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 > > . > Thanks, Keqian
