Consult on ARM SMMU debugfs

[Russell King - ARM Linux admin]

On Mon, Jan 11, 2021 at 08:01:48PM +0000, Robin Murphy wrote:
> 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.

I do think that the IOMMU stuff needs better debugging. I've hit the
WARN_ON() in __arm_lpae_map(), and it's been pretty much undebuggable,
so I've resorted to putting the IOMMU into bypass mode permanently to
work around the issue.

The reason that it's undebuggable is if one puts printk() or trace
statements in the code, boots the platform, you get flooded with those
debugging messages, because every access to the rootfs generates and
tears down a mapping.

It would be nice to be able to inspect the IOMMU page tables and state
of the IOMMU, rather than having to resort to effectively disabling
the IOMMU.

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