APM smmu implementation

Robin Murphy robin.murphy at arm.com
Mon Jan 9 09:55:26 PST 2017

On 09/01/17 12:03, Will Deacon wrote:
> On Mon, Jan 09, 2017 at 11:34:42AM +0000, Robin Murphy wrote:
>> On 06/01/17 23:21, Feng Kan wrote:
>>> The APM IOMMU implementation is mostly just the ARM SMMU 500 variant.
>> "Mostly"? Have APM actually modified it (which I strongly doubt) or do
>> you mean it's simply been integrated with the upper address lines tied
>> off? MMU-500 reports a 48-bit IAS because MMU-500 has 48-bit-wide slave
>> interfaces; that's all there is to it. Whether or not you use all of
>> those bits is up to you as a system integrator.
> That's a good point; MMU-500 doesn't appear to let you change the IAS
> anyway. That should also mean that UBS and OAS are unchanged.
>>> However, our internal bus is only 42 bits wide. Our IAS field is coded
>>> as 48 bits, which cause IPA to truncated to 42 bits on the physical
>>> bus. In order for our system to work with the arm-smmu.c, there needs
>>> to be a way to force the ipa_size to 42. The current internal solution
>>> is to use the cpuid, but that is quite ugly. I was thinking of using
>>> the model
>>> as indication to right the ipa_size, but I am not too sure of the ACPI
>>> side. Would it be okay to add an APM MMU500 variant? I would also
>>> appreciated it if you guys have any alternate solutions.
>> This is something we've been axpecting to run into for a while now - the
>> appropriate solution is to use a "dma-ranges" property on the master
>> device(s) to describe that they have 42 bits of address wired up, from
>> which they will then inherit the appropriate DMA mask. The outstanding
>> issue which remains is that we're still missing some way of preventing
>> drivers simply clobbering that with a 64-bit mask later, but that's a
>> more general problem[1].
> I wonder if the driver is actually using IAS, OAS and UBS incorrectly.
> We're using them to parameterise the DMA aperture, which is then used
> to size the IOVA domain, but that's wrong because the IAS, OAS and UBS
> are upper bounds and we can still end up allocating unusable/unreachable
> addresses.

It's not incorrect, it's simply all we know at that point. From inside
the SMMU, we can't tell how many of the bits we have are actually wired
up externally, which is why we always take the intersection of the DMA
aperture and the given device's DMA mask at the point of IOVA allocation.

You can reproduce much the same thing on your Juno if you fancy - just
make the HDLCD or PL330 driver set a DMA mask larger than the default 32
bits and the top 8 bits of the MMU-401's 40-bit input being tied off to
0 will become apparent (the only difference being there's not actually
anything at the master end they could be wired to either).

> So I do think that this should be fixed on the SMMU firmware node, rather
> than restricting the range of each master device.

In general, it's a per-master thing which "dma-ranges" is exactly the
correct tool to describe - a property on the SMMU would just be a weird
nonstandard shorthand for a very particular case (it breaks as soon as
you have some *more* limited, e.g. 32-bit, device in the same system).
The fact that every master in this case apparently has the same
capability is just happenstance.


> Will

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