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Date: Mon, 1 Dec 2008 06:41:46 -0500 From: Mathieu Desnoyers <compudj@...stal.dyndns.org> To: Russell King <rmk+lkml@....linux.org.uk> 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? * Russell King (rmk+lkml@....linux.org.uk) wrote: > 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. > This can be dealt with by refusing the get_trace_clock(), which should probably be allowed to fail if the underlying infrastructure is not ready when the trace buffers are created. The tracer would therefore not accept to start any sort of tracing too soon for the architecture being used. Mathieu -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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