linux-kernel - Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console

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Date:   Thu, 12 Nov 2020 09:17:07 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        shreyasjoshi15@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there
 is no console

On (20/11/11 14:54), Petr Mladek wrote:
[..]
> diff --git a/init/main.c b/init/main.c
> index 130376ec10ba..24413c055a85 100644
> --- 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();
> +

A nit, this probably can be done in console_device() function.

For several reasons:

- we will get covered all the future cases when something other than
  console_on_rootfs() will filp_open("/dev/console")

- filp_open() failure does not necessarily mean that we have console=
  and hence must register null-tty driver

- this will keep low level printk/tty details within the printk

Something like this?

---
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 08c50d8ba110..202fa87eb991 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2633,6 +2633,12 @@ struct tty_driver *console_device(int *index)
 	struct tty_driver *driver = NULL;
 
 	console_lock();
+	if (unlikely(!console_drivers)) {
+		console_unlock();
+		register_ttynull_console();
+		console_lock();
+	}
+
 	for_each_console(c) {
 		if (!c->device)
 			continue;