Re: [PATCH v5 1/1] serial/8250: Use fifo in 8250 console driver

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 1 Apr 2022 06:35:58 +0200
Jiri Slaby <jirislaby@kernel.org> wrote:

> > With the help of the function tracer, I then noticed the serial
> > controller was taking around 410us seconds to dispatch one single byte:
> > 
> > $ trace-cmd record -p function_graph -g serial8250_console_write \
> >     ./sertest -n 1 /tmp/serco
> > 
> > $ trace-cmd report
> > 

Note, the function graph tracer can add a noticeable amount of overhead to
these timings. If you want a more accurate time for a function, just trace
that one function:

	trace-cmd record -p function_graph -l serial8250_console_write ...

As that will only trace the serial8250_console_write() function (think of
-l as "limit"), and the overhead of function graph for tracing a single
function becomes negligible, and then you can see the true time of that
function.

-- Steve