linux-kernel - Re: use of volatile in iounmap()?

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Message-ID: <47ED5A40.9020307@zytor.com>
Date:	Fri, 28 Mar 2008 13:51:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sam Ravnborg <sam@...nborg.org>
CC:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: use of volatile in iounmap()?

Sam Ravnborg wrote:
> While reviewing some CAN driver I stumbled on iounmap
> which has following prototype on x86:
> 
> extern void iounmap(volatile void __iomem *addr);
> 
> I argued that the driver should not use volatile
> but then I cannot explain why the argument to
> iounmap takes a volatile.
> 
> The same goes for many other functions in
> the io*.h headers.
> 
> Grepping the other archs they mostly follow
> same pattern.
> 
> Can anyone explain the rational for volatile in this case.
> 

Yes.  The use of volatile in a function prototype like this means that 
it is valid to pass a volatile pointer to that function -- in other 
words, we're telling gcc that we're not going to do anything with the 
pointer that is invalid for a volatile pointer.

A lot of the "volatile considered harmful" stuff that has been bandied 
about is explicitly about marking *data* items volatile (it does have 
its uses, but it's easy to get wrong); Linus has explicitly made the 
distinction between volatile *data* and volatile *operations*.

	-hpa
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