[PATCH wireless] wifi: b43: back off the N-PHY TX power per rate

[Rosen Penev <rosenp@gmail.com>]

On N-PHY rev 3+ the hardware TX power control aims at the highest
per-rate target, and an adjusted power table backs each rate group off
from it. b43 fills that table from nphy->tx_power_offset[], but never
sets the offsets, so they stay zero and every rate is sent at the power
allowed for CCK.

The SPROM power offsets exist to avoid this: on boards with external
PAs, OFDM and MCS rates have to be sent a few dB lower than CCK to
keep the PA linear. Sent at full power, 16-QAM and 64-QAM frames are
too distorted to decode. On an ASUS RT-N16 (BCM4718, N-PHY rev 5,
radio 2056, ofdm2gpo 0x66666666) the router could only send at 6-18
Mbit/s and MCS 0-2, even at -50 dBm, and 24 Mbit/s and up or MCS 3 and
up delivered nothing.

Set the offsets from the per-rate targets b43 already computes from
the SPROM, as brcmsmac does, before the table is built.

With this, the RT-N16 sends at 54 Mbit/s, and at MCS 7 with HT
enabled, and TCP from the router to a client goes from 9 to 17-18
Mbit/s.

Fixes: 8615eb2870f1 ("b43: N-PHY: support setting custom TX power")
Assisted-by: LLM
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/net/wireless/broadcom/b43/phy_n.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/wireless/broadcom/b43/phy_n.c b/drivers/net/wireless/broadcom/b43/phy_n.c
index 22359bd9db47..c9b2b7496983 100644
--- a/drivers/net/wireless/broadcom/b43/phy_n.c
+++ b/drivers/net/wireless/broadcom/b43/phy_n.c
@@ -5834,6 +5834,7 @@ static enum b43_txpwr_result b43_nphy_op_recalc_txpower(struct b43_wldev *dev,
 	struct ieee80211_channel *channel = dev->wl->hw->conf.chandef.chan;
 	struct b43_ppr *ppr = &nphy->tx_pwr_max_ppr;
 	u8 max; /* qdBm */
+	int i;
 
 	if (nphy->tx_pwr_last_recalc_freq == channel->center_freq &&
 	    nphy->tx_pwr_last_recalc_limit == phy->desired_txpower)
@@ -5868,6 +5869,16 @@ static enum b43_txpwr_result b43_nphy_op_recalc_txpower(struct b43_wldev *dev,
 	/* Make sure we didn't go too low */
 	b43_ppr_apply_min(dev, ppr, INT_TO_Q52(8));
 
+	/*
+	 * The hardware power control aims at the highest per-rate target.
+	 * The adjusted power table backs other rates off from it, so that
+	 * e.g. 64-QAM isn't sent at the power allowed for CCK.
+	 */
+	max = b43_ppr_get_max(dev, ppr);
+	memset(nphy->tx_power_offset, 0, sizeof(nphy->tx_power_offset));
+	for (i = 0; i < B43_PPR_RATES_NUM; i++)
+		nphy->tx_power_offset[i] = max - ppr->__all_rates[i];
+
 	/* Apply */
 	b43_mac_suspend(dev);
 	b43_nphy_tx_power_ctl_setup(dev);
-- 
2.56.0