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

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Message-ID: <739046.56993.qm@web26915.mail.ukl.yahoo.com>
Date:	Mon, 16 Jul 2007 15:31:44 +0200 (CEST)
From:	Etienne Lorrain <etienne_lorrain@...oo.fr>
To:	"H. Peter Anvin" <hpa@...or.com>, Chuck Ebbert <cebbert@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: x86 setup code rewrite in C - revised

--- "H. Peter Anvin" <hpa@...or.com> wrote:
> >>  - The VGA recalc has the same bug as the assembly version where a VGA
> >> write protected register is written (Overflow register) without setting
> >> the enable bit (see VGA docs).
> 
> I dug into this, and it turns out you're incorrect.  Both the assembly
> code and the C code are, in fact, 100% correct:
> 
> The only instance of writing the vertical overflow register is this code
> in vga_set_480_scanlines():
> 
> 	out_idx(0x0c, crtc, 0x11); /* Vertical sync end, unlock CR0-7 */
> 	out_idx(0x0b, crtc, 0x06); /* Vertical total */
> 	out_idx(0x3e, crtc, 0x07); /* Vertical overflow */
> 	out_idx(0xea, crtc, 0x10); /* Vertical sync start */
> 	out_idx(end,  crtc, 0x12); /* Vertical display end */
> 	out_idx(0xe7, crtc, 0x15); /* Vertical blank start */
> 	out_idx(0x04, crtc, 0x16); /* Vertical blank end */
> 
> Register 0x11 has the Protect (not enable!) bit in it, it is bit 7.  As
> you can see, it is cleared (meaning writable) at the beginning of this
> sequence, and the fact that it's being done is even documented.

  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.

  Etienne.

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