Bluetooth: Use 3-DH5 payload size for default ERTM max PDU size

The previous value of 672 for L2CAP_DEFAULT_MAX_PDU_SIZE is based on
the default L2CAP MTU.  That default MTU is calculated from the size
of two DH5 packets, minus ACL and L2CAP b-frame header overhead.

ERTM is used with newer basebands that typically support larger 3-DH5
packets, and i-frames and s-frames have more header overhead.  With
clean RF conditions, basebands will typically attempt to use 1021-byte
3-DH5 packets for maximum throughput.  Adjusting for 2 bytes of ACL
headers plus 10 bytes of worst-case L2CAP headers yields 1009 bytes
of payload.

This PDU size imposes less overhead for header bytes and gives the
baseband the option to choose 3-DH5 packets, but is small enough for
ERTM traffic to interleave well with other L2CAP or SCO data.
672-byte payloads do not allow the most efficient over-the-air
packet choice, and cannot achieve maximum throughput over BR/EDR.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h
index 16e412f3722b1e7e90e45aef7c10119ac23cb8fa..6c241444f902800baf1c4cd16aa89d767d751065 100644 (file)
--- a/include/net/bluetooth/l2cap.h
+++ b/include/net/bluetooth/l2cap.h
@@ -35,7 +35,7 @@
 #define L2CAP_DEFAULT_MAX_TX           3
 #define L2CAP_DEFAULT_RETRANS_TO       2000    /* 2 seconds */
 #define L2CAP_DEFAULT_MONITOR_TO       12000   /* 12 seconds */
-#define L2CAP_DEFAULT_MAX_PDU_SIZE     672
+#define L2CAP_DEFAULT_MAX_PDU_SIZE     1009    /* Sized for 3-DH5 packet */
 #define L2CAP_DEFAULT_ACK_TO           200
 #define L2CAP_LOCAL_BUSY_TRIES         12