linux-kernel - Re: [PATCH v2 03/15] drivers: thermal: tsens: Add __func_

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Date:   Thu, 29 Aug 2019 21:44:04 +0530
From:   Amit Kucheria <amit.kucheria@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Eduardo Valentin <edubezval@...il.com>,
        Andy Gross <agross@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Brian Masney <masneyb@...tation.org>,
        Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2 03/15] drivers: thermal: tsens: Add __func__ identifier
 to debug statements

On Thu, Aug 29, 2019 at 8:49 PM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Thu, Aug 29, 2019 at 07:58:45PM +0530, Amit Kucheria wrote:
> > On Thu, Aug 29, 2019 at 7:35 PM Daniel Thompson
> > <daniel.thompson@...aro.org> wrote:
> > >
> > > On Tue, Aug 27, 2019 at 05:43:59PM +0530, Amit Kucheria wrote:
> > > > Printing the function name when enabling debugging makes logs easier to
> > > > read.
> > > >
> > > > Signed-off-by: Amit Kucheria <amit.kucheria@...aro.org>
> > > > Reviewed-by: Stephen Boyd <swboyd@...omium.org>
> > > > Reviewed-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> > >
> > > This should need to be manually added at each call site; it is already
> > > built into the logging system (the f flag for dynamic debug)?
> >
> > I assume you meant "shouldn't".
>
> Quite so. Sorry about that.
>
> > I haven't yet integrated dynamic debug into my daily workflow.
> >
> > Last time I looked at it, it was a bit bothersome to use because I
> > needed to lookup exact line numbers to trigger useful information. And
> > those line numbers constantly keep changing as I work on the driver,
> > so it was a bit painful to script. Not to mention the syntax to frob
> > the correct files in debugfs to enable this functionality.
> >
> > As opposed to this, adding the following to the makefile is so easy. :-)
> >
> > CFLAGS_tsens-common.o          := -DDEBUG
> >
> > Perhaps I am using it all wrong? How would I go about using dynamic
> > debug instead of this patch?
>
> Throwing dyndbg="file <fname>.c +pf" onto the kernel command line is a
> good start (+p enables debug level prints, +f causes messages to include
> the function name).

That's useful to know.

$ git grep __func__ | wc -l
30914

Want to send some patches? :-)

> When the C files map to module names (whether the modules are actually
> built-in or not) then <module>.dyndbg=+pf is a bit cleaner and allows
> you to debug the whole of a driver without how it is decomposed into
> files.

And if changing the kernel cmdline options isn't possible or is inconvenient?

> There are (many) other controls to play with[1] but the above should be
> sufficient to simulate -DDEBUG .

The "hard" bit is explicitly poking the line number in a file to
activate a paricular pr_dbg statement. Even if I scripted it, those
lines numbers keep changing in an actively developed driver.

Somehow, I've always felt dyndbg was more useful to debug a production
system where recompiling the kernel wasn't an option e.g. reporting an
issue back to a distro-kernel vendor.

> Daniel.
>
> [1]
> https://www.kernel.org/doc/html/latest/admin-guide/dynamic-debug-howto.html