linux-kernel - Re: [PATCH] Tracing: do export trace_set_clr

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Date:	Mon, 08 Nov 2010 10:14:19 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Christoph Hellwig <hch@...radead.org>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] Tracing: do export trace_set_clr_event

On Mon, 8 Nov 2010 04:56:10 -0500, Christoph Hellwig <hch@...radead.org> wrote:
> On Mon, Nov 08, 2010 at 02:05:12PM +0800, Yuanhan Liu wrote:
> > Trace event belongs to a module does exist only when the module is
> > loaded. Well, we can use trace_set_clr_event funtion to enable some
> > trace event at the module init routine, so that we will not miss
> > something while loading then module.
> > 
> > So, Export the trace_set_clr_event function so that module can use it.
> 
> Which module?

drm/i915. The scenario is that we want to enable a tracepoint to record
(selected) register writes both at runtime and module-load time. mmiotrace
is not quite sufficient for our needs, since we want to be enable
recording around a troublesome application from within X (i.e. without
having to ask the user to reload the module after enabling mmiotrace). We
also want to exclude the bit-banging registers from being traced since
they generate voluminous noise. So we settled on adding a tracepoint to
the our register read/write routines.

The question then became how to enable register tracing upon module load?
This was the simpler proposal, add a module parameter to enable the
tracepoints - hence the exporting of this routine. The alternative that we
are mulling over is whether it is possible for ftrace to parse a trace= on
every module load to manipulate the tracepoints within that module.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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