Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220601135528.3176471-1-michael@walle.cc>
In-Reply-To: <20220601135528.3176471-1-michael@walle.cc>
From:   Rob Herring <robh+dt@kernel.org>
Date:   Wed, 1 Jun 2022 11:39:04 -0500
Message-ID: <CAL_JsqK6DjLTXqu475XTJXBOFihJEePw40zQcx+sCF7tG3GkSA@mail.gmail.com>
Subject: Re: [PATCH] earlycon: prevent multiple register_console()
To:     Michael Walle <michael@walle.cc>
Cc:     Frank Rowand <frowand.list@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>, devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Jun 1, 2022 at 8:55 AM Michael Walle <michael@walle.cc> wrote:
>
> If the earlycon parameter is given twice, the kernel will spit out a
> WARN() in register_console() because it was already registered. The
> non-dt variant setup_earlycon() already handles that gracefully. The dt
> variant of_setup_earlycon() doesn't. Add the check there and propagate
> it through early_init_dt_scan_chosen_stdout().
>
> FWIW, this doesn't happen if CONFIG_ACPI_SPCR_TABLE is set. In that case
> the registration is delayed until after earlycon parameter(s) are
> parsed.
>
> Signed-off-by: Michael Walle <michael@walle.cc>
> ---
>
> I'm not sure if this should have a Fixes tag or not. If so I guess it
> should be the very first commit which introduced the support (commit
> fb11ffe74c79 ("of/fdt: add FDT serial scanning for earlycon")).

Considering no one noticed or cared in 8 years and that earlycon is
supposed to be a debug option, I'd say no.

> For the curious, here is the backtrace:
>
> [    0.000000] ------------[ cut here ]------------
> [    0.000000] WARNING: CPU: 0 PID: 0 at kernel/printk/printk.c:3328 register_console+0x2b4/0x364
> [    0.000000] console 'atmel_serial0' already registered
> [    0.000000] Modules linked in:
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 5.18.0-next-20220601+ #652
> [    0.000000] Hardware name: Generic DT based system
> [    0.000000] Backtrace:
> [    0.000000]  dump_backtrace from show_stack+0x18/0x1c
> [    0.000000]  show_stack from dump_stack_lvl+0x48/0x54
> [    0.000000]  dump_stack_lvl from dump_stack+0x18/0x1c
> [    0.000000]  dump_stack from __warn+0xd0/0x148
> [    0.000000]  __warn from warn_slowpath_fmt+0x9c/0xc4
> [    0.000000]  warn_slowpath_fmt from register_console+0x2b4/0x364
> [    0.000000]  register_console from of_setup_earlycon+0x29c/0x2ac
> [    0.000000]  of_setup_earlycon from early_init_dt_scan_chosen_stdout+0x154/0x18c
> [    0.000000]  early_init_dt_scan_chosen_stdout from param_setup_earlycon+0x40/0x48
> [    0.000000]  param_setup_earlycon from do_early_param+0x88/0xc4
> [    0.000000]  do_early_param from parse_args+0x1a4/0x404
> [    0.000000]  parse_args from parse_early_options+0x40/0x48
> [    0.000000]  parse_early_options from parse_early_param+0x38/0x48
> [    0.000000]  parse_early_param from setup_arch+0x114/0x7a4
> [    0.000000]  setup_arch from start_kernel+0x74/0x6dc
> [    0.000000]  start_kernel from 0x0
> [    0.000000] ---[ end trace 0000000000000000 ]---
>
>  drivers/of/fdt.c              | 6 ++++--
>  drivers/tty/serial/earlycon.c | 5 ++++-
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index a8f5b6532165..7f3524213b43 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -1025,6 +1025,7 @@ int __init early_init_dt_scan_chosen_stdout(void)
>         int l;
>         const struct earlycon_id *match;
>         const void *fdt = initial_boot_params;
> +       int ret;
>
>         offset = fdt_path_offset(fdt, "/chosen");
>         if (offset < 0)
> @@ -1057,8 +1058,9 @@ int __init early_init_dt_scan_chosen_stdout(void)
>                 if (fdt_node_check_compatible(fdt, offset, match->compatible))
>                         continue;
>
> -               if (of_setup_earlycon(match, offset, options) == 0)
> -                       return 0;
> +               ret = of_setup_earlycon(match, offset, options);
> +               if (!ret || ret == -EALREADY)
> +                       return ret;

Wouldn't just doing a 'return 0' here be simpler? The only other
caller of this function doesn't check the error.

Rob