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

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Message-ID: <4697EC66.2030404@zytor.com>
Date:	Fri, 13 Jul 2007 14:19:34 -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,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: RE : Re: x86 setup code rewrite in C - revised

Etienne Lorrain wrote:
> --- "H. Peter Anvin" <hpa@...or.com> wrote:
>> Chuck Ebbert wrote:
> 
>   Wrong name.
> 
>>>>  Have fun, this code:
>>>>  - do not open the fast A20 gate before checking if the slow A20 gate is open or
>> closed.
>>
>> As does the current code; this is highly intentional behaviour since
>> there are machines (in particular a whole series of machines made by
>> Olivetti) which lock up if you do it differently.
> 
>  There was some discussion on this list about some machine which would not wake-up
> correctly if slow A20 was not closed, long time ago. I did not really follow
> the code after that discussion.
>  I wonder if you should do a "outb(0xFF, 0x64);" after "outb(0xdf, 0x60);" like
> the HIMEM.SYS driver, to force an immediate update of the I/O ports - I think
> I also read that in this initial chip docs, long time ago also.

Well, the code we have now has worked for quite some years as-is.  This
code is not an algorithmic change.  I haven't seen any machines which
need outb(0xff, 0x64) -- I wonder if that has the same effect as our
io_delay() or if it is actually potent.  I shall look into it.

>>>>  - Does not save and restore %ds when printing a char on the screen (%ds is
>>>>  destroyed only when the content of the screen scroll - only for some video cards)
>> %ds?  Aren't you confusing it with the old bug which would destroy %bp?
>>  If you have any references to %ds being destroyed I would be very
>> surprised.  I can guarantee that very little if any assembly code I've
>> ever seen that deals with INT 10h -- and I've seen a lot of it -- guards
>> against %ds being randomly trashed.
>>
>> However, the trashing of %bp is a well-known bug (although only for
>> machines older than the ones that can run Linux) -- the Interrupt List has:
>>
>> BUGS:   some implementations (including the original IBM PC) have a bug
>>  	which destroys BP
> 
>  That is on Trident cards, old card but may still be used, and BIOS may have
> been copied to other cards.
>  Detected and documented on Gujin (boot.c and vgabios.h scroll)

Are you talking about BP or DS?  As I said, the BP is well-known, and
the code accounts for it in the form of the INT10 macro.

>>>>  and probably few other problems - just seen those by reading the patches (the
>>>>  asm("") are inlined in the C code - I find it more difficult to check).
>>>>
>>>>  Also, I do not know if "m" is right in here:
>>>> static inline u8 rdfs8(addr_t addr)
>>>> {
>>>> 	u8 v;
>>>> 	asm("movb %%fs:%1,%0" : "=r" (v) : "m" (*(u8 *)addr));
>>>> 	return v;
>>>> }
>> The "m" is correct right there.
> 
>  strange, "g" would mean anything can go there - and this assembly instruction
> should accept every access modes.

Not with an %fs: prefix.  It would also allow the compiler to do a move
into a register "on its own", which would be disastrous, since it would
lack the prefix.  So "m" is correct.

	-hpa
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