Why have another variable deciding a tracepoint?

[Steven Rostedt]

I've been reviewing different users of tracepoints and I stumbled
across this:

drivers/gpu/host1x/cdma.c: host1x_cdma_push()

	if (host1x_debug_trace_cmdbuf)
		trace_host1x_cdma_push(dev_name(cdma_to_channel(cdma)->dev),
				       op1, op2);

That host1x_debug_trace_cmdbuf is a variable that gets set by another
debugfs file "trace_cmdbuf" that is custom to this driver.

Why?

The tracepoint host1x_cdma_push is already controlled by either ftrace
or perf. If it gets enabled by perf or ftrace, it still wont be traced
unless we also enable this trace_cmdbuf. Is there some reason for this?
I can't figure it out from the change log: 6236451d83a720 ("gpu:
host1x: Add debug support").

As tracepoints uses jump labels, there is no branch cost associated
with them. That is, they are either a direct jump, or a nop (in most
cases a nop). But here you added the overhead of a conditional branch
depending on this variable.

If this is truly needed, then use TRACE_EVENT_CONDITION() for that
tracepoint.


/me is baffled

-- Steve


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