netdev - Re: [PATCH 4/6] net/macb: add RX checksum offload feature

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Message-ID: <53CE9109.9040407@atmel.com>
Date:	Tue, 22 Jul 2014 18:27:53 +0200
From:	Cyrille Pitchen <cyrille.pitchen@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	<nicolas.ferre@...el.com>, <davem@...emloft.net>,
	<linux-arm-kernel@...ts.infradead.org>, <netdev@...r.kernel.org>,
	<soren.brinkmann@...inx.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] net/macb: add RX checksum offload feature

Le 18/07/2014 17:36, Eric Dumazet a écrit :
> On Fri, 2014-07-18 at 16:21 +0200, Cyrille Pitchen wrote:
>> Signed-off-by: Cyrille Pitchen <cyrille.pitchen@...el.com>
>> ---
>>  drivers/net/ethernet/cadence/macb.c | 29 ++++++++++++++++++++++++++++-
>>  drivers/net/ethernet/cadence/macb.h |  2 ++
>>  2 files changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/cadence/macb.c b/drivers/net/ethernet/cadence/macb.c
>> index 9bdcd1b..6acd6e2 100644
>> --- a/drivers/net/ethernet/cadence/macb.c
>> +++ b/drivers/net/ethernet/cadence/macb.c
>> @@ -766,6 +766,8 @@ static int gem_rx(struct macb *bp, int budget)
>>  
>>  		skb->protocol = eth_type_trans(skb, bp->dev);
>>  		skb_checksum_none_assert(skb);
>> +		if (bp->dev->features & NETIF_F_RXCSUM)
>> +			skb->ip_summed = CHECKSUM_UNNECESSARY;
>>  
> 
> 
> Really ?
> 
> If this is what you meant, this deserves a big and fat comment.
> 
> 
> 
Hi Eric,

Isn't it the proper way to do? According to Cadence documentation about RX checksum offload:
"If any of the checksums (IP, TCP or UDP) are verified incorrect by the GEM, the packet is discarded."

If I understand, when RX checksum offload is enabled setting bit 24 of the Network Configuration Register, the driver only receives RX frames with correct checksum. Then it tells the kernel that there's no need to verify the checksum again in software. This is done setting skb->ip_summed to CHECKSUM_UNNECESSARY. Intel e1000 driver does the same in e1000_rx_checksum() function.

Also bit 24 of the Network Configuration Register is updated by macb_open() and macb_set_features() so it always matches the state of NETIF_F_RXCSUM flag in dev->features, once the network interface is up. That's why I'd rather read from dev->features than read from register.

Did I make a mistake? Is it the kind of comment you expect to be added?

Regards,

Cyrille
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