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

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Date:	Mon, 1 Dec 2008 10:59:56 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	ltt-dev@...ts.casi.polymtl.ca, linux-kernel@...r.kernel.org
Subject: Re: [ltt-dev] keypad/touchscreen driver events latencies using LTTng on ARM?

On Mon, Dec 01, 2008 at 05:57:37AM -0500, Mathieu Desnoyers wrote:
> Ok, so for LTTng on ARM it would make sense to use the same clock source
> as sched_clock for :
> mach-pxa, mach-realview, mach-sa1100, mach-versatile, plat-omap.
> 
> We basically have, for each of these build scenarios, to take the mmio
> clock used by sched_clock and use it through
> kernel/trace/trace-clock-32-to-64 to extend it to 64-bits. Therefore we
> won't suffer from any of the constrains linked to cnt32_to_63 and we
> would be sure that the trace clock is correct wrt SMP wrt memory
> barriers and cache line bouncing because it uses per-cpu data to keep
> the counters.

Just remember that MMIO clock sources are not available until after
setup_arch() has finished - in other words, they don't work when the
kernel initially boots.  Accessing them too early will cause a page
fault and hang the kernel.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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