packet: tx_ring: allow the user to choose tx data offset

The tx data offset of packet mmap tx ring used to be :
(TPACKET2_HDRLEN - sizeof(struct sockaddr_ll))

The problem is that, with SOCK_RAW socket, the payload (14 bytes after
the beginning of the user data) is misaligned.

This patch allows to let the user gives an offset for it's tx data if
he desires.

Set sock option PACKET_TX_HAS_OFF to 1, then specify in each frame of
your tx ring tp_net for SOCK_DGRAM, or tp_mac for SOCK_RAW.

Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt
index 1c08a4b0981f..7cd879eba5dc 100644
--- a/Documentation/networking/packet_mmap.txt
+++ b/Documentation/networking/packet_mmap.txt
@@ -163,6 +163,19 @@ As capture, each frame contains two parts:
 
  A complete tutorial is available at: http://wiki.gnu-log.net/
 
+By default, the user should put data at :
+ frame base + TPACKET_HDRLEN - sizeof(struct sockaddr_ll)
+
+So, whatever you choose for the socket mode (SOCK_DGRAM or SOCK_RAW),
+the beginning of the user data will be at :
+ frame base + TPACKET_ALIGN(sizeof(struct tpacket_hdr))
+
+If you wish to put user data at a custom offset from the beginning of
+the frame (for payload alignment with SOCK_RAW mode for instance) you
+can set tp_net (with SOCK_DGRAM) or tp_mac (with SOCK_RAW). In order
+to make this work it must be enabled previously with setsockopt()
+and the PACKET_TX_HAS_OFF option.
+
 --------------------------------------------------------------------------------
 + PACKET_MMAP settings
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