netdev - Re: Single socket with TX_RING and RX

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Message-Id: <20130515224452.7CAA6224C4@mail.nwl.cc>
Date:	Thu, 16 May 2013 00:44:45 +0200
From:	Phil Sutter <phil@....cc>
To:	Ricardo Tubío <rtpardavila@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Single socket with TX_RING and RX_RING

On Wed, May 15, 2013 at 12:53:55PM +0000, Ricardo Tubío wrote:
> Once I tell kernel to export the TX_RING through setsockopt() (see code
> below) I always get an error (EBUSY) if i try to tell kernel to export the
> RX_RING with the same socket descriptor. Therefore, I have to open an
> additional socket for the RX_RING and I require of two sockets when I though
> that I would only require of one socket for both TX and RX using mmap()ed
> memory.
> 
> Do I need both sockets or am I doing something wrong?

After requesting the rings, a single mmap() call suffices for both. So
pseudo-code basically looks like this:

| setsockopt(fd, SOL_PACKET, PACKET_RX_RING, p, sizeof(p));
| setsockopt(fd, SOL_PACKET, PACKET_TX_RING, p, sizeof(p));
| rx_ring = mmap(NULL, ring_len * 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
| tx_ring = rx_ring + ring_len;

Note that packet_mmap() in net/packet/af_packet.c always maps the TX
ring memory right after the RX one.

HTH, Phil
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