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

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Message-ID: <47B9ECE0.70000@keyaccess.nl>
Date:	Mon, 18 Feb 2008 21:38:56 +0100
From:	Rene Herman <rene.herman@...access.nl>
To:	"David P. Reed" <dpreed@...d.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.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 19:58, David P. Reed wrote:

> --- linux-2.6.orig/include/asm-x86/i8253.h
> +++ linux-2.6/include/asm-x86/i8253.h
> @@ -12,7 +12,25 @@ extern struct clock_event_device *global
>  
>  extern void setup_pit_timer(void);
>  
> -#define inb_pit		inb_p
> -#define outb_pit	outb_p
> +/* accesses to PIT registers need careful delays on some platforms. Define
> +   them here in a common place */
> +static inline unsigned char inb_pit(unsigned int port)
> +{
> +	/* delay for some accesses to PIT on motherboard or in chipset must be
> +	   at least one microsecond, but be safe here. */
> +	unsigned char value = inb(port);
> +	udelay(2);
> +	return value;
> +}

With the remark that (at least) the PIT is accessed at a time that 
microseconds and hence udelay are still a total fiction, this looks obvious 
otherwise.

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.

Rene.
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