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

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Date:	Fri, 28 Mar 2008 22:04:22 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: use of volatile in iounmap()?

On Fri, Mar 28, 2008 at 01:51:12PM -0700, H. Peter Anvin wrote:
> 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.
If I understand you correct then it is then not wrong to say
that we have the argument volatile to avoid warnings from gcc
when we pass a volatile pointer.

And then having the pointer marked volatile put a few restrictions
on iounmap().

> 
> 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*.

Yes - but unfortunately the volatile-considered-harmful.txt
does many deal with the data part.

Thanks,
	Sam
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