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Subject: Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be,
 le}_to_cpu() and cpu_to_{be, le}() macros
From: Joe Perches <joe@perches.com>
To: Cody P Schafer <dev@codyps.com>
Date: Wed, 28 May 2014 16:00:25 -0700
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Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>,
 David Laight <David.Laight@aculab.com>,
 Linux PPC <linuxppc-dev@lists.ozlabs.org>, LKML <linux-kernel@vger.kernel.org>
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On Wed, 2014-05-28 at 17:11 -0500, Cody P Schafer wrote:
> On Wed, May 28, 2014 at 5:05 PM, Cody P Schafer <dev@codyps.com> wrote:
> > On Wed, May 28, 2014 at 3:45 AM, David Laight <David.Laight@aculab.com> wrote:
> >> From: Cody P Schafer
> >>> Rather manually specifying the size of the integer to be converted, key
> >>> off of the type size. Reduces duplicate size info and the occurance of
> >>> certain types of bugs (using the wrong sized conversion).
> >> ...
> >>> +#define be_to_cpu(v) \
> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint8_t) , v, \
> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint16_t), be16_to_cpu(v), \
> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint32_t), be32_to_cpu(v), \
> >>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint64_t), be64_to_cpu(v), \
> >>> +             (void)0))))
> >> ...
> >>
> >> I'm not at all sure that using the 'size' of the constant will reduce
> >> the number of bugs - it just introduces a whole new category of bugs.
> >
> > Certainly, if you mis-size the argument (and thus have missized one of
> > the variables containing the be value, probably a bug anyhow), there
> > will be problems.
> >
> > I put this interface together because of an actual bug I wrote into
> > the initial code of the hv_24x7 driver (resized a struct member
> > without adjusting the be*_to_cpu() sizing).
> > Having this "auto sizing" macro means I can avoid encoding the size of
> > a struct field in multiple places.
> 
> To clarify, the point I'm making here is that this simply cuts out 1
> more place we can screw up endianness conversion sizing.

It does screw up other types when you do things like:

	u8 foo = some_function();

	cpu_to_be(foo + 1);

the return value is sizeof(int) not u8