Re: [PATCH v2] console: use first console if stdout-path device doesn't appear

[Paul Burton <paul.burton@imgtec.com>]

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On Monday, 31 October 2016 12:14:55 GMT Paul Burton wrote:
> If a device tree specified a preferred device for kernel console output
> via the stdout-path or linux,stdout-path chosen node properties there's
> no guarantee that it will have specified a device for which we have a
> driver. It may also be the case that we do have a driver but it doesn't
> call of_console_check() to register as a preferred console (eg. offb
> driver as used on powermac systems). In these cases try to ensure that
> we provide some console output by enabling the first usable registered
> console, which we keep track of with the of_fallback_console variable.
> 
> Tested in QEMU with a PowerPC pseries_defconfig kernel.

Actually whilst this fixes the output in QEMU it has other problems. I'm still 
digging...

Thanks,
    Paul

> 
> Signed-off-by: Paul Burton <paul.burton@imgtec.com>
> Fixes: 05fd007e4629 ("console: don't prefer first registered if DT specifies
> stdout-path") Reported-by: Andreas Schwab <schwab@linux-m68k.org>
> Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
> Reported-by: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Larry Finger <Larry.Finger@lwfinger.net>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> 
> ---
> 
> Changes in v2:
> - Split enable_console() out of register_console() & call in the fallback
> case. - Track the console we would have enabled as of_fallback_console.
> 
>  kernel/printk/printk.c | 60
> +++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 55
> insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index de08fc9..b02c00a 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -264,6 +264,13 @@ void console_set_by_of(void)
>  # define of_specified_console false
>  #endif
> 
> +/*
> + * The first usable console, which we'll fall back to using if the system
> + * uses a device tree which indicates a stdout-path device for which we
> + * have no driver, or for which our driver doesn't call of_console_check().
> + */
> +static struct console *of_fallback_console;
> +
>  /* Flag: console code may call schedule() */
>  static int console_may_schedule;
> 
> @@ -2598,6 +2605,8 @@ static int __init keep_bootcon_setup(char *str)
> 
>  early_param("keep_bootcon", keep_bootcon_setup);
> 
> +static void enable_console(struct console *newcon);
> +
>  /*
>   * The console driver calls this routine during kernel initialization
>   * to register the console printing procedure with printk() and to
> @@ -2620,7 +2629,6 @@ early_param("keep_bootcon", keep_bootcon_setup);
>  void register_console(struct console *newcon)
>  {
>  	int i;
> -	unsigned long flags;
>  	struct console *bcon = NULL;
>  	struct console_cmdline *c;
> 
> @@ -2657,7 +2665,9 @@ void register_console(struct console *newcon)
>  	 *	didn't select a console we take the first one
>  	 *	that registers here.
>  	 */
> -	if (preferred_console < 0 && !of_specified_console) {
> +	if (preferred_console < 0 && of_specified_console) {
> +		of_fallback_console = of_fallback_console ?: newcon;
> +	} else if (preferred_console < 0) {
>  		if (newcon->index < 0)
>  			newcon->index = 0;
>  		if (newcon->setup == NULL ||
> @@ -2705,8 +2715,18 @@ void register_console(struct console *newcon)
>  		break;
>  	}
> 
> -	if (!(newcon->flags & CON_ENABLED))
> -		return;
> +	if (newcon->flags & CON_ENABLED)
> +		enable_console(newcon);
> +}
> +EXPORT_SYMBOL(register_console);
> +
> +static void enable_console(struct console *newcon)
> +{
> +	struct console *bcon = NULL;
> +	unsigned long flags;
> +
> +	if (console_drivers && console_drivers->flags & CON_BOOT)
> +		bcon = console_drivers;
> 
>  	/*
>  	 * If we have a bootconsole, and are switching to a real console,
> @@ -2777,7 +2797,6 @@ void register_console(struct console *newcon)
>  				unregister_console(bcon);
>  	}
>  }
> -EXPORT_SYMBOL(register_console);
> 
>  int unregister_console(struct console *console)
>  {
> @@ -2839,10 +2858,41 @@ EXPORT_SYMBOL(unregister_console);
>   * intersects with the init section. Note that code exists elsewhere to get
> * rid of the boot console as soon as the proper console shows up, so there
> * won't be side-effects from postponing the removal.
> + *
> + * Additionally we may be using a device tree which specifies valid
> + * stdout-path referencing a device for which we don't have a driver, or
> for + * which we have a driver that doesn't register itself as preferred
> console + * using of_console_check(). In these cases we attempt here to
> enable the + * first usable registered console, which we assigned to
> of_fallback_console. */
>  static int __init printk_late_init(void)
>  {
>  	struct console *con;
> +	bool any_enabled = false;
> +
> +	for_each_console(con) {
> +		if (!(con->flags & CON_ENABLED))
> +			continue;
> +
> +		any_enabled = true;
> +		break;
> +	}
> +
> +	if (!any_enabled && of_fallback_console) {
> +		if (of_fallback_console->index < 0)
> +			of_fallback_console->index = 0;
> +
> +		if (!of_fallback_console->setup ||
> +		    !of_fallback_console->setup(of_fallback_console, NULL)) {
> +			of_fallback_console->flags |= CON_ENABLED;
> +			if (of_fallback_console->device) {
> +				of_fallback_console->flags |= CON_CONSDEV;
> +				preferred_console = 0;
> +			}
> +		}
> +
> +		enable_console(of_fallback_console);
> +	}
> 
>  	for_each_console(con) {
>  		if (!keep_bootcon && con->flags & CON_BOOT) {


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