linux-kernel - Re: [PATCH 1/3] printk: use CONFIG_CONSOLE_LOGLEVEL

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Date:   Wed, 3 Feb 2021 16:23:06 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Shevchenko <andy@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Darren Hart <dvhart@...radead.org>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Mike Travis <mike.travis@....com>,
        Peter Jones <pjones@...hat.com>,
        Russ Anderson <russ.anderson@....com>,
        Steve Wahl <steve.wahl@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        dri-devel@...ts.freedesktop.org, linux-efi@...r.kernel.org,
        linux-fbdev@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH 1/3] printk: use CONFIG_CONSOLE_LOGLEVEL_* directly

On Tue 2021-02-02 09:44:22, John Ogness wrote:
> On 2021-02-02, Masahiro Yamada <masahiroy@...nel.org> wrote:
> > CONSOLE_LOGLEVEL_DEFAULT is nothing more than a shorthand of
> > CONFIG_CONSOLE_LOGLEVEL_DEFAULT.
> >
> > When you change CONFIG_CONSOLE_LOGLEVEL_DEFAULT from Kconfig, almost
> > all objects are rebuilt because CONFIG_CONSOLE_LOGLEVEL_DEFAULT is
> > used in <linux/printk.h>, which is included from most of source files.
> >
> > In fact, there are only 4 users of CONSOLE_LOGLEVEL_DEFAULT:
> >
> >   arch/x86/platform/uv/uv_nmi.c
> >   drivers/firmware/efi/libstub/efi-stub-helper.c
> >   drivers/tty/sysrq.c
> >   kernel/printk/printk.c
> >
> > So, when you change CONFIG_CONSOLE_LOGLEVEL_DEFAULT and rebuild the
> > kernel, it is enough to recompile those 4 files.
> >
> > Remove the CONSOLE_LOGLEVEL_DEFAULT definition from <linux/printk.h>,
> > and use CONFIG_CONSOLE_LOGLEVEL_DEFAULT directly.
> 
> With commit a8fe19ebfbfd ("kernel/printk: use symbolic defines for
> console loglevels") it can be seen that various drivers used to
> hard-code their own values. The introduction of the macros in an
> intuitive location (include/linux/printk.h) made it easier for authors
> to find/use the various available printk settings and thresholds.
> 
> Technically there is no problem using Kconfig macros directly. But will
> authors bother to hunt down available Kconfig settings? Or will they
> only look in printk.h to see what is available?
> 
> IMHO if code wants to use settings from a foreign subsystem, it should
> be taking those from headers of that subsystem, rather than using some
> Kconfig settings from that subsystem. Headers exist to make information
> available to external code. Kconfig (particularly for a subsystem) exist
> to configure that subsystem.

I agree with this this view.

What about using default_console_loglevel() in the external code?
It reads the value from an array. This value is initialized to
CONSOLE_LOGLEVEL_DEFAULT and never modified later.

Best Regards,
Petr