Date:   Fri, 21 Aug 2020 20:07:12 +0800
From:   Nicolas Boichat <drinkcat@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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" <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, Aug 21, 2020 at 7:32 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Nicolas Boichat
> > Sent: 21 August 2020 11:28
> >
> > On Fri, Aug 21, 2020 at 4:48 PM David Laight <David.Laight@...lab.com> wrote:
> > >
> > > From: Steven Rostedt
> > > > Sent: 21 August 2020 01:36
> > > > 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.
> > >
> > > Is it worth having three compile-time options:
> > > 1) trace_printk() ignored.
> >
> > CONFIG_TRACE=n (now)
> >
> > > 2) trace_printk() enabled.
> >
> > CONFIG_TRACE=y (now)
> >
> > > 3) trace_printk() generates a compile time error.
> >
> > CONFIG_TRACE=y and CONFIG_TRACING_ALLOW_PRINTK=n (my patch)
> >
> > >
> > > Normal kernel builds would ignore calls.
> > > Either a config option or a module option (etc) would enable it.
> > > A config option that 'rand-config' builds would turn on would
> > > generate compile-time errors.
> >
> > Yes, a config option is exactly what my patch 2/2 does. And see
> > Steven's argument.
>
> But you were adding #ifs to you code to enable the traces.
> That is just horrid.

Yeah this patch 3/3 is not the best (if I understand what you mean),
1/3 type of #ifdef is actually fairly common in the kernel (an ifdef
that you can enable manually in the same file, _not_ a config option).

Steven's point is that both 1/3 and 3/3 should be replaced by trace
events anyway.

>
> What you want is CONFIG_HJHJVLKHCVKIYVKIIYVYKIYVLUCLUCL=y (default n)
> that would only ever get set by a 'rand-config' build and would
> never be tested in any source code.

What I have now is CONFIG_TRACING_ALLOW_PRINTK=n (default y). That's
the same thing?

Also, I'd want to enable this option on Chromium OS (i.e. set your
CONFIG_HJHJVLKHCVKIYVKIIYVYKIYVLUCLUCL=y, or my
CONFIG_TRACING_ALLOW_PRINTK=n), and it's likely some distros would
make that choice (because they'd also do not want to see that big
trace_printk warning on their production kernels).

And then you end up with Steven's point (developers inconvenience when
trying to add trace_printk using a config that somebody else provided
to them) ,-(

>
> You might also want a #define that can set temporarily
> to enable traces in a specific file/module even though
> CONFIG_TRACE=n.

I don't understand how traces are supposed to work with CONFIG_TRACE=n?


> (But rand-config builds would still fail if they enabled the
> 'special' option.)


>
>         David.
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

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