netdev - Re: [PATCH] ixgbe: Introduce new 10GbE driver for Intel 82598 based PCI Express adapters...

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Message-ID: <46891A99.7090003@garzik.org>
Date:	Mon, 02 Jul 2007 11:32:41 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	"Kok, Auke" <auke-jan.h.kok@...el.com>
CC:	Arjan van de Ven <arjan@...ux.intel.com>,
	Ayyappan Veeraiyan <ayyappan.veeraiyan@...el.com>,
	netdev@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] ixgbe: Introduce new 10GbE driver for Intel 82598 based
 PCI	Express adapters...

Kok, Auke wrote:
> Maybe this is not most important for ixgbe, where we only have 8 or so 
> flags, but the new e1000 driver that I posted this weekend currently has 
> 63 (you wanted flags ;)) of them. Do you want me to use 63 integers or 
> just 2 ?

Don't be silly.  We are talking about single-bit feature flags 
implemented using machine ints (a la tg3 with 32 flags per int), versus 
bitfields.

Bitfields are to be avoided for many reasons:
* more difficult, in general, for a compiler to generate optimal code
* in particular, known to generate worse code on various architectures
* often causes endian problems
* often enhances programmer confusion, when trying to review structs and 
determine optimal layout and alignment
* programmers have proven they will screw up bitfields in e.g. cases 
with 1-bit and signedness.

I can probably think up more reasons to avoid bitfields if given another 
5 minutes :)

	Jeff

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