netdev - Re: [PATCH net-next 1/4] cxgb4: Use mask & shift while returning vlan

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Message-Id: <20151216.232008.805617588685597196.davem@davemloft.net>
Date:	Wed, 16 Dec 2015 23:20:08 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	leedom@...lsio.com
Cc:	hariprasad@...lsio.com, netdev@...r.kernel.org,
	nirranjan@...lsio.com
Subject: Re: [PATCH net-next 1/4] cxgb4: Use mask & shift while returning
 vlan_prio

From: Casey Leedom <leedom@...lsio.com>
Date: Wed, 16 Dec 2015 17:40:41 -0800

> 
>> On Dec 16, 2015, at 4:07 PM, David Miller <davem@...emloft.net> wrote:
>> 
>> From: Hariprasad Shenai <hariprasad@...lsio.com>
>> Date: Wed, 16 Dec 2015 13:16:25 +0530
>> 
>>> @@ -66,7 +66,7 @@ struct l2t_data {
>>> 
>>> static inline unsigned int vlan_prio(const struct l2t_entry *e)
>>> {
>>> -	return e->vlan >> 13;
>>> +	return (e->vlan & VLAN_PRIO_MASK) >> VLAN_PRIO_SHIFT;
>> 
>> e->vlan is a u16, the vlan priotity is the top 3 bits of the 16-bit
>> value, and finally the right shift will be non-signed.
>> 
>> Therefore this change is absolutely not necessary.
>> 
>> Please remove this patch from the series and resend.
> 
> I assume that you only meant that the masking portion is
> unnecessary.  Doing the shift with the symbolic constant
> VLAN_PRIO_SHIFT instead of the literal constant “13” is still a
> reasonable change.  The masking was almost certainly from me because
> once one uses the symbolic constants, weren’t not supposed to “know”
> about the internal structure of the operation.  Modern compilers are
> of course free to optimize away the mask, etc.

Yes I'm only objecting to the unnecessary mask operation.
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