[ltt-dev] keypad/touchscreen driver events latencies using LTTng on ARM?

Mathieu Desnoyers mathieu.desnoyers at polymtl.ca
Mon Dec 1 05:35:02 EST 2008

* Trilok Soni (soni.trilok at gmail.com) wrote:
> Hi,
> I would like to try LTTng to measure the keypad and touchscreen
> drivers events latency on ARM based board, which typically runs less
> than 1GHz frequency.  I have recently tried LTTng patches as per
> compatibility list on LTTng website on 2.6.25 kernel on ARM11 based
> board, but on LTTv it shows the IRQ min/max interval times on
> nanoseconds precision, would that be correct? Because I thought it
> could be possible to microseconds format until with hrtimers support
> right? Just guide so that I can use this tool to effectively measure
> driver latencies. Thanks.

Hi Trilok,

The current Trace Clock implementation uses the generic fallback in
include/asm-generic/trace-clock.h as a time source on ARM. It has a 1
jiffy precision and uses a logical clock in the least significant bits,
incremented a each event, to keep track of the event order. Therefore,
you should not rely on timing information more precise than 1 HZ.

However, I wonder if some of the newer ARM boards would happen to have a
cycle counter (timestamp counter), so we could implement a get_cycles()
and trace clock for them ?


Mathieu Desnoyers
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