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

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Message-ID: <47BA0A3D.1060708@zytor.com>
Date:	Mon, 18 Feb 2008 14:44:13 -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:
> On 18-02-08 23:07, Rene Herman wrote:
> 
>> On 18-02-08 23:01, H. Peter Anvin wrote:
>>
>>> Rene Herman wrote:
>>>>
>>>> Yes, but generally not any P5+ system is going to need the PIT delay 
>>>> in the first place meaning it just doesn't matter. There were the 
>>>> VIA issues with the PIC but unless I missed it not with the PIT.
>>>>
>>>
>>> Uhm, I'm not sure I believe that's safe.
>>>
>>> The PIT is particularly pissy in this case -- the semantics of the 
>>> PIT are ill-defined if there hasn't been a PIT clock between two 
>>> adjacent accesses, so I fully expect that there are chipsets out 
>>> there which will do very bad things in this case.
>>
>> Okay. Now that they're isolated, do you have a suggestion for 
>> {in,out}b_pit? You say a PIT clock, so do you think we can bounce of 
>> the PIT iself in this case after all?
> 
> Am I correct that channel 1 is never used? A simple read from 0x41?
> 

Channel 1 is available for the system.  In modern systems, it's pretty 
much available for the OS, although that's never formally stated (in the 
original PC, it was used for DRAM refresh.)

However, I could very easily see a chipset have issues with that kind of 
stuff.

	-hpa
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