[PATCH 11/12] carl9170: skip cross-band channel changes during software scan

[Masi Osmani <mas-i@hotmail.de>]

The carl9170 relies on mac80211 software scanning because it does not
implement a hw_scan callback.  During a scan, mac80211 iterates all
supported channels across both bands, calling carl9170_op_config()
with IEEE80211_CONF_CHANGE_CHANNEL for each one.

Every channel change triggers a full baseband cold reset, RF bank
re-initialisation and AGC calibration via the firmware RF_INIT command
with a 200 ms timeout.  Cross-band switches (2.4 GHz <-> 5 GHz) are
especially expensive and error-prone: the AGC calibration frequently
times out (firmware returns error code 2), leaving the PHY in a
degraded state.  Subsequent channel changes -- even within the same
band -- then also fail, and after three consecutive failures the
driver restarts the device, causing a multi-second connectivity gap.

When the adapter is associated on a specific band, scanning channels
on the other band produces no useful roaming candidates for the
current BSS.  Add sw_scan_start/sw_scan_complete callbacks to track
the scanning state and skip cross-band channel changes while a
software scan is in progress.  Intentional cross-band association
changes (e.g. roaming from 2.4 GHz to 5 GHz on a dual-band SSID)
are not affected because they occur outside the scanning window.

Tested on Fritz\!WLAN N (AR9170) with 2.4 GHz association and
concurrent full-band scans: no channel change failures, no device
restarts, no PHY corruption.

Signed-off-by: Masi Osmani <mas-i@hotmail.de>
---
 drivers/net/wireless/ath/carl9170/carl9170.h |  1 +
 drivers/net/wireless/ath/carl9170/main.c     | 42 +++++++++++++++++++
 2 files changed, 43 insertions(+)

--- a/drivers/net/wireless/ath/carl9170/carl9170.h	2026-03-15 23:51:23.598565789 +0100
+++ b/drivers/net/wireless/ath/carl9170/carl9170.h	2026-03-15 23:51:39.769123563 +0100
@@ -333,6 +333,7 @@ struct ar9170 {
 	/* PHY */
 	struct ieee80211_channel *channel;
 	unsigned int num_channels;
+	bool scanning;
 	int noise[4];
 	unsigned int chan_fail;
 	unsigned int total_chan_fail;
--- a/drivers/net/wireless/ath/carl9170/main.c	2026-03-15 23:51:23.597355728 +0100
+++ b/drivers/net/wireless/ath/carl9170/main.c	2026-03-15 23:52:02.845563524 +0100
@@ -916,6 +916,33 @@ static int carl9170_op_config(struct iee
 		enum nl80211_channel_type channel_type =
 			cfg80211_get_chandef_type(&hw->conf.chandef);
 
+		/*
+		 * Skip cross-band channel changes during software scan.
+		 *
+		 * mac80211 sw_scan iterates all channels including the
+		 * other band.  Each channel change requires a full BB
+		 * cold reset and AGC calibration via the firmware RF_INIT
+		 * command (200 ms timeout).  Cross-band switches
+		 * frequently cause AGC calibration timeouts (firmware
+		 * returns error 2), leaving the PHY in a degraded state
+		 * that cascades into failures on subsequent intra-band
+		 * channel changes and ultimately triggers a device
+		 * restart after three consecutive failures.
+		 *
+		 * When associated, scanning the other band yields no
+		 * useful roaming candidates for the current BSS.  Skip
+		 * the channel change so mac80211 advances to the next
+		 * scan channel harmlessly.
+		 */
+		if (ar->scanning && ar->channel &&
+		    hw->conf.chandef.chan->band != ar->channel->band) {
+			wiphy_dbg(ar->hw->wiphy,
+				  "skip cross-band scan: %d MHz -> %d MHz\n",
+				  ar->channel->center_freq,
+				  hw->conf.chandef.chan->center_freq);
+			goto out;
+		}
+
 		/* adjust slot time for 5 GHz */
 		err = carl9170_set_slot_time(ar);
 		if (err)
@@ -954,6 +981,27 @@ out:
 	return err;
 }
 
+static void carl9170_op_sw_scan_start(struct ieee80211_hw *hw,
+				      struct ieee80211_vif *vif,
+				      const u8 *mac_addr)
+{
+	struct ar9170 *ar = hw->priv;
+
+	mutex_lock(&ar->mutex);
+	ar->scanning = true;
+	mutex_unlock(&ar->mutex);
+}
+
+static void carl9170_op_sw_scan_complete(struct ieee80211_hw *hw,
+					 struct ieee80211_vif *vif)
+{
+	struct ar9170 *ar = hw->priv;
+
+	mutex_lock(&ar->mutex);
+	ar->scanning = false;
+	mutex_unlock(&ar->mutex);
+}
+
 static u64 carl9170_op_prepare_multicast(struct ieee80211_hw *hw,
 					 struct netdev_hw_addr_list *mc_list)
 {
@@ -1723,6 +1771,8 @@ static const struct ieee80211_ops carl91
 	.add_interface		= carl9170_op_add_interface,
 	.remove_interface	= carl9170_op_remove_interface,
 	.config			= carl9170_op_config,
+	.sw_scan_start		= carl9170_op_sw_scan_start,
+	.sw_scan_complete	= carl9170_op_sw_scan_complete,
 	.prepare_multicast	= carl9170_op_prepare_multicast,
 	.configure_filter	= carl9170_op_configure_filter,
 	.conf_tx		= carl9170_op_conf_tx,