Messages in this thread | | | Date | Fri, 5 Feb 2021 10:51:14 +0100 | From | Jean-Philippe Brucker <> | Subject | Re: [RFC PATCH 01/11] iommu/arm-smmu-v3: Add feature detection for HTTU |
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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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