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Subject: Re: [PATCH] IBM Lanstreamer bugfixes
To: yoder1@us.ibm.com (Kent E Yoder)
Date: Fri, 1 Feb 2002 23:38:38 +0000 (GMT)
Cc: alan@lxorguk.ukuu.org.uk (Alan Cox),
        jgarzik@mandrakesoft.com (Jeff Garzik), linux-kernel@vger.kernel.org
In-Reply-To: <OFB91B46B3.DE7448D0-ON85256B53.0071158D@raleigh.ibm.com> from "Kent E Yoder" at Feb 01, 2002 02:53:43 PM
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From: Alan Cox <alan@lxorguk.ukuu.org.uk>
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> what purpose does the volatile below serve? 
> 
> io.h:122:#define writew(b,addr) (*(volatile unsigned short *) 
> __io_virt(addr) = (b))X
> 
> Is this a sort of "go do this now" command to flush it from the CPU to the 
> PCI bus, while the readw() makes sure its flushed out of the PCI cache? 

The compiler isnt obliged to actually generate the assignment in order
otherwise, so given

	writew(1, foo+2);
	writew(2, foo);

it might have the urge to reverse them - perhaps to optimise in using
postincrement modes

volatile says "stuff happens here beyond the compilers direct knowledge of
events".

Simple example

	i=0;
	while(i<100000) i++;

can be optimised to i=100000 if i is not volatile

Alan
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