linux-kernel - Re: x86 setup code rewrite in C

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Message-ID: <469BAC68.50409@zytor.com>
Date:	Mon, 16 Jul 2007 10:35:36 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Etienne Lorrain <etienne_lorrain@...oo.fr>
CC:	Chuck Ebbert <cebbert@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: x86 setup code rewrite in C - revised

Etienne Lorrain wrote:
> 
>   The only time I ever needed this "end line recalculation" was when the heigh in
>  graphic lines was not a multiple of the character heigh - i.e. 640x350 with 8x16
>  or 8x8 chars - some VGA adapters do not hide the bottom graphic lines.
>   The function vga_set_480_scanlines() is not called, and the protect bit is never
>  cleared - the video BIOS leaving those low index register protected.
>   The function vga_recalc_vertical() (or its assembler equivalent) is probably
>  perfectly called but because the protect bit is never cleared, the few graphic
>  line are displayed during the whole Linux text session...
>   I have myself never seen any other problems when the graphic heigh is a multiple
>  of the character heigh - tested on ~40 video boards.
> 

OK, I see what you mean.  This would be a problem if:

- the VGA BIOS leaves the protected bit set
- the user enables vertical recalculation
- the size crosses a multiple of 256

It is a bug (ported from the assembly) and fortunately quite easy to
fix.  I don't know why I missed this when I looked before.

	-hpa
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