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

Rob Clark
2014-07-16T20:24:29Z

On Tue, Jul 15, 2014 at 9:25 PM, Olav Haugan <ohaugan@codeaurora.org> wrote:
> On 7/13/2014 4:43 AM, Rob Clark wrote:
>> On Sun, Jul 13, 2014 at 5:43 AM, Will Deacon <will.deacon@arm.com> 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@arndb.de> 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 ;-)
>
> Is the reason you want to do it this way because you want to guarantee
> that all masters (and stream IDs) have been identified before the first
> attach call? I am just wondering why you cannot continue doing the
> master/streamID discovery during add_device() callback?

it was mostly because I couldn't think of a sane way to differentiate
between first and second time a device attaches (without keeping a
reference to the device). But I guess it is ok to assume no hotplug
(since walking the device tree also seems acceptable)

BR,
-R

>>>
>>> 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.
>>
>
> The msm-v0 IOMMU is not compatible with ARM SMMUv1 specification.
> However, it is a close cousin. The hardware was designed before the ARM
> SMMUv1 specification was available I believe. But it shares many of the
> same concepts as the ARM SMMUv1.
>
> msm-v0 IOMMU supports V7S page table format only. The ARM SMMU driver
> does not support V7S at this time. However, I believe we need to support
> this.
>
> Will, this reminds me. We definitely have a need to use different page
> tables in the ARM SMMU driver vs. the ARM CPU. We have an SoC with ARMv8
> cores (and thus ARMv8 page tables) but the SMMUs (SMMUv1) on this SoC
> only have support for V7S/V7L page table format. So we cannot use the
> same page table format as the CPU.
>
> Thanks,
>
> Olav
>
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