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

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Message-ID: <47B9FC3A.2010508@zytor.com>
Date:	Mon, 18 Feb 2008 13:44:26 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rene Herman <rene.herman@...access.nl>
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

Rene Herman wrote:
> 
> 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.
> 

That doesn't make any sense at all.  The whole point why the two jumps 
were obsoleted with the P5 (or even late P4, if I'm not mistaken) was 
because they were utterly insufficient when the CPU ran at something 
much higher than the external speed.

> 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...

Uhm... no.  Quite the contrary, I would say.

	-hpa
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