[src/trunk]: src/sys/dev/pci add comments about the situation wm(4) use MSI-X...

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

details:   https://anonhg.NetBSD.org/src/rev/4c95388aa04f
branches:  trunk
changeset: 352287:4c95388aa04f
user:      knakahara <knakahara%NetBSD.org@localhost>
date:      Fri Mar 24 10:02:35 2017 +0000

description:
add comments about the situation wm(4) use MSI-X but does not use multiqueue.

diffstat:

 sys/dev/pci/if_wm.c |  21 ++++++++++++++++-----
 1 files changed, 16 insertions(+), 5 deletions(-)

diffs (64 lines):

diff -r 3de6fd4a691d -r 4c95388aa04f sys/dev/pci/if_wm.c
--- a/sys/dev/pci/if_wm.c       Fri Mar 24 10:01:55 2017 +0000
+++ b/sys/dev/pci/if_wm.c       Fri Mar 24 10:02:35 2017 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: if_wm.c,v 1.502 2017/03/24 10:01:55 knakahara Exp $    */
+/*     $NetBSD: if_wm.c,v 1.503 2017/03/24 10:02:35 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 1.502 2017/03/24 10:01:55 knakahara Exp $");
+__KERNEL_RCSID(0, "$NetBSD: if_wm.c,v 1.503 2017/03/24 10:02:35 knakahara Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_net_mpsafe.h"
@@ -2561,10 +2561,21 @@
        ifp->if_ioctl = wm_ioctl;
        if ((sc->sc_flags & WM_F_NEWQUEUE) != 0) {
                ifp->if_start = wm_nq_start;
+               /*
+                * When the number of CPUs is one and the controller can use
+                * MSII-X, wm(4) use MSI-X but *does not* use multiqueue.
+                * That is, wm(4) use two interrupts, one is used for Tx/Rx
+                * and the other is used for link status changing.
+                * In this situation, wm_nq_transmit() is disadvantageous
+                * because of wm_select_txqueue() and pcq(9) overhead.
+                */
                if (wm_is_using_multiqueue(sc))
                        ifp->if_transmit = wm_nq_transmit;
        } else {
                ifp->if_start = wm_start;
+               /*
+                * wm_transmit() has the same disadvantage as wm_transmit().
+                */
                if (wm_is_using_multiqueue(sc))
                        ifp->if_transmit = wm_transmit;
        }
@@ -2833,7 +2844,7 @@
 
        /*
         * There are still some upper layer processing which call
-        * ifp->if_start(). e.g. ALTQ
+        * ifp->if_start(). e.g. ALTQ or one CPU system
         */
        /* Try to get more packets going. */
        ifp->if_start(ifp);
@@ -7611,12 +7622,12 @@
        }
 
        if ((sc->sc_flags & WM_F_NEWQUEUE) != 0) {
-               /* XXX need for ALTQ */
+               /* XXX need for ALTQ or one CPU system */
                if (qid == 0)
                        wm_nq_start_locked(ifp);
                wm_nq_transmit_locked(ifp, txq);
        } else {
-               /* XXX need for ALTQ */
+               /* XXX need for ALTQ or one CPU system */
                if (qid == 0)
                        wm_start_locked(ifp);
                wm_transmit_locked(ifp, txq);