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: <475E7DC2.4060509@davidnewall.com>
Date:	Tue, 11 Dec 2007 22:38:34 +1030
From:	David Newall <david@...idnewall.com>
To:	Rene Herman <rene.herman@...access.nl>
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

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:

    "Some devices require that accesses to their ports are slowed down. 
    This functionality is provided by appending a _p to the end of the
    function."
    -- Documentation/DocBook/deviceiobook.tmpl


(I've not seen any other formal definition.)

Most architectures (Alpha, Arm, Arm2, Blackfin, FRV, h8300, IA64, 
PA-RISC, PowerPC, Sparc, Sparc64, V850 and Xtensa) do no pause.  M68k 
does no pause except in one configuration, when it's the same as i386.  
On m32r it's a push and a pop.  On SuperH it's similar to i386, only 
using 16-bit input.  X86-64 is the same as i386!

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?
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