linux-kernel - Re: [PATCH v2] printk: Fix preferred console selection with multiple matches

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Date:   Thu, 19 Dec 2019 14:50:53 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] printk: Fix preferred console selection with multiple
 matches

On Mon 2019-12-16 12:08:14, Benjamin Herrenschmidt wrote:
> In the following circumstances, the rule of selecting the console
> corresponding to the last "console=" entry on the command line as
> the preferred console (CON_CONSDEV, ie, /dev/console) fails. This
> is a specific example, but it could happen with different consoles
> that have a similar name aliasing mechanism.
> 
> v2. Use a different logic to avoid calling match/setup multiple
>     times as discussed with Petr.
> 
> NOTE: This may look convoluted because I'm trying to keep the existing
> behaviour identical when it comes to things like Braille selection,
> setup failures, on Braille consoles, or setup failures on normal consoles
> which all have subtly different results in the current code.
> 
> Some of those behaviour are a bit dubious and we might be able to simply
> rely on CON_ENABLED and CON_BRL flags in newcon after the search but I
> don't want to change those corner cases in this patch.

Yes, it is dubious. IMHO, the 5 error codes make it even harder to
see what happens in which case.

The code really need simplification. I would prefer to take the risk
and reduce the amount of added conditions as much as possible.
I have an idea, see below.


> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2542,6 +2545,53 @@ static int __init keep_bootcon_setup(char *str)
>  
>  early_param("keep_bootcon", keep_bootcon_setup);
>  
> +enum con_match {
> +	con_matched,
> +	con_matched_preferred,
> +	con_braille,
> +	con_failed,
> +	con_no_match,
> +};

Please, replace this with int, where:

   + con_matched -> 0
   + con_matched_preferred -> 0 and make "has_preferred" global variable
   + con_braile -> 0		later check for CON_BRL flag
   + con_failed -> -EFAULT
   + con_no_match -> -ENOENT

> @@ -2615,41 +2664,19 @@ void register_console(struct console *newcon)
> +	/* See if this console matches one we selected on the command line */
> +	match = try_match_new_console(newcon, true);
> +	/* If it didn't, try matching the platform ones */
> +	if (match == con_no_match)
> +		match = try_match_new_console(newcon, false);
> +	/* If we matched a Braille console, bail out */
> +	if (match == con_braille)
> +		return;
> +	/* Check if we found a preferred one */
> +	if (match == con_matched_preferred)
> +		has_preferred = true;
>  
> +	/* If we don't have an enabled console, bail out */
>  	if (!(newcon->flags & CON_ENABLED))
>  		return;

Some of the comments describe what is obvious. I would simplify
it the following way:

	/* Prefer command line over platform specific defaults. */
	err = try_match_new_console(newcon, true);
	if (err = -ENOENT)
		err = try_match_new_console(newcon, false);

	/* printk() messages are not printed to Braille consoles. */
	if (err || console->flags | CON_BRL)
		return;


Finally, please split the change into two patches:

1st patch will "just" introduce try_match_new_console(console) and
 use it the following way:

	err = try_match_new_console(newcon);

	/* printk() messages are not printed to the Braille console. */
	if (err || console->flags | CON_BRL)
		return;

2nd patch will add the user_specified logic.

This way bisection will distinguish regressions caused
by the refactoring and by the changed search order.

Best Regards,
Petr

PS: I have vacation between December 23 and January 1. I believe
that v3 will be ready for merging. Anyway, I will not push it
into linux-next before I am back from vacation. I would like
to stay off the computer and do not want to eventually break
linux-next for too long.