On 2022-11-15 02:28, leo-hou@xxxxxxxxxxx wrote:
Hi,
Why doesn't the arm smmu driver for Linux support debugfs ?
Because nobody's ever written any debugfs code for it.
Are there any historical reasons?
Only that so far nobody's needed to.
TBH, arm-smmu is actually quite straightforward, and none of the
internal driver state is really all that interesting (other than the
special private Adreno stuff, but we leave it up to Rob to implement
whatever he needs there). Given the kernel config, module parameters,
and the features logged at probe, you can already infer how it will set
up context banks etc. for regular IOMMU API work; there won't be any
surprises. At this point there shouldn't be any need to debug the driver
itself, it's mature and stable. For debugging *users* of the driver,
I've only dealt with the DMA layer, where a combination of the IOMMU API
tracepoints, CONFIG_DMA_API_DEBUG, and my own hacks to iommu-dma have
always proved sufficient to get enough insight into what's being mapped
where.
I think a couple of people have previously raised the idea of
implementing some kind of debugfs dumping for io-pgtable, but nothing's
ever come of it. As above, it often turns out that you can find the
information you need from other existing sources, thus the effort of
implementing and maintaining a load of special-purpose debug code can be
saved. In particular it would not be worth having driver-specific code
that only helps debug generic IOMMU API usage - that would be much
better implemented at the generic IOMMU API level.
Thanks,
Robin.
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