linux-kernel - Re: [PATCH] x86: use explicit timing delay for pit accesses in kernel and pcspkr driver

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Message-ID: <47B9F2EC.4070308@keyaccess.nl>
Date:	Mon, 18 Feb 2008 22:04:44 +0100
From:	Rene Herman <rene.herman@...access.nl>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"David P. Reed" <dpreed@...d.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Dmitry Torokhov <dtor_core@...ritech.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: use explicit timing delay for pit accesses in kernel
 and pcspkr driver

On 18-02-08 21:43, H. Peter Anvin wrote:

> Rene Herman wrote:
>>
>> Now with respect to the original pre port 80 "jmp $+2" I/O delay 
>> (which the Pentium obsoleted) I suppose it'll probably be okay even 
>> without fixing that specifically but do note such -- it's a vital part 
>> of the problem.
>>
> 
> Sorry, that paragraph didn't parse for me.

I mean that before the linux kernel used a port 0x80 write as an I/O delay 
it used a short jump (two in a row actually...) as such and this was at the 
time that it actually ran on the old legacy stuff that is of most concern here.

No, if I'm not mistaken, those two jumps are actually what the udelay() is 
going to do anyway as part of delay_loop() at that early stage so that even 
before loops_per_jiffy calibration, I believe we should still be okay.

Yes, it's a bit of a "well, hrrm" thing, but, well... loops_per_jiffy can be 
initialised a bit more conservatively then today as well (and as discussed 
earlier, possibly per CPU family) but I believe it's actually sort of fine 
not too worry much about it...

Rene.
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