netdev - Re: [PATCH][net-next] gianfar: Fix reported sent bytes to BQL for Tx timestamping

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Message-ID: <517E9742.1010002@freescale.com>
Date:	Mon, 29 Apr 2013 18:52:34 +0300
From:	Claudiu Manoil <claudiu.manoil@...escale.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	<netdev@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH][net-next] gianfar: Fix reported sent bytes to BQL for
 Tx timestamping

On 4/29/2013 6:28 PM, Eric Dumazet wrote:
> On Mon, 2013-04-29 at 08:23 -0700, Eric Dumazet wrote:
>> On Mon, 2013-04-29 at 17:27 +0300, Claudiu Manoil wrote:
>>> Hi,
>>>
>>> Unfortunately fixing this the other way around would imply changes
>>> in both xmit and clean_tx, and additional overhead in clean_tx.
>>> On xmit skb->len gets incremented with FCB_LEN and/or TXPAL_LEN,
>>> on a case by case basis. This would imply to add extra checks on
>>> clean_tx to identify whether only FCB_LEN has been added, or
>>> FCB+TXPAL or neither, all these just to report the bytes on wire
>>> for BQL.
>>> Does BQL really need to measure the bytes-on-wire or the bytes consumed
>>> for buffering?
>>
>> Yes.
>>
>> What about :
>>
>> diff --git a/drivers/net/ethernet/freescale/gianfar.c b/drivers/net/ethernet/freescale/gianfar.c
>> index 2375a01..bb727e1 100644
>> --- a/drivers/net/ethernet/freescale/gianfar.c
>> +++ b/drivers/net/ethernet/freescale/gianfar.c
>> @@ -2065,6 +2065,7 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>   	u32 bufaddr;
>>   	unsigned long flags;
>>   	unsigned int nr_frags, nr_txbds, length, fcb_length = GMAC_FCB_LEN;
>> +	unsigned int len;
>>
>>   	/* TOE=1 frames larger than 2500 bytes may see excess delays
>>   	 * before start of transmission.
>> @@ -2130,7 +2131,8 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>   	}
>>
>>   	/* Update transmit stats */
>> -	tx_queue->stats.tx_bytes += skb->len;
>> +	len = skb->len;

Already tried this change in xmit, but it breaks the UDP/TCP cases (eg. 
ftp). Due to (skb->ip_summed == CHECKSUM_PARTIAL) condition FCB is
added on xmit (skb->len increased with FCB_LEN only), and Tx completion
does not make this check.

> Idea is to use this value in TX completion.
>
> If at tx completion skb->len might be different, then store the true
> skb->len in skb->cb[]
>

Interesting but non-trivial, I think. Is this allowed? I mean, doesn't
the net stack overwrite this field?

Thanks,
Claudiu


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