Re: [PATCH v2 2/3] perf cs-etm: Remove callback cs_etm_find_snapshot()

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Jul 01, 2021 at 05:35:36PM +0800, Leo Yan escreveu:
> The callback cs_etm_find_snapshot() is invoked for snapshot mode, its
> main purpose is to find the correct AUX trace data and returns "head"
> and "old" (we can call "old" as "old head") to the caller, the caller
> __auxtrace_mmap__read() uses these two pointers to decide the AUX trace
> data size.
> 
> This patch removes cs_etm_find_snapshot() with below reasons:
> 
> - The first thing in cs_etm_find_snapshot() is to check if the head has
>   wrapped around, if it is not, directly bails out.  The checking is
>   pointless, this is because the "head" and "old" pointers both are
>   monotonical increasing so they never wrap around.
> 
> - cs_etm_find_snapshot() adjusts the "head" and "old" pointers and
>   assumes the AUX ring buffer is fully filled with the hardware trace
>   data, so it always subtracts the difference "mm->len" from "head" to
>   get "old".  Let's imagine the snapshot is taken in very short
>   interval, the tracers only fill a small chunk of the trace data into
>   the AUX ring buffer, in this case, it's wrongly to copy the whole the
>   AUX ring buffer to perf file.
> 
> - As the "head" and "old" pointers are monotonically increased, the
>   function __auxtrace_mmap__read() handles these two pointers properly.
>   It calculates the reminders for these two pointers, and the size is
>   clamped to be never more than "snapshot_size".  We can simply reply on
>   the function __auxtrace_mmap__read() to calculate the correct result
>   for data copying, it's not necessary to add Arm CoreSight specific
>   callback.

Thanks, applied.

- Arnaldo