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

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Message-ID: <475E9B3A.7050403@keyaccess.nl>
Date:	Tue, 11 Dec 2007 15:14:18 +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 14:50, David Newall wrote:

> Rene Herman wrote:

>> 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.
> 
> That could be true if outb_p were used only in architecture dependent

It not only could be, it _is_ true. Not using an output to port 0x80 is what 
this discussion is about.

> code, but it's not.  It's used in drivers that are supposed to run on 
> all sorts of platforms.  Why does a megaraid controller need delays on 
> i386 but not on Sparc, PowerPC, Alpha and others?  Is it buggy on most 
> platforms, or just unnecessarily slow on Intel?

The latter probably and I don't bleedin' well care. In a discussion about 
removing the out to 0x80 the only thing that is relevant is what it should 
be replaced with. Usually, "nothing" will do but generally, udelay(1) will.

>>> is needed, wouldn't you use a real delay; one that says how long it
>>> should be?
>> Thinking that _p gives a pause is perhaps too PC-centric. Why, if a delay
>>
>> Because any possible outb_p delay should be synced to the bus-clock, 
>> not to any wall-clock.
> 
> You misunderstand.  A delay can be counted in bus cycles.

No damnit, you misunderstand. I'm saying that an outb_p _should_ be defined 
in terms of the bus clock since if you want a wall-clock delay you should be 
using just that.

The _hardware_ is synced to the bus clock and therefore, having a delay 
available that is synced to the bus clock as well makes some sense. And 
again again again again not withstanding that, a udelay will still be an 
okay replacement in practice.

Rene.
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