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

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Message-ID: <48B5C4E3.3000002@goop.org>
Date:	Wed, 27 Aug 2008 14:19:31 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	David Vrabel <david.vrabel@....com>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	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

David Vrabel wrote:
> 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;
>   

Why not just

	drp_ie->drp_control |= tiebreaker & UWB_DRP_IE_DRP_CTRL_TIEBREAKER;

then?  Doesn't matter which random bit you pick, does it?

    J
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