netdev - Re: [PATCH v2] gianfar: Fall back to software tcp/udp checksum on oldercontrollers

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Message-Id: <20110128.115939.104064843.davem@davemloft.net>
Date:	Fri, 28 Jan 2011 11:59:39 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	scottwood@...escale.com
Cc:	David.Laight@...LAB.COM, netdev@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] gianfar: Fall back to software tcp/udp checksum on
 oldercontrollers

From: Scott Wood <scottwood@...escale.com>
Date: Fri, 28 Jan 2011 10:56:10 -0600

> On Fri, 28 Jan 2011 09:10:46 +0000
> David Laight <David.Laight@...LAB.COM> wrote:
> 
>>  
>> > +		if (unlikely(gfar_has_errata(priv, GFAR_ERRATA_12)
>> > +			     && ((unsigned long)fcb % 0x20) > 0x18)) {
>> 
>> You need to check the generated code, but I think you need:
>> 
>>     if (unlikely(gfar_has_errata(priv, GFAR_ERRATA_12))
>> 	     && unlikely(((unsigned long)fcb % 0x20) > 0x18))
>> 
>> ie unlikely() around both the primitive comparisons.
> 
> Is the first condition actually unlikely?  If you've got affected
> hardware, you'll hit it every time.
> 
> If packets with the problematic alignment are rare, seems like it'd be
> better to check that first.

In cases like this gfar_has_errata() case, better to leave it's
likelyhood unmarked.

And yes, since it's cheaper, checking the alignment should be done
first.
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