linux-kernel - Re: [PATCH v4 3/3] media: atomisp: Only use trace

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Message-ID: <20200820203601.4f70bf98@oasis.local.home>
Date:   Thu, 20 Aug 2020 20:36:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        devel@...verdev.osuosl.org, lkml <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v4 3/3] media: atomisp: Only use trace_printk if allowed

On Fri, 21 Aug 2020 08:13:00 +0800
Nicolas Boichat <drinkcat@...omium.org> wrote:

> On Thu, Aug 20, 2020 at 10:23 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Thu, 20 Aug 2020 17:14:12 +0800
> > Nicolas Boichat <drinkcat@...omium.org> wrote:
> >  
> > > Technically, we could only initialize the trace_printk buffers
> > > when the print env is switched, to avoid the build error and
> > > unconditional boot-time warning, but I assume this printing
> > > framework will eventually get removed when the driver moves out
> > > of staging?  
> >
> > Perhaps this should be converting into a trace event. Look at what bpf
> > did for their bpf_trace_printk().
> >
> > The more I think about it, the less I like this series.  
> 
> To make it clear, the primary goal of this series is to get rid of
> trace_printk sprinkled in the kernel by making sure some randconfig
> builds fail. Since my v2, there already has been one more added (the
> one that this patch removes), so I'd like to land 2/3 ASAP to prevent
> even more from being added.
> 
> Looking at your reply on 1/3, I think we are aligned on that goal? Is
> there some other approach you'd recommend?
> 
> Now, I'm not pretending my fixes are the best possible ones, but I
> would much rather have the burden of converting to trace events on the
> respective driver maintainers. (btw is there a short
> documentation/tutorial that I could link to in these patches, to help
> developers understand what is the recommended way now?)
>

I like the goal, but I guess I never articulated the problem I have
with the methodology.

trace_printk() is meant to be a debugging tool. Something that people
can and do sprinkle all over the kernel to help them find a bug in
areas that are called quite often (where printk() is way too slow).

The last thing I want them to deal with is adding a trace_printk() with
their distro's config (or a config from someone that triggered the bug)
only to have the build to fail, because they also need to add a config
value.

I add to the Cc a few developers I know that use trace_printk() in this
fashion. I'd like to hear their view on having to add a config option
to make trace_printk work before they test a config that is sent to
them.

-- Steve