Re: [patch 2.6.22-rc2-git] spidev compiler warning gone

[Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>]

On Tue, 29 May 2007 19:05:34 -0700 David Brownell <david-b-yBeKhBN/0LDR7s880joybQ@public.gmane.org> wrote:

> 
> > > > > Get rid of annoying GCC warning on 32-bit platforms.  The trick is to
> > > > > add an extra cast using "ptrdiff_t" to convert the u64 to the correct
> > > > > size integer, and only then casting it into a "void *" pointer.
> > > > > 
> > > > >	...
> > > > > -			if (__copy_to_user((u8 __user *)u_tmp->rx_buf, buf,
> > > > > +			if (__copy_to_user((u8 __user *)
> > > > > +					(ptrdiff_t) u_tmp->rx_buf, buf,
> > > > > 	...
> > > > 
> > > > This seems overly tricky.  The way we normally squish that warning is
> > > > via a (long) cast.
> > > 
> > > Well, "ptrdiff_t" seems equivalent to "long" in common cases.
> > > 
> > > Presumably C99 guarantees pointer-to-long conversions work in
> > > both directions?  It seemed there must necessarily be such a
> > > guarantee for "ptrdiff_t", even if it weren't true for "long".
> > > I didn't have a copy of a C spec to consult.  ;)
> > > 
> > 
> > Thing is, the kernel does a cast of an integer-type to a pointer-type in a
> > *lot* of places.  And the standard way in which we squish the warning is
> > via a (long) cast.
> 
> Good to know, but I didn't notice any examples anywhere in the source
> code of doing that.  I used the first solution I found, when I went
> hunting for a fix.  It wasn't "long".  And it appears that "long" has
> fewer portability guarantees...

Kernel assumes that sizeof(long)==sizeof(foo *) in a tremendous number of 
places.  That's why we safely use (long) for this.

box:/usr/src/linux-2.6.22-rc3> fgrep -r '*)(long)' . | wc -l
95
box:/usr/src/linux-2.6.22-rc3> fgrep -r '(int)(long)' . | wc -l 
45
box:/usr/src/linux-2.6.22-rc3> fgrep -r '(ptrdiff_t)' . | wc -l         
7

> Something in Documentation/* should talk about 64-bit cleanliness,
> and maybe mention such issues.

spose so.
 
> 
> > So it's a standard pattern and people understand it.  Whereas the ptrdiff_t
> > thing will make readers go "wtf?".  Even if it deos happen to work.
> 
> At this point, "long" would surprise *me*!  I notice you didn't
> answer my question about C specs and "long".

oh.  I wasn't very interested, sorry ;)  I'm just pointing out that
(long) will cause least-surprise to most-people.

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