linux-kernel - Re: [PATCH v9 3/3] printk: fix double printing with earlycon

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Date:   Mon, 10 Apr 2017 16:22:14 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Aleksey Makarov <aleksey.makarov@...aro.org>
Cc:     linux-serial@...r.kernel.org, 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>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH v9 3/3] printk: fix double printing with earlycon

On Wed 2017-04-05 23:20:00, 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, introduce an invariant "The last non-braille console
> is always the preferred one" on the entries of the console_cmdline
> array.  Then traverse it in reverse order to be sure that if
> the console is preferred then it will be the first matching entry.
> Introduce variable console_cmdline_cnt that keeps the number
> of elements of the console_cmdline array (Petr Mladek).  It helps
> to get rid of the loop that searches for the end of this array.
> 
> Reported-by: Sudeep Holla <sudeep.holla@....com>
> Signed-off-by: Aleksey Makarov <aleksey.makarov@...aro.org>

This version looks fine to me. Just a small nitpick below.
Anyway:

Reviewed-by: Petr Mladek <pmladek@...e.com>

> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index fd752f0c8ef1..be657af45758 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -269,6 +269,7 @@ static struct console *exclusive_console;
>  #define MAX_CMDLINECONSOLES 8
>  
>  static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
> +static int console_cmdline_cnt;
>  
>  static int preferred_console = -1;
>  int console_set_on_cmdline;
> @@ -1905,12 +1906,26 @@ static int __add_preferred_console(char *name, int idx, char *options,
>  	 *	See if this tty is not yet registered, and
>  	 *	if we have a slot free.
>  	 */
> -	for (i = 0, c = console_cmdline;
> -	     i < MAX_CMDLINECONSOLES && c->name[0];
> -	     i++, c++) {
> +	for (i = 0, c = console_cmdline; i < console_cmdline_cnt; i++, c++) {
>  		if (strcmp(c->name, name) == 0 && c->index == idx) {
> -			if (!brl_options)
> -				preferred_console = i;
> +

This extra new line is non-standard and looks slightly weird to me.
I just point it out. I personally do not mind ;-)


> +			if (brl_options)
> +				return 0;
> +
> +			/*
> +			 * Maintain an invariant that will help to find if
> +			 * the matching console is preferred, see
> +			 * register_console():
> +			 *
> +			 * The last non-braille console is always
> +			 * the preferred one.
> +			 */
> +			if (i != console_cmdline_cnt - 1)
> +				swap(console_cmdline[i],
> +				     console_cmdline[console_cmdline_cnt - 1]);
> +
> +			preferred_console = console_cmdline_cnt - 1;
> +
>  			return 0;
>  		}
>  	}
> @@ -2457,12 +2473,24 @@ void register_console(struct console *newcon)
>  	}
>  
>  	/*
> -	 *	See if this console matches one we selected on
> -	 *	the command line.
> +	 * See if this console matches one we selected on the command line.
> +	 *
> +	 * There may be several entries in the console_cmdline array matching
> +	 * with the same console, one with newcon->match(), another by
> +	 * name/index:
> +	 *
> +	 *	pl011,mmio,0x87e024000000,115200 -- added from SPCR
> +	 *	ttyAMA0 -- added from command line
> +	 *
> +	 * Traverse the console_cmdline array in reverse order to be
> +	 * sure that if this console is preferred then it will be the first
> +	 * matching entry.  We use the invariant that is maintained in
> +	 * __add_preferred_console().
>  	 */
> -	for (i = 0, c = console_cmdline;
> -	     i < MAX_CMDLINECONSOLES && c->name[0];
> -	     i++, c++) {
> +	for (i = console_cmdline_cnt - 1; i >= 0; i--) {
> +

Same here.

> +		c = console_cmdline + i;
> +
>  		if (!newcon->match ||
>  		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
>  			/* default matching */

Best Regards,
Petr