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

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Message-ID: <Pine.LNX.4.64.0712261525490.23059@dut46.unminc.com>
Date:	Wed, 26 Dec 2007 15:29:22 -0800 (PST)
From:	Dhananjay Phadke <dhananjay@...xen.com>
To:	Al Viro <viro@...IV.linux.org.uk>
cc:	netdev@...r.kernel.org, jeff@...zik.org
Subject: Re: [patch 7/7] netxen: fix byte-swapping in tx and rx

I agree for tx desc, the compiler would optimize.

But for rx (status) desc, I didn't like multiple le64_to_cpu() 
for extracting various fields out of same status dword.

-Dhananjay

On Wed, 26 Dec 2007, Al Viro wrote:

> 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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