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

On 2021/2/5 17:51, Jean-Philippe Brucker wrote:
> 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.
OK, and Robin said that the latest IORT spec literally states it.

> 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).
Yeah, that would be more flexible. ;-)

> Thanks,
> Jean
> .

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