On Sun, Dec 04, 2016 at 02:06:23PM +0530, Srinivas Ramana wrote:

On 12/02/2016 04:38 PM, Will Deacon wrote:

quoted

On Fri, Dec 02, 2016 at 01:44:55PM +0530, Srinivas Ramana wrote:

quoted

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.

Having cycle counter would help in the cases where we want to correlate the
time with other subsystems which are outside cpu subsystem.

Do you have an example of these subsystems? Can they be used to generate
trace data with mainline?

local_clock or even the perf track_clock uses sched_clock which gets
suspended during system suspend. Yes, they are backed up by the
architected counter but they ignore the cycles spent in suspend.i

Does mono_raw solve this (also hangs off the architected counter and is
supported in the vdso)?

so, when comparing with monotonically increasing cycle counter, other
clocks doesn't help. It seems X86 uses the TSC counter to help such cases.

Does this mean we need a way to expose the frequency to userspace, too?

Will