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SubjectRe: [RFC PATCH 01/11] iommu/arm-smmu-v3: Add feature detection for HTTU
Hi Keqian,

On Fri, Feb 05, 2021 at 05:13:50PM +0800, Keqian Zhu wrote:
> > We need to accommodate the firmware override as well if we need this to be meaningful. Jean-Philippe is already carrying a suitable patch in the SVA stack[1].
> Robin, Thanks for pointing it out.
>
> Jean, I see that the IORT HTTU flag overrides the hardware register info unconditionally. I have some concern about it:
>
> If the override flag has HTTU but hardware doesn't support it, then driver will use this feature but receive access fault or permission fault from SMMU unexpectedly.
> 1) If IOPF is not supported, then kernel can not work normally.
> 2) If IOPF is supported, kernel will perform useless actions, such as HTTU based dma dirty tracking (this series).
>
> As the IORT spec doesn't give an explicit explanation for HTTU override, can we comprehend it as a mask for HTTU related hardware register?

To me "Overrides the value of SMMU_IDR0.HTTU" is clear enough: disregard
the value of SMMU_IDR0.HTTU and use the one specified by IORT instead. And
that's both ways, since there is no validity mask for the IORT value: if
there is an IORT table, always ignore SMMU_IDR0.HTTU.

That's how the SMMU driver implements the COHACC bit, which has the same
wording in IORT. So I think we should implement HTTU the same way.

One complication is that there is no equivalent override for device tree.
I think it can be added later if necessary, because unlike IORT it can be
tri state (property not present, overriden positive, overridden negative).

Thanks,
Jean

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