linux-kernel - Re: [RFC PATCH] printk: console: Allow each console to have its own loglevel

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Message-ID: <CAMuHMdU5=XW28XBdEx9ofrrysUCqqtoaS7=0U8Mr7TF3XdntWw@mail.gmail.com>
Date:   Thu, 19 May 2022 19:48:46 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kernel Team <kernel-team@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [RFC PATCH] printk: console: Allow each console to have its own loglevel

Hi Chris,

CC DT

On Thu, May 19, 2022 at 4:37 PM Chris Down <chris@...isdown.name> wrote:
> Geert Uytterhoeven writes:
> >All of the above options are appropriate for "classic" systems,
> >where the console device is selected using the "console=" option.
> >
> >On systems using Device tree, the serial console device is selected
> >using the "chosen/stout-path" property in DT, and the graphical
> >console is usually auto-detected and auto-enabled through DRM.
> >Do you envision a way to specify a specific console loglevel on the
> >kernel command line on such systems?
>
> Interesting question! I hadn't really thought about device tree. I actually
> have very little understanding of how it works to be honest :-)
>
> I'm happy to add loglevel support to device tree, I assume I'd add another
> property under the chosen node, like chosen/stdout-loglevel.

Please do not add a new property there.
IMHO, the loglevel should be specified on the kernel command line,
and not be fixed using a DT property.

Typically, boot loaders (e.g. U-Boot) replace the command line
specified in chosen/stdout-path in DT by something retrieved from
the boot environment (e.g. user-specifief bootargs in NVMEM).

> I have some questions, though:
>
> 1. It looks like DT is standardised. Do I need to get standards approval first?
> 2. Is there documentation on how to reliably test DT changes?

The DT specification can be found at
https://www.devicetree.org/specifications/

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds