[PATCH v1 06/14] iommu/arm-smmu-v3: Unset corresponding STE fields when s2_cfg is NULL

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy at arm.com]
> Sent: 09 March 2023 13:13
> To: Nicolin Chen <nicolinc at nvidia.com>; jgg at nvidia.com; will at kernel.org
> Cc: eric.auger at redhat.com; kevin.tian at intel.com; baolu.lu at linux.intel.com;
> joro at 8bytes.org; Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi at huawei.com>; jean-philippe at linaro.org;
> linux-arm-kernel at lists.infradead.org; iommu at lists.linux.dev;
> linux-kernel at vger.kernel.org
> Subject: Re: [PATCH v1 06/14] iommu/arm-smmu-v3: Unset corresponding
> STE fields when s2_cfg is NULL
> 
> On 2023-03-09 10:53, Nicolin Chen wrote:
> > From: Eric Auger <eric.auger at redhat.com>
> >
> > Despite the spec does not seem to mention this, on some implementations,
> > when the STE configuration switches from an S1+S2 cfg to an S1 only one,
> > a C_BAD_STE error would happen if dst[3] (S2TTB) is not reset.
> 
> Can you provide more details, since it's not clear whether this is a
> hardware erratum workaround or a bodge around the driver itself doing
> something wrong like not doing a proper break-before-make transition of
> the STE. The architecture explicitly states that all the STE.S2* fields
> except S2VMID and potentially S2S are ignored when Stage 2 is bypassed.

Took a while to locate the email thread where this was discussed,
https://patchwork.kernel.org/cover/11449895/#23244457

This was observed on a HiSilicon implementation where, if the SMMUv3 is configured with
both Stage 1 and Stage 2 (nested) mode once, then it is not possible to configure it back
for Stage 1 mode for the same device(stream id).

IIRC, the SMMUv3 implementation on these boards expects to set the S2TTB field in STE to zero
when using S1, otherwise it reports C_BAD_STE error. :(

You are right that the specification doesn't demand this and I am not sure there are any other
Hardware that requires this.

Could we please have this with a comment added in the code?

Thanks,
Shameer