Consult on ARM SMMU debugfs

[Robin Murphy]

On 2021-01-07 02:45, chenxiang (M) wrote:
> Hi Will,  Robin or other guys,
> 
> When debugging SMMU/SVA issue on huawei ARM64 board, we find that it 
> lacks of enough debugfs for ARM SMMU driver (such as
> 
> the value of STE/CD which we need to check sometimes). Currently it 
> creates top-level iommu directory in debugfs, but there is no debugfs
> 
> for ARM SMMU driver specially. Do you know whether ARM have the plan to 
> do that recently?

FWIW I don't think I've ever felt the need to need to inspect the Stream 
Table on a live system. So far the nature of the STE code has been 
simple enough that it's very hard for any given STE to be *wrong* - 
either it's set up as expected and thus works fine, or it's not 
initialised at all and you get C_BAD_STE, where 99% of the time you then 
just cross-reference the Stream ID against the firmware and find that 
the DT/IORT is wrong.

Similarly I don't think I've even even *seen* an issue that could be 
attributed to a context descriptor, although I appreciate that as we 
start landing more PASID and SVA support the scope for that starts to 
widen considerably.

Feel free to propose a patch if you believe it would be genuinely useful 
and won't just bit-rot into a maintenance burden, but it's not something 
that's on our roadmap here.

Thanks,
Robin.