linux-kernel - Re: [patches] [PATCH] [21/22] x86_64: Extend bzImage protocol for relocatable bzImage

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Message-ID: <m1tzumu10k.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 08 May 2007 21:30:35 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	yhlu <yhlu.kernel@...il.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, vgoyal@...ibm.com,
	Gerd Hoffmann <kraxel@...hat.com>,
	Jeff Garzik <jeff@...zik.org>, patches@...-64.org,
	linux-kernel@...r.kernel.org,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patches] [PATCH] [21/22] x86_64: Extend bzImage protocol for relocatable bzImage

"H. Peter Anvin" <hpa@...or.com> writes:

> Eric W. Biederman wrote:
>> 
>> I expect I can find a few more examples where we specify
>> video_cols and video_lines but we use video_mode == 0.
>> 
>> Going farther mode 0x00 is a BIOS 40x25 mode.  So the patch below is
>> not always safe even if we boot the bzImage.  It is just highly
>> unlikely anyone would start the kernel in 40x25 text mode. 
>> 
>
> Mode 0x00 is, at least theoretically, BIOS 40x25 *grayscale*; this mode
> (and mode 0x02 which is the same thing in 80x25) were as far as I know
> only ever used with composite monitors off CGA cards, i.e. functionally
> never.  Actual monochrome monitors used mode 0x07.

I agree.  We are not at all likely to see it in practice.  Even
if my memory is correct and vga cards and non-monochrome cga
cards supported that mode.

That doesn't mean checking for 0x00 is sufficient to detect
an initialized struct screen_info, or a lack of a video screen.

We have in kernel historical precedent for using 0x00 as just meaning
a text mode.  I'm fairly certain that I looked I more closely I could
find this convention of using 0x00 to mean a text mode on ia64, mips,
and ppc, in addition to the instances I found on alpha, arm,

Since the whole point is to detect the case where we don't have
a screen at all it makes sense to check several additional variables
and make certain that they are all 0.  Agreed?

Eric
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