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packet: doc: describe PACKET_MMAP with one packet socket for rx and tx

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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>
This commit is contained in:
Norbert van Bolhuis 2014-01-10 10:22:37 +01:00 committed by David S. Miller
parent 6daaf0de2f
commit 7e11daa7c1
1 changed files with 18 additions and 0 deletions
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18
Documentation/networking/packet_mmap.txt
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@ -98,6 +98,11 @@ by the kernel.
The destruction of the socket and all associated resources
is done by a simple call to close(fd).
Similarly as without PACKET_MMAP, it is possible to use one socket
for capture and transmission. This can be done by mapping the
allocated RX and TX buffer ring with a single mmap() call.
See "Mapping and use of the circular buffer (ring)".
Next I will describe PACKET_MMAP settings and its constraints,
also the mapping of the circular buffer in the user process and
the use of this buffer.
@ -414,6 +419,19 @@ tp_block_size/tp_frame_size frames there will be a gap between
the frames. This is because a frame cannot be spawn across two
blocks.
To use one socket for capture and transmission, the mapping of both the
RX and TX buffer ring has to be done with one call to mmap:
...
setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &foo, sizeof(foo));
setsockopt(fd, SOL_PACKET, PACKET_TX_RING, &bar, sizeof(bar));
...
rx_ring = mmap(0, size * 2, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
tx_ring = rx_ring + size;
RX must be the first as the kernel maps the TX ring memory right
after the RX one.
At the beginning of each frame there is an status field (see
struct tpacket_hdr). If this field is 0 means that the frame is ready
to be used for the kernel, If not, there is a frame the user can read