packet: doc: describe PACKET_MMAP with one packet socket for rx and tx
Document how to use one AF_PACKET mmap socket for RX and TX. Signed-off-by: Norbert van Bolhuis <nvbolhuis@aimvalley.nl> Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -98,6 +98,11 @@ by the kernel.
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The destruction of the socket and all associated resources
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is done by a simple call to close(fd).
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Similarly as without PACKET_MMAP, it is possible to use one socket
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for capture and transmission. This can be done by mapping the
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allocated RX and TX buffer ring with a single mmap() call.
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See "Mapping and use of the circular buffer (ring)".
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Next I will describe PACKET_MMAP settings and its constraints,
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also the mapping of the circular buffer in the user process and
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the use of this buffer.
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@ -414,6 +419,19 @@ tp_block_size/tp_frame_size frames there will be a gap between
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the frames. This is because a frame cannot be spawn across two
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blocks.
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To use one socket for capture and transmission, the mapping of both the
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RX and TX buffer ring has to be done with one call to mmap:
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...
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setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &foo, sizeof(foo));
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setsockopt(fd, SOL_PACKET, PACKET_TX_RING, &bar, sizeof(bar));
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...
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rx_ring = mmap(0, size * 2, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
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tx_ring = rx_ring + size;
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RX must be the first as the kernel maps the TX ring memory right
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after the RX one.
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At the beginning of each frame there is an status field (see
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struct tpacket_hdr). If this field is 0 means that the frame is ready
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to be used for the kernel, If not, there is a frame the user can read
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