linux-kernel - Re: [PATCH v3] x86: Do not write into VGA memory space if there is no VGA device in the system

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Message-ID: <j2l84144f021004260735ib574b5f1t6c00a3a8a8f886ee@mail.gmail.com>
Date:	Mon, 26 Apr 2010 17:35:16 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Guenter Roeck <guenter.roeck@...csson.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3] x86: Do not write into VGA memory space if there is no 
	VGA device in the system

On Mon, Apr 26, 2010 at 5:05 PM, Guenter Roeck
<guenter.roeck@...csson.com> wrote:
> On Mon, Apr 26, 2010 at 05:36:05AM -0400, Pekka Enberg wrote:
>> Guenter Roeck kirjoitti:
>> > Ensure that early_printk() does not write into VGA memory space
>> > if there is not VGA device in the system.
>> >
>> > Signed-off-by: Guenter Roeck <guenter.roeck@...csson.com>
>> > ---
>> > v3:
>> > Changes are now limited to early_printk.c. Functionality is retained, meaning
>> > there will still be output on the VGA console before setup_early_printk()
>> > is called, but only if a VGA device is known to exist in the system.
>> >
>> >  arch/x86/kernel/early_printk.c |   17 +++++++++++++----
>> >  1 files changed, 13 insertions(+), 4 deletions(-)
>> >
>> > diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
>> > index b9c830c..783cb25 100644
>> > --- a/arch/x86/kernel/early_printk.c
>> > +++ b/arch/x86/kernel/early_printk.c
>> > @@ -170,10 +170,13 @@ asmlinkage void early_printk(const char *fmt, ...)
>> >     int n;
>> >     va_list ap;
>> >
>> > -   va_start(ap, fmt);
>> > -   n = vscnprintf(buf, sizeof(buf), fmt, ap);
>> > -   early_console->write(early_console, buf, n);
>> > -   va_end(ap);
>> > +   if (early_console && (early_console_initialized ||
>> > +                         boot_params.screen_info.orig_video_isVGA == 1)) {
>> > +           va_start(ap, fmt);
>> > +           n = vscnprintf(buf, sizeof(buf), fmt, ap);
>> > +           early_console->write(early_console, buf, n);
>> > +           va_end(ap);
>> > +   }
>> >  }
>> >
>> >  static inline void early_console_register(struct console *con, int keep_early)
>> > @@ -233,6 +236,12 @@ static int __init setup_early_printk(char *buf)
>> >  #endif
>> >             buf++;
>> >     }
>> > +
>> > +   /* If there is no VGA device, don't try to use it as early console. */
>> > +   if (early_console == &early_vga_console &&
>> > +       boot_params.screen_info.orig_video_isVGA != 1)
>> > +           early_console = NULL;
>>
>> Wouldn't it be better to switch to, say, serial console here to avoid
>> the NULL check in early_printk()?
>>
> This would be another possibility.
>
> However, since the serial console was not explicitly selected, it would possibly
> not be initialized. My understanding is that Peter had a problem with that.
>
> One can argue that it would still be better to select the serial console in that case,
> even though it might not be initialized. Actually, most likely it is (if it exists),
> since the system must presumably have some means to communicate with the world.
>
> I am pretty much open to either option, if that results in the patch being accepted.
> Just let me know which way to go.

Peter?
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