linux-kernel - Re: [PATCH] trace: extend trace_clock to support arch

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Message-ID: <20161202110845.GC8266@arm.com>
Date:   Fri, 2 Dec 2016 11:08:45 +0000
From:   Will Deacon <will.deacon@....com>
To:     Srinivas Ramana <sramana@...eaurora.org>
Cc:     catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] trace: extend trace_clock to support arch_arm clock
 counter

On Fri, Dec 02, 2016 at 01:44:55PM +0530, Srinivas Ramana wrote:
> Extend the trace_clock to support the arch timer cycle
> counter so that we can get the monotonic cycle count
> in the traces. This will help in correlating the traces with the
> timestamps/events in other subsystems in the soc which share
> this common counter for driving their timers.

I'm not sure I follow this reasoning. What's wrong with nanoseconds? In
particular, the "perf" trace_clock hangs off sched_clock, which should
be backed by the architected counter anyway. What does the cycle counter in
isolation tell you, given that the frequency isn't architected?

I think I'm missing something here.

Will