Re: [PATCH v2 11/14] perf auxtrace: cs-etm: Capture wrapped snapshots

> snapshot_has_wrapped() isn't implemented for Coresight, so when a wrap
> occurs we only save from offset 0 to current head, modulo buffer size,
> rather than capturing the whole buffer. Coresight drivers currently use
> a monotonic head pointer even in snapshot mode, which gives a lot more
> information for taking snapshots than a wrapped pointer. It means we
> don't need to search for data at the end of the buffer to guess if a
> wrap has occurred, and we can go back to saving partial buffers after a
> wrap by checking if the next size to take is less than the buffer
> size.
> 
> The drivers will always be able to generate a monotonic head because
> TRBE has a bytes written counter, so even if we stop generating an AUX
> record on every overflow, we can check if the counter indicated an
> overwrite and offset head by a whole buffer size if it wrapped. This
> forces userspace to read the whole buffer rather than between last and
> current head. ETR and other sinks use software double buffering, so can
> continue as they are.
> 
> Add a much simpler 'monotonic_snapshot_head' path for tracers to use,
> and use it for Coresight.
> 
> Signed-off-by: James Clark <[email protected]>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260821-james-cs-unformatted-per-thread-fix-v2-0-00c4fd0701b4@linaro.org?part=11