This diff enables the short guard interval (SGI) feature of 802.11n.
In theory this should raise the max data rate from 65Mbit/s to 72Mbit/s.
To check if your AP supports SGI, associate to your AP and then run:
tcpdump -n -i iwm0 -s 1500 -y IEEE802_11_RADIO -v type mgt subtype beacon
and look for the SGI@20MHz HT capability: "htcaps=<...,SGI@20MHz,...>"
Performance tests welcome.
Index: if_iwm.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_iwm.c,v
retrieving revision 1.100
diff -u -p -r1.100 if_iwm.c
--- if_iwm.c 1 Sep 2016 09:47:47 -0000 1.100
+++ if_iwm.c 1 Sep 2016 10:37:40 -0000
@@ -6082,7 +6082,7 @@ iwm_setrates(struct iwm_node *in)
struct iwm_softc *sc = IC2IFP(ic)->if_softc;
struct iwm_lq_cmd *lq = &in->in_lq;
struct ieee80211_rateset *rs = &ni->ni_rates;
- int i, ridx, ridx_min, j, tab = 0;
+ int i, ridx, ridx_min, j, sgi_ok, tab = 0;
struct iwm_host_cmd cmd = {
.id = IWM_LQ_CMD,
.len = { sizeof(in->in_lq), },
@@ -6092,9 +6092,12 @@ iwm_setrates(struct iwm_node *in)
memset(lq, 0, sizeof(*lq));
lq->sta_id = IWM_STATION_ID;
- /* For HT, always enable RTS/CTS to avoid excessive retries. */
- if (ni->ni_flags & IEEE80211_NODE_HT)
+ /* For HT, enable RTS/CTS, and SGI (if supported). */
+ if (ni->ni_flags & IEEE80211_NODE_HT) {
lq->flags |= IWM_LQ_FLAG_USE_RTS_MSK;
+ sgi_ok = (ni->ni_htcaps & IEEE80211_HTCAP_SGI20);
+ } else
+ sgi_ok = 0;
/*
* Fill the LQ rate selection table with legacy and/or HT rates
@@ -6120,6 +6123,8 @@ iwm_setrates(struct iwm_node *in)
if (ridx == iwm_mcs2ridx[i]) {
tab = iwm_rates[ridx].ht_plcp;
tab |= IWM_RATE_MCS_HT_MSK;
+ if (sgi_ok)
+ tab |= IWM_RATE_MCS_SGI_MSK;
break;
}
}
@@ -8000,8 +8005,7 @@ iwm_attach(struct device *parent, struct
IEEE80211_C_SHSLOT | /* short slot time supported */
IEEE80211_C_SHPREAMBLE; /* short preamble supported */
- /* No optional HT features supported for now, */
- ic->ic_htcaps = 0;
+ ic->ic_htcaps = IEEE80211_HTCAP_SGI20;
ic->ic_htxcaps = 0;
ic->ic_txbfcaps = 0;
ic->ic_aselcaps = 0;
