linux-kernel - Re: [PATCH v2 3/3] printk: fix double printing with earlycon

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Date:   Thu, 2 Mar 2017 16:58:59 +0300
From:   Aleksey Makarov <aleksey.makarov@...aro.org>
To:     linux-serial@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Hurley <peter@...leysoftware.com>,
        Jiri Slaby <jslaby@...e.com>,
        Robin Murphy <robin.murphy@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Nair, Jayachandran" <Jayachandran.Nair@...ium.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH v2 3/3] printk: fix double printing with earlycon



On 03/02/2017 04:11 PM, Aleksey Makarov wrote:
> If a console was specified by ACPI SPCR table _and_ command line
> parameters like "console=ttyAMA0" _and_ "earlycon" were specified,
> then log messages appear twice.
> 
> The root cause is that the code traverses the list of specified
> consoles (the `console_cmdline` array) and stops at the first match.
> But it may happen that the same console is referred by the elements
> of this array twice:
> 
> 	pl011,mmio,0x87e024000000,115200 -- from SPCR
> 	ttyAMA0 -- from command line
> 
> but in this case `preferred_console` points to the second entry and
> the flag CON_CONSDEV is not set, so bootconsole is not deregistered.
> 
> To fix that, match the console against the `console_cmdline` entry
> pointed by `preferred_console` instead of the first match.
> 
> Signed-off-by: Aleksey Makarov <aleksey.makarov@...aro.org>
> ---
>  kernel/printk/printk.c | 49 ++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 34 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ed2a9b31f214..ad35011f8374 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2380,6 +2380,24 @@ static int __init keep_bootcon_setup(char *str)
>  
>  early_param("keep_bootcon", keep_bootcon_setup);
>  
> +static int match_console(struct console *newcon, struct console_cmdline *c)
> +{
> +	if (!newcon->match ||
> +	    newcon->match(newcon, c->name, c->index, c->options) != 0) {
> +		/* default matching */
> +		BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
> +		if (strcmp(c->name, newcon->name) != 0)
> +			return -ENODEV;
> +		if (newcon->index >= 0 &&
> +		    newcon->index != c->index)
> +			return -ENODEV;
> +		if (newcon->index < 0)
> +			newcon->index = c->index;
> +	}
> +
> +	return 0;
> +}
> +
>  /*
>   * The console driver calls this routine during kernel initialization
>   * to register the console printing procedure with printk() and to
> @@ -2460,17 +2478,9 @@ void register_console(struct console *newcon)
>  	for (i = 0, c = console_cmdline;
>  	     i < MAX_CMDLINECONSOLES && c->name[0];
>  	     i++, c++) {
> -		if (!newcon->match ||
> -		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
> -			/* default matching */
> -			BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
> -			if (strcmp(c->name, newcon->name) != 0)
> -				continue;
> -			if (newcon->index >= 0 &&
> -			    newcon->index != c->index)
> -				continue;
> -			if (newcon->index < 0)
> -				newcon->index = c->index;
> +		if (match_console(newcon, c)) {
> +			continue;
> +		} else {

My bad.  The first version of this patch was closer to the original logic,
despite this patch looks simpler.

The problem is that newcon->setup() is called twice, firstly from
newcon->match(), then explicitly.  And it could be called third time later 
in another call to newcon->match().

The correct sequence is 1) try to match against preferred_console,
2) if that fails check other entries of console_cmdline.  That would require
some work so I will send a correction tomorrow.

All the best
Aleksey Makarov