linux-kernel - Re: keypad/touchscreen driver events latencies using LTTng on ARM?

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Date:	Mon, 1 Dec 2008 05:35:02 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Trilok Soni <soni.trilok@...il.com>,
	Russell King <rmk@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, ltt-dev@...ts.casi.polymtl.ca
Subject: Re: keypad/touchscreen driver events latencies using LTTng on ARM?

* Trilok Soni (soni.trilok@...il.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

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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