Re: [PATCH v3 15/18] perf auxtrace: cs-etm: Capture wrapped snapshots

[sashiko-bot@kernel.org]

> find_snapshot() 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 auxtrace_record__monotonic_find_snapshot() and use it for Coresight.
> A monotonic head can't be auto-detected in the default find_snapshot()
> by checking if head > buffer size, because before the first wrap a
> monotonic head looks the same as a wrapped one. The search would then
> falsely detect a wrap when head is near the end of the buffer, and the
> wrapped pointer fixup moves 'old' and 'head' a whole buffer ahead of the
> driver's head. With a wrapped head that doesn't matter because only the
> offset within the buffer is used, but with a monotonic head the next
> snapshot would either save already saved data again or drop up to a
> buffer's worth of new data.
> 
> Signed-off-by: James Clark <james.clark@linaro.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261102-james-cs-unformatted-per-thread-fix-v3-0-dbcb0c281b9b@linaro.org?part=15