MFC r287775:

Update TSO limits to include all headers.

To make driver programming easier the TSO limits are changed to
reflect the values used in the BUSDMA tag a network adapter driver is
using. The TCP/IP network stack will subtract space for all linklevel
and protocol level headers and ensure that the full mbuf chain passed
to the network adapter fits within the given limits. See r287775
for a more detailed description.

Differential Revision:	https://reviews.freebsd.org/D3477
Reviewed by:		rmacklem

1 files changed, 6 insertions, 5 deletions

diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index 58391a1..1ff7a4b 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -236,11 +236,12 @@ struct ifnet {
 	 * count limit does not apply. If all three fields are zero,
 	 * there is no TSO limit.
 	 *
-	 * NOTE: The TSO limits only apply to the data payload part of
-	 * a TCP/IP packet. That means there is no need to subtract
-	 * space for ethernet-, vlan-, IP- or TCP- headers from the
-	 * TSO limits unless the hardware driver in question requires
-	 * so.
+	 * NOTE: The TSO limits should reflect the values used in the
+	 * BUSDMA tag a network adapter is using to load a mbuf chain
+	 * for transmission. The TCP/IP network stack will subtract
+	 * space for all linklevel and protocol level headers and
+	 * ensure that the full mbuf chain passed to the network
+	 * adapter fits within the given limits.
 	 */
 	u_int	if_hw_tsomax;