward you'll get for using it will come
> eventually when the DMA ops are automatically swizzled for devices using the
> generic binding.
>
> My plan for the ARM SMMU driver is:
>
>   (1) Change ->probe() to walk the device-tree looking for all masters with
>       phandles back to the SMMU instance being probed
>
>   (2) For each master, extract the Stream IDs and add them to the internal
>       SMMU driver data structures (an rbtree per SMMU instance). For
>       hotpluggable buses, we'll need a way for the bus controller to
>       reserve a range of IDs -- this will likely be a later extension to
>       the binding.
>
>   (3) When we get an ->add() call, warn if it's a device we haven't seen
>       and reject the addition.
>
> That way, ->attach() should be the same as it is now, I think.
>
> Have you tried implementing something like that? We agreed that (1) isn't
> pretty, but I don't have a good alternative and it's only done at
> probe-time.

I haven't tried implementing that yet, but I'm sure it would work.  I
was just hoping to avoid having to do that ;-)

I suppose perhaps there is room for a shared helper here, to at least
avoid duplicating that in each IOMMU driver which needs the
stream-id's up front.

> Will
>
> BTW: Is the msm-v0 IOMMU compatible with the ARM SMMU driver, or is it a
> completely different design requiring a different driver?

My understanding is that it is different from msm v1 IOMMU, although I
think it shares the same pagetable format with v1.  Not sure if that
is the same as arm-smmu?   If so it might be nice to try to extract
out some shared helper fxns for map/unmap as well.

I expect Olav knows better the similarities/differences.

BR,
-R
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