linux-kernel - Re: [rft] s2ram wakeup moves to .c, could fix few machines

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Message-ID: <20080208210905.GB12923@elf.ucw.cz>
Date:	Fri, 8 Feb 2008 22:09:05 +0100
From:	Pavel Machek <pavel@....cz>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	kernel list <linux-kernel@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.osdl.org>
Subject: Re: [rft] s2ram wakeup moves to .c, could fix few machines

On Fri 2008-02-08 13:02:57, H. Peter Anvin wrote:
> Pavel Machek wrote:
>> On Fri 2008-02-08 17:23:15, Rafael J. Wysocki wrote:
>>> On Friday, 8 of February 2008, Pavel Machek wrote:
>>>> Hi!
>>> Hi,
>>>
>>>>>> I really need the entry point to be at offset 0, so that I can get
>>>>>> pointers to my data. I could not figure out how to do it any other
>>>>>> way. And if 0 is taken, I thought I'd put header at the end.
>>>>>>
>>>>> Why not just put the structure at 0, and put pointers in the structure 
>>>>> to everything else you need?
>>>> segments:offsets rear its ugly head here. I need %ds to point to my
>>>> data, and the way to do it is copy it from %cs; that needs start to be
>>>> at 0.
>>> Hm, why exactly is that necessay?
>>
>> It is not _neccessary_. Try to come up with another method that gets
>> relocations right. I could not :-(.
>>
>> (Actually, putting table at the offset 0 and short jump at beggining
>> of the table would probably do the trick. But that still keeps code at
>> offset 0 :-).
>
> Why not just put a pointer to the start of the code in the table, and make 
> an indirect call to it?

Indirect call from where?

BIOS jumps to address you provide.

> Where is this code?

arch/x86/kernel/acpi/realmode/wakeup.S

BIOS jumps to wakeup_code. With CS=something, IP=0. If wakeup code is
at other address than zero, I'll not be able to easily address data
below it.
								Pavel

		.globl	wakeup_header
wakeup_header:
...
realmode_flags:	.long	0
signature:	.long	0x51ee1111
...
	.text
	.globl	_start
	.code16
wakeup_code:
_start:
	cli
	cld

	/* Set up segments */
	movw	%cs,%ax
	movw	%ax,%ds
	movw	%ax,%es
	movw	%ax,%ss

...
	/* Check header signature... */
	movl	signature, %eax
	cmpl	$0x51ee1111, %eax
	jne	bogus_real_magic



									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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