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 15:39:04 -0700
David Brownell <david-b-yBeKhBN/0LDR7s880joybQ@public.gmane.org> wrote:

> On Tuesday 29 May 2007, Andrew Morton wrote:
> > On Fri, 25 May 2007 10:35:07 -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.

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.


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