netdev - Re: [patch 7/7] netxen: fix byte-swapping in tx and rx

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Date:	Wed, 26 Dec 2007 22:38:03 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	dhananjay@...xen.com
Cc:	netdev@...r.kernel.org, jeff@...zik.org
Subject: Re: [patch 7/7] netxen: fix byte-swapping in tx and rx

On Wed, Dec 26, 2007 at 10:23:59AM -0800, dhananjay@...xen.com wrote:
> This cleans up some unnecessary byte-swapping while setting up tx and
> interpreting rx desc. The 64 bit rx status data should be converted
> to host endian format only once and the macros just need to extract
> bitfields.
 
> -	(cmd_desc)->flags_opcode |= cpu_to_le16((val) & 0x7f))
> +	((cmd_desc)->flags_opcode &= ~(0x7f), \
> +	(cmd_desc)->flags_opcode |= (val) & 0x7f)
>  #define netxen_set_cmd_desc_opcode(cmd_desc, val)	\
> -	((cmd_desc)->flags_opcode &= ~cpu_to_le16(0x3f<<7), \
> -	(cmd_desc)->flags_opcode |= cpu_to_le16(((val & 0x3f)<<7)))
> +	((cmd_desc)->flags_opcode &= ~(0x3f<<7), \
> +	(cmd_desc)->flags_opcode |= ((val) & 0x3f) << 7)

_oh_?

That's actually a pessimisation and I'd rather see that conversion in
place gone - the thing will be less brittle that way.

BTW, note that cpu_to_le16() et.al. get evaluated by compiler when the
argument is constant.
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