[src/trunk]: src/sys/dev/pci add comments about transmit context descriptor

[knakahara <knakahara%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/8927b4074a95
branches:  trunk
changeset: 352235:8927b4074a95
user:      knakahara <knakahara%NetBSD.org@localhost>
date:      Wed Mar 22 08:44:41 2017 +0000

description:
add comments about transmit context descriptor

diffstat:

 sys/dev/pci/if_wm.c |  19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)

diffs (47 lines):

diff -r 6c77c45c7cde -r 8927b4074a95 sys/dev/pci/if_wm.c
--- a/sys/dev/pci/if_wm.c       Wed Mar 22 07:14:18 2017 +0000
+++ b/sys/dev/pci/if_wm.c       Wed Mar 22 08:44:41 2017 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: if_wm.c,v 1.499 2017/03/22 02:56:12 knakahara Exp $    */
+/*     $NetBSD: if_wm.c,v 1.500 2017/03/22 08:44:41 knakahara Exp $    */
 
 /*
  * Copyright (c) 2001, 2002, 2003, 2004 Wasabi Systems, Inc.
@@ -84,7 +84,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: if_wm.c,v 1.499 2017/03/22 02:56:12 knakahara Exp $");
+__KERNEL_RCSID(0, "$NetBSD: if_wm.c,v 1.500 2017/03/22 08:44:41 knakahara Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_net_mpsafe.h"
@@ -6575,6 +6575,13 @@
                    WTX_TCPIP_TUCSE(0) /* rest of packet */;
        }
 
+       /*
+        * We don't have to write context descriptor for every packet
+        * except for 82574. For 82574, we must write context descriptor
+        * for every packet when we use two descriptor queues.
+        * It would be overhead to write context descriptor for every packet,
+        * however it does not cause problems.
+        */
        /* Fill in the context descriptor. */
        t = (struct livengood_tcpip_ctxdesc *)
            &txq->txq_descs[txq->txq_next];
@@ -7176,6 +7183,14 @@
                *fieldsp |= NQTXD_FIELDS_TUXSM;
        }
 
+       /*
+        * We don't have to write context descriptor for every packet to
+        * NEWQUEUE controllers, that is 82575, 82576, 82580, I350, I354,
+        * I210 and I211. It is enough to write once per a Tx queue for these
+        * controllers.
+        * It would be overhead to write context descriptor for every packet,
+        * however it does not cause problems.
+        */
        /* Fill in the context descriptor. */
        txq->txq_nq_descs[txq->txq_next].nqrx_ctx.nqtxc_vl_len =
            htole32(vl_len);