From: Joe Perches <joe@perches.com>
Subject: Re: [PATCH 2/9] ext4: Use pr_fmt and pr_<level>
Date: Tue, 20 Mar 2012 02:44:36 -0700
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On Tue, 2012-03-20 at 09:47 +0100, Jiri Slaby wrote:
> Joe writes: "notify a particular set of per subsystem messages that
> pr_<level> could easily provide"
> 
> Maybe the concept is not explained well enough that we do not follow?

Basically, when pr_fmt(fmt) is #defined to something
other than "#define pr_fmt(fmt) fmt", it could be used
as a mechanism to do more than standardize prefixes.

__builtin_strcmp is pretty useful.

You can do things like:
(I'm just typing, this may have errors)

#define _first_macro_arg(arg1) arg1
#define first_macro_arg(arg1, ...) _first_macro_arg(arg1)

#define pr_info(fmt, ...)					\
do {								\
	if (!__builtin_strcmp(first_macro_arg(pr_fmt(fmt)),	\
			      KBUILD_MODNAME ": " fmt)))	\
		advanced_printk(fmt, ##__VA_ARGS__);		\
	else							\
		printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__);	\
while (0)

etc...

Anyway, it's been a couple of years since I prototyped
something like that and tested file sizes.

The size delta of an allyesconfig was:

-rwxr-xr-x 1 joe joe 926968471 2010-06-29 10:39 vmlinux.allyesconfig.printk_level_with_module
-rwxr-xr-x 1 joe joe 927196127 2010-06-23 02:40 vmlinux.allyesconfig.pr_level

It reduced an allyesconfig by .1%
It wasn't a huge saving.

I'll play with it again and send an rfc
patch in a couple of days.

Maybe I'll prototype a pr_notify mechanism too.

cheers, Joe