netdev - Re: [PATCH] af_packet: add interframe drop cmsg (v6)

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Date:	Tue, 06 Oct 2009 12:26:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
CC:	Neil Horman <nhorman@...driver.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: add interframe drop cmsg (v6)

Oliver Hartkopp a écrit :

>> +	gap = check_packet_gap(skb);
>> +	if (gap)
>> +		put_cmsg(msg, SOL_PACKET, PACKET_GAPDATA, sizeof(__u32), &gap);
>> +
> 
> I wonder, if using SOL_PACKET with a packet specific sockopt PACKET_GAPDATA is
> the right approach here, as this functionality should probably be made
> available to all DGRAM/RAW sockets ...
> 
> E.g. getting the gap information is also interesting for CAN_RAW sockets as i
> polled from the SocketCAN ML.
> 
> So i could easily implement something like this in net/can/raw.c :
> 
> gap = check_packet_gap(skb);
> if (gap)
> 	put_cmsg(msg, SOL_CAN_RAW, CAN_RAW_GAPDATA, sizeof(__u32), &gap);
> 
> Is this a good approach or should we better find a general handling within
> SOL_SOCKET here?
> 



Given that datagrams are usually lost by network before hitting your socket, I am not sure
it is useful at all. 

But yes, we could do the following for free :

Use existing sock->sk_drops total counter, and to store its value at time datagram 
is received and stored into socket queue (store sock->sk_drops into skb->mark)

At recv() time, copy this saved value into a CMSG item if user set a given SOL_SOCKET option.

(It would not be a relative gap, but absolute. Your application would have to compute
deltas itself)


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