linux-kernel - Re: [PATCH][RESEND] x86: Do not write to VGA memory space if CONFIG_VGA

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Date:	Mon, 05 Apr 2010 13:25:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	guenter.roeck@...csson.com
CC:	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	mingo@...hat.com, x86@...nel.org
Subject: Re: [PATCH][RESEND] x86: Do not write to VGA memory space if CONFIG_VGA_CONSOLE
 is undefined

On 04/05/2010 01:02 PM, Guenter Roeck wrote:
>>
>> You didn't answer my question (c).
>>
>> I want to know how you ended up with
>> boot_params.screen_info.orig_video_isVGA == 1 on a system with no VGA,
>> which seems like it would have resolved this.
>>
>> I am *not* inclined to add a compile-time test for what should have been
>> handed with a runtime test already.
>>
> Sorry, I thought I did answer it.
>

You didn't.  You still haven't!

> The problem is that early_printk() can be called prior to the call to
> setup_early_printk(). Since early_console is currently pre-initialized
> with early_vga_console, output can be written to VGA memory space even
> if there is no VGA controller in the system (and even if
> boot_params.screen_info.orig_video_isVGA == 0). This happens for all
> early_printk() calls executed prior to the call to setup_early_printk().

If boot_params.screen_info.orig_video_isVGA == 0, at least this bit of
your patch has no effect:

> > -		if (!strncmp(buf, "vga", 3) &&
> > +		if (have_vga_console && !strncmp(buf, "vga", 3) &&
> >  		    boot_params.screen_info.orig_video_isVGA == 1) {

Now, we have at least two ways to report a non-VGA console at runtime:

boot_params.screen_info.orig_video_isVGA != 1
boot_params.screen_info.orig_video_lines == 0

The former is zero for CGA/MDA/EGA, but early_vga_write() doesn't work
right for MDA at least, so keying on isVGA is probably right.

early_printk() being called before setup_early_printk() is a problem,
and it's not immediately obvious to me how to fix it.  We can of course
make early_vga_write() simply return if boot_params.screen_info.isVGA ==
0, of course, but it really is a bigger problem than that in many ways.

	-hpa
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