linux-kernel - Re: [Buildroot] ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

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Message-ID: <20210107152724.GL1485369@scaer>
Date:   Thu, 7 Jan 2021 16:27:24 +0100
From:   "Yann E. MORIN" <yann.morin.1998@...e.fr>
To:     Vineet Gupta <vgupta@...opsys.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        linux-arch@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        linux-kernel@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        buildroot@...ybox.net, arcml <linux-snps-arc@...ts.infradead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        shreyasjoshi15@...il.com, Guenter Roeck <linux@...ck-us.net>
Subject: Re: [Buildroot] ARC no console output (was Re: [PATCH 1/2]
 init/console: Use ttynull as a fallback when there is no console)

Vineet, All,

On 2021-01-06 15:36 -0800, Vineet Gupta spake thusly:
> On 11/11/20 5:54 AM, Petr Mladek wrote:
[--SNIP--]
> >Make sure that stdin, stdout, stderr, and /dev/console are always
> >available by a fallback to the existing ttynull driver. It has
> >been implemented for exactly this purpose but it was used only
> >when explicitly configured.
> >
> >Signed-off-by: Petr Mladek <pmladek@...e.com>
> 
> >--- a/init/main.c
> >+++ b/init/main.c
> >@@ -1470,8 +1470,14 @@ void __init console_on_rootfs(void)
> >  	struct file *file = filp_open("/dev/console", O_RDWR, 0);
> >  	if (IS_ERR(file)) {
> >-		pr_err("Warning: unable to open an initial console.\n");
> >-		return;
> >+		pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
> >+		register_ttynull_console();
> >+
> >+		file = filp_open("/dev/console", O_RDWR, 0);
> >+		if (IS_ERR(file)) {
> >+			pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
> >+			return;
> >+		}
> 
> This breaks ARC booting (no output on console).
> 
> Our Buildroot based setup has dynamic /dev where /dev/console doesn't exist
> statically and there's a primoridla /init shell script which does following
> 
> /bin/mount -t devtmpfs devtmpfs /dev
> exec 0</dev/console
> exec 1>/dev/console
> exec 2>/dev/console
> exec /sbin/init "$@"

I guess you are speaking about the initramfs (cpio) case, right?

We've changed that code last August:

    https://git.buildroot.org/buildroot/commit/fs/cpio/init?id=b9026e83f

I.e. if we can't do the redirection, then we don't redirect anything.
The change was done for people who explicitly pass an empty console= on
their kernel command line.

Now, I haven't looked at nulltty yet, and I have (so far) no idea on how
it works. Thanks for the hint, I'll have a look.

> Buildroot has had this way of handling missing /dev/console since 2011 [1]
> and [2].

See also more archaelogy on that topic, referenced in that commit:
    https://git.buildroot.org/buildroot/commit/fs/cpio/?id=98a6f1fc02e41

> Please advise what needs to be done to unbork boot.

This has been present since the 2020.08 release, and has been backported
to the maintenance branches:
    2020.02.x (LTS) -> f1a83afe2df2a
    2020.05.x       -> 797f9e40224c9

> Otherwise this seems
> like a kernel change which breaks user-space and needs to be backed-out (or
> perhaps conditionalize on CONFIG_NULL_TTY. I'm surprised it hasn't been
> reported by any other  embedded folks

I won't speak about whether this is a kernel regression or not, not my
call.

Regards,
Yann E. MORIN.

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