Re: [PATCH] iommu/arm-smmu-v3: Maintain valid access attributes for non-coherent SMMU

[Robin Murphy]

On 2026-01-05 2:53 pm, Jason Gunthorpe wrote:
> On Mon, Jan 05, 2026 at 01:33:34PM +0000, Robin Murphy wrote:
> > The assumption is that if the SMMU is not I/O-coherent, then the Normal
> > Cacheable attribute will inherently degrade to a non-snooping (and thus
> > effectively Normal Non-Cacheable) one, as that's essentially what AXI will
> > do in practice, and thus the attribute doesn't actually matter all that much
> > in terms of functional correctness. If the SMMU _is_ capable of snooping but
> > is not described as such then frankly firmware is wrong.
>
> Sadly I am aware of people doing this.. Either a HW bug or some other
> weird issue forces the FW to set a non-coherent FW attribute even
> though the HW is partially or fully able to process cachable AXI
> attributes.

What I've seen more often is that firmware authors ignore (or don't even 
realise) I/O-coherency, then someone ends up trying to "fix" Linux with 
wacky DMA API patches when things start going wrong on mis-described 
hardware... Yes, they might happen to get away with it with stuff like 
SMMUs or Mali GPUs where a "well-behaved" driver might have control of 
the source attributes, but where AxCACHE/AxDOMAIN are hard-wired there's 
really no option other than to describe the hardware correctly, hence 
that should always be the primary consideration.

And as I say, if there *is* some reason to specifically avoid using 
certain attributes, then deliberately describing the hardware 
incorrectly, in the hope that OS drivers might be "well-behaved" in a 
way that just happens to lead to them not using those attributes, is 
really not a robust workaround anyway.

> It is reasonable that Linux will set the attributes properly based on
> what it is doing. Setting the wrong attributes and expecting the HW to
> ignore them seems like a hacky direction.

Oh, I'm not saying that we *shouldn't* set our attributes more exactly - 
this would still be needed for doing things the "right" way too - I just 
want to be very clear on the reasons why. The current assumption is not 
a bug per se, and although it's indeed not 100% robust, the cases where 
it doesn't hold are more often than not for the wrong reason. Therefore 
I would say doing this purely for the sake of working around bad 
firmware - and especially errata - is just as hacky if not more so.

> I didn't see anything in the spec that says COHACC means the memory
> attributes are ignored and forced to non-coherent, even though that is
> the current assumption of the driver.

It kinda works the other way round: COHACC==1 says that the Cacheability 
and Shareability attributes *can* be configured to snoop CPU caches 
(although do not have to be); therefore the "If a system does not 
support IO-coherent access from the SMMU, SMMU_IDR0.COHACC must be 0" 
case implies that the SMMU is incapable of snooping regardless of how 
those attributes are set, thus must at least be in a different Inner 
Shareability domain from the CPUs, at which point the Cacheability 
domains probably aren't too meaningful either. I would consider it would 
be unexpectedly incorrect behaviour for an SMMU reporting COHACC==0 to 
actually be capable of snooping.

> > If prople have a good reason for wanting to use a coherent SMMU
> > non-coherently (and/or control of allocation hints), then that should really
> > be some kind of driver-level option - it would need a bit of additional DMA
> > API work (which has been low down my to-do list for a few years now...), but
> > it's perfectly achievable, and I think it's still preferable to abusing the
> > COHACC override in firmware.
>
> IMHO, this is a different topic, and something that will probably
> become interesting this year. I'm aware of some HW/drivers that needs
> optional non-coherent mappings for isochronous flows - but it is not
> the DMA API that is the main issue but the page table walks :\

Hmm, yeah, potentially configuring PTW attributes for a DMA domain is 
something I hadn't even thought about - the DMA API aspect I mean is 
that in general we need some sort of DMA_ATTR_NO_SNOOP when 
mapping/allocating such isochronous buffers/pagetables etc., to make the 
DMA layer still do the cache maintenance/non-cacheable remaps despite 
dev_is_dma_coherent() being true.

Thanks,
Robin.