netdev - Re: [RFC PATCH] bridge: inherit slave devices needed

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Message-Id: <20130822.130635.86687384282537453.davem@davemloft.net>
Date:	Thu, 22 Aug 2013 13:06:35 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	stephen@...workplumber.org
Cc:	f.fainelli@...il.com, netdev@...r.kernel.org,
	eric.dumazet@...il.com, vyasevic@...hat.com
Subject: Re: [RFC PATCH] bridge: inherit slave devices needed_headroom

From: Stephen Hemminger <stephen@...workplumber.org>
Date: Wed, 21 Aug 2013 21:58:29 -0700

> On Tue, 20 Aug 2013 10:21:45 +0100
> "Florian Fainelli" <f.fainelli@...il.com> wrote:
> 
>> Some slave devices may have set a dev->needed_headroom value which is
>> different than the default one, most likely in order to prepend a
>> hardware descriptor in front of the Ethernet frame to send. Whenever a
>> new slave is added to a bridge, ensure that we update the
>> needed_headroom value accordingly to account for the slave
>> needed_headroom value.
>> 
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>>  net/bridge/br_if.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
>> index aa6c9a8..c41d5fb 100644
>> --- a/net/bridge/br_if.c
>> +++ b/net/bridge/br_if.c
>> @@ -383,6 +383,9 @@ int br_add_if(struct net_bridge *br, struct net_device *dev)
>>  
>>  	netdev_update_features(br->dev);
>>  
>> +	if (br->dev->needed_headroom < dev->needed_headroom)
>> +		br->dev->needed_headroom = dev->needed_headroom;
>> +
>>  	spin_lock_bh(&br->lock);
>>  	changed_addr = br_stp_recalculate_bridge_id(br);
>>  
> 
> I am okay with this but it only helps locally generated traffic
> which is the minority on most bridge devices. It does nothing for the case
> where one port needs more headroom than the packet received on the
> other port. That is why a device has to always work when it receives
> a packet with less headroom, usually by copying.
> 

Right so this is basically an optimization.

Florian please submit this formally as a non-RFC patch so Stephen
can ACK it and I can apply it, thanks.
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