linux-kernel - Re: [PATCH] xen: Enable Xen console by default in domU

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Message-Id: <1207903945.26445.4.camel@muff>
Date:	Fri, 11 Apr 2008 09:52:25 +0100
From:	Mark McLoughlin <markmc@...hat.com>
To:	Markus Armbruster <armbru@...hat.com>
Cc:	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com,
	linux-fbdev-devel@...ts.sourceforge.net, adaplas@....net,
	Jeremy Fitzhardinge <jeremy@...p.org>, mingo@...hat.com
Subject: Re: [PATCH] xen: Enable Xen console by default in domU

On Thu, 2008-04-10 at 17:46 +0200, Markus Armbruster wrote:
...
> Problem: when tty registers, we can't yet know whether the PVFB is
> enabled.  By the time we can know (xenstore is up), the console setup
> game is over.
> 
> Enable the Xen console hvc by default, and make it the preferred
> console.  Change the preferred console to tty when PVFB probes
> successfully, unless we've been given console kernel parameters.
...
> The previous version of this patch had the PVFB change the preferred
> console *always*.  That's wrong, because it blithely overwrites any
> preferred console the user might have set up with console kernel
> parameters.  Unfortunately, I couldn't see how to fix that cleanly
> without messing with printk.c.  If you can think of a better way, let
> me know.
...
> --- a/drivers/video/xen-fbfront.c
> +++ b/drivers/video/xen-fbfront.c

> @@ -358,6 +361,28 @@ static int __devinit xenfb_probe(struct xenbus_device *dev,
>  	return ret;
>  }
>  
> +static __devinit void
> +xenfb_make_preferred_console(void)
> +{
> +	struct console *c;
> +
> +	if (console_set_on_cmdline)
> +		return;
> +
> +	acquire_console_sem();
> +	for (c = console_drivers; c; c = c->next) {
> +		if (!strcmp(c->name, "tty") && c->index == 0)
> +			break;
> +	}

How about adding a new console flag so that e.g. you could do:

        if (!strcmp(c->name, "tty") && c->index == 0 &&
            !(c->flags & CON_SET_ON_CMDLINE))
                break;

> +	release_console_sem();
> +	if (c) {
> +		unregister_console(c);
> +		c->flags |= CON_CONSDEV;
> +		c->flags &= ~CON_PRINTBUFFER; /* don't print again */
> +		register_console(c);
> +	}
> +}
> +
...
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -107,6 +107,8 @@ struct console {
>  	struct	 console *next;
>  };
>  
> +extern int console_set_on_cmdline;
> +
>  extern int add_preferred_console(char *name, int idx, char *options);
>  extern int update_console_cmdline(char *name, int idx, char *name_new, int idx_new, char *options);
>  extern void register_console(struct console *);
> diff --git a/kernel/printk.c b/kernel/printk.c
> index c46a20a..c07bfc1 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -118,6 +118,7 @@ struct console_cmdline
>  static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
>  static int selected_console = -1;
>  static int preferred_console = -1;
> +int console_set_on_cmdline;
>  
>  /* Flag: console code may call schedule() */
>  static int console_may_schedule;
> @@ -829,6 +830,7 @@ static int __init console_setup(char *str)
>  	*s = 0;
>  
>  	add_preferred_console(buf, idx, options);
> +	console_set_on_cmdline = 1;
>  	return 1;
>  }
>  __setup("console=", console_setup);

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