linux-kernel - Re: [PATCH] init: Don't proxy console= to earlycon

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Date:   Mon, 10 Jul 2023 09:30:19 -0600
From:   Raul Rangel <rrangel@...omium.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Mario Limonciello <mario.limonciello@....com>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, kramasub@...omium.org,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Li Zhe <lizhe.67@...edance.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Zhou jie <zhoujie@...china.com>
Subject: Re: [PATCH] init: Don't proxy console= to earlycon

On Sun, Jul 9, 2023 at 8:43 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
>
>
> On 7/9/23 18:15, Mario Limonciello wrote:
> > On 7/9/23 18:46, Randy Dunlap wrote:
> >>
> >>
> >> On 7/7/23 18:17, Raul E Rangel wrote:
> >>> Right now we are proxying the `console=XXX` command line args to the
> >>> param_setup_earlycon. This is done because the following are
> >>> equivalent:
> >>>
> >>>      console=uart[8250],mmio,<addr>[,options]
> >>>      earlycon=uart[8250],mmio,<addr>[,options]
> >>>
> >>> In addition, when `earlycon=` or just `earlycon` is specified on the
> >>> command line, we look at the SPCR table or the DT to extract the device
> >>> options.
> >>>
> >>> When `console=` is specified on the command line, it's intention is to
> >>> disable the console. Right now since we are proxying the `console=`
> >>
> >> How do you figure this (its intention is to disable the console)?
> >

https://www.kernel.org/doc/html/v6.1/admin-guide/kernel-parameters.html
says the following:
console=
    { null | "" }
            Use to disable console output, i.e., to have kernel
            console messages discarded.
            This must be the only console= parameter used on the
            kernel command line.

        earlycon=       [KNL] Output early console device and options.

            When used with no options, the early console is
            determined by stdout-path property in device tree's
            chosen node or the ACPI SPCR table if supported by
            the platform.

The reason this bug showed up is that ChromeOS has set `console=` for a
very long time:
https://chromium.googlesource.com/chromiumos/platform/crosutils/+/main/build_kernel_image.sh#282
I'm not sure on the exact history, but AFAIK, we don't have the ttyX devices.

Coreboot recently added support for the ACPI SPCR table which in
combination with the
`console=` arg, we are now seeing earlycon enabled when it shouldn't be.

> > I read that as "it's intention is to disable the default console (tty0)".
>
> Yes, that "default" word should be there IMO.
>
> Does this patch affect behavior if someone uses
>         console=tty0
> i.e., the default?
>

No, it shouldn't. This change makes it so that the
param_setup_earlycon function gets
skipped if there is no value.
See https://chromium.googlesource.com/chromiumos/third_party/kernel/+/v6.1/drivers/tty/serial/earlycon.c#223

> >
> > IE if I add console=ttyS0,115200,n8 to my kernel command line then I don't get the output on tty0 anymore.  If I want it on both then I do
> >
> > console=ttyS0,115200,n8 console=tty0.
> >
> >>
> >>> flag to the earlycon handler, we enable the earlycon_acpi_spcr_enable
> >>> variable when an SPCR table is present. This means that we
> >>> inadvertently enable the earlycon.
> >>>
> >>> This change makes it so we only proxy the console= command if it's
> >>> value is not empty. This way we can correctly handle both cases.
> >>>
> >>> Signed-off-by: Raul E Rangel <rrangel@...omium.org>
> >>> ---
> >>>
> >>>   init/main.c | 3 +--
> >>>   1 file changed, 1 insertion(+), 2 deletions(-)
> >>>
> >>> diff --git a/init/main.c b/init/main.c
> >>> index aa21add5f7c54..f72bf644910c1 100644
> >>> --- a/init/main.c
> >>> +++ b/init/main.c
> >>> @@ -738,8 +738,7 @@ static int __init do_early_param(char *param, char *val,
> >>>       for (p = __setup_start; p < __setup_end; p++) {
> >>>           if ((p->early && parameq(param, p->str)) ||
> >>>               (strcmp(param, "console") == 0 &&
> >>> -             strcmp(p->str, "earlycon") == 0)
> >>> -        ) {
> >>> +             strcmp(p->str, "earlycon") == 0 && val && val[0])) {
> >>>               if (p->setup_func(val) != 0)
> >>>                   pr_warn("Malformed early option '%s'\n", param);
> >>>           }
> >>
> >
>
> --
> ~Randy