linux-kernel - Re: [patch] Add helper macros for little-endian bitfields

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Date:	Wed, 27 Aug 2008 16:20:54 +0100
From:	David Vrabel <david.vrabel@....com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Roland Dreier <rdreier@...co.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	linux-usb <linux-usb@...r.kernel.org>,
	torvalds@...ux-foundation.org
Subject: Re: [patch] Add helper macros for little-endian bitfields

Al Viro wrote:
> On Sun, Aug 24, 2008 at 06:37:43PM -0700, Roland Dreier wrote:
>>  > + * NOTE: When using multibyte bitfields, you need to convert the data
>>  > + *       from Little Endian to CPU before you can access the bitfield
>>  > + *       (to make it simpler):
> 
> NOTE: When tempted to use multibyte bitfields on fixed-layout data, you
> need to look in the mirror, ask yourself "what will they do to me during
> code review for that?", shudder and decide that some temptations are
> just not worth the pain.

But why is this worthy of a crispy flaming?  I've not seen anything
definite beyond a somewhat vague 'some compilers don't optimize
bitfields very well'.

The structure definition and the DECL_BF_LEx() macros might be ugly but
the code using the structures is clearer.  For example,

    get_random_bytes(&tiebreaker, sizeof(unsigned));
    drp_ie->tiebreaker = tiebreaker & 1;

versus

    get_random_bytes(&tiebreaker, sizeof(unsigned));
    drp_ie->drp_control |= (tiebreaker & 1)
            ? UWB_DRP_IE_DRP_CTRL_TIEBREAKER : 0;

David
-- 
David Vrabel, Senior Software Engineer, Drivers
CSR, Churchill House, Cambridge Business Park,  Tel: +44 (0)1223 692562
Cowley Road, Cambridge, CB4 0WZ                 http://www.csr.com/
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