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Subject: Re: Help adding trace events to xHCI
From: Steven Rostedt <rostedt@goodmis.org>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>,
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	Kalle Valo <kvalo@qca.qualcomm.com>
Date: Thu, 11 Jul 2013 15:29:15 -0400
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On Thu, 2013-07-11 at 19:08 +0200, Johannes Berg wrote:

> > > lets say that we want the tracepoint function to have the prototype:
> > > 
> > > void trace_cmd_address_device(const char *fmt, ...).
> 
> I'm not sure this is possible. I think we (wireless) do this with
> 
> void trace_cmd_address_device(struct va_format *vaf)
> 
> instead only.
> 
> Note that there's no easy way to dynamically allocate the right amount
> of space in the ringbuffer, or at least I haven't found one. We
> therefore have a static size, which is somewhat inefficient.

Can you add a helper function? If these trace events can't nest (one in
interrupt context and another in normal context with interrupts
enabled), then you can just allocate a per-cpu buffer and store the
string in that, and then move the string into the buffer.

	vsnprintf(this_cpu_ptr(trace_buf), MAX_BUF_LEN, fmt, va);
	__assign_str(str, trace_buf);

You could even use the reg, unreg, methods for TRACE_EVENT_FN() that
will allocate the buffers when the tracepoint is created. This will mean
that you don't waste memory when not tracing.

-- Steve