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

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Date:   Thu, 4 Feb 2021 06:51:09 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Linux Kernel Mailing List <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 <dri-devel@...ts.freedesktop.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 1/3] printk: use CONFIG_CONSOLE_LOGLEVEL_* directly

On Thu, Feb 4, 2021 at 12:23 AM Petr Mladek <pmladek@...e.com> wrote:
>
> 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.


I have never seen a policy to restrict
the use of CONFIG options in relevant
subsystem headers.



> 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.

I do not think default_console_loglevel()
is a perfect constant
because it can be modified via
/proc/sys/kernel/printk


I am not sure if it works either.

Some code may not be linked to vmlinux.
drivers/firmware/efi/libstub/efi-stub-helper.c




> Best Regards,
> Petr




--
Best Regards
Masahiro Yamada