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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Fri, 21 Aug 2020 20:37:15 +0800
Message-ID: <CANMq1KCc-KObgJTe0vWXGHcsffMD=41VMe=GjtsKG0iobJES0Q@mail.gmail.com>
Subject: Re: [PATCH v4 3/3] media: atomisp: Only use trace_printk if allowed
To:     David Laight <David.Laight@aculab.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        "devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
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On Fri, Aug 21, 2020 at 8:18 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Nicolas Boichat
> > Sent: 21 August 2020 13:07
> ...
> > > 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?
>
> Probably because I meant something different :-)
>
> You want the kernel built so that there are no (expanded)
> calls to trace_printf() but with support for modules that
> contain them.
>
> Then I can load a module into a distro kernel that
> contains trace_printf() calls for debug testing.

Gotcha. I think it already works this way ,-)

So if you have CONFIG_TRACE=y, but no trace_printk in your
vmlinux/kernel, no memory is used, and no warning splat
(https://elixir.bootlin.com/linux/v5.8/source/kernel/trace/trace.c#L3160)
is displayed. But then when you load a module with trace_printk, the
buffers are allocated and the warning splat is printed.

The magic is here:
https://elixir.bootlin.com/linux/v5.8/source/kernel/trace/trace_printk.c#L53

My option wouldn't really change that. I mean, if you have
CONFIG_TRACING_ALLOW_PRINTK=n when you compile your module, it'd fail
at build time, but if you set it to =y, your module could happily
build and load (with the big warning splat), no matter how you built
your kernel (I mean, you still need CONFIG_TRACE=y, but
CONFIG_TRACING_ALLOW_PRINTK doesn't matter).

> Which is why I was suggesting a config option that
> only rand-config builds would ever set that would
> cause the calls to generate compile-time errors.

I think I already answered that one above. We'd want that config
option enabled on Chrome OS and we're not a rand-config build (I mean,
we're a very carefully selected random config ,-P).

Thanks,

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