linux-kernel - Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

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Date:	Tue, 11 Dec 2007 14:16:01 +0100
From:	Rene Herman <rene.herman@...access.nl>
To:	David Newall <david@...idnewall.com>
CC:	Paul Rolland <rol@...917.net>, "H. Peter Anvin" <hpa@...or.com>,
	Krzysztof Halasa <khc@...waw.pl>, Pavel Machek <pavel@....cz>,
	Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"David P. Reed" <dpreed@...d.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, rol@...be.net
Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64
 with MCP51 laptops

On 11-12-07 13:08, David Newall wrote:

> Rene Herman wrote:
>> On 11-12-07 08:40, Paul Rolland wrote:
>>
>>> Well, if the delay is so much unspecified, what about _reading_ port 
>>> 0x80 ?
>>> Will the delay be shorter ?
>>
>> The delay is completely and fully specified in terms of the ISA/LPC clock
> 
> That would be the delay on the i386 (sic) architecture.  In general, 
> though, the delay is:

This particular discussion isn't about anything in general but solely about 
the delay an outb_p gives you on x86 since what is under discussion is not 
using an output to port 0x80 on that platform to generate it.

> Thinking that _p gives a pause is perhaps too PC-centric. Why, if a delay
> is needed, wouldn't you use a real delay; one that says how long it
> should be?

Because any possible outb_p delay should be synced to the bus-clock, not to 
any wall-clock. Drivers that want to sync to wall-clock need to use an outb, 
delay pair as you'd expect.

In the real world, driver authors aren't perfect and will have used outb_p 
as a wall-clock delay which they have gotten away with since it's a nicely 
specified delay in terms of the ISA/LPC clock and the ISA/LPC clock being 
fairly (old) to very (new) constant.

The delay it gives is very close to 1 us on a spec ISA/LPC bus (*) and as 
such, even though it may not be the right thing to do from an theoretical 
standpoint, generally a udelay(1) is going to be a fine replacement from a 
practical one -- as soon as we _can_ use udelay(), as I also wrote.

Rene.

(*) some local testing shows it to be almost exactly that for both out and 
in on my own PC -- a little over. If anyone cares, see attached little test 
program. The "little over" I don't worry about. 0 us delay is also fine for 
me and if any code was _that_ fragile it would have broken long ago.

View attachment "port80.c" of type "text/plain" (635 bytes)