linux-kernel - Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be, le}_to_cpu() and cpu_to

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Message-ID: <CAPoQQ-17OHotfbczJvEoq=vAzu_+6-sV3M1ZAhDaQ45wB3NB4A@mail.gmail.com>
Date:	Wed, 28 May 2014 17:05:32 -0500
From:	Cody P Schafer <dev@...yps.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC <linuxppc-dev@...ts.ozlabs.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be,
 le}_to_cpu() and cpu_to_{be, le}() macros

On Wed, May 28, 2014 at 3:45 AM, David Laight <David.Laight@...lab.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.
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