wifi: iwlwifi: mvm: select ptp cross timestamp from multiple reads

iwl_mvm_get_sync_time() reads the gp2 from the device and then
reads the system clock. Since the two reads are not done atomically,
unexpected delays may happen between the two reads (e.g. context
switch) which make it inaccurate.
In order to improve the accuracy of the cross timestamp, call
iwl_mvm_get_sync_time() multiple times in a loop and take the
result in which the difference between the two clock is the smallest.
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230320122330.d9e6f8f8998a.I569939ec4ddf0c6c64c112e7d0c30583f5509d9a@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/ptp.c

@@ -18,6 +18,8 @@
 #define SCALE_FACTOR	65536000000ULL
 #define IWL_PTP_WRAP_THRESHOLD_USEC	(5000)
 
+#define IWL_PTP_GET_CROSS_TS_NUM	5
+
 static void iwl_mvm_ptp_update_new_read(struct iwl_mvm *mvm, u32 gp2)
 {
 	/* If the difference is above the threshold, assume it's a wraparound.
@@ -122,6 +124,28 @@ iwl_mvm_get_crosstimestamp_fw(struct iwl_mvm *mvm, u32 *gp2, u64 *sys_time)
 	return ret;
 }
 
+static void iwl_mvm_phc_get_crosstimestamp_loop(struct iwl_mvm *mvm,
+						ktime_t *sys_time, u32 *gp2)
+{
+	u64 diff = 0, new_diff;
+	u64 tmp_sys_time;
+	u32 tmp_gp2;
+	int i;
+
+	for (i = 0; i < IWL_PTP_GET_CROSS_TS_NUM; i++) {
+		iwl_mvm_get_sync_time(mvm, CLOCK_REALTIME, &tmp_gp2, NULL,
+				      &tmp_sys_time);
+		new_diff = tmp_sys_time - ((u64)tmp_gp2 * NSEC_PER_USEC);
+		if (!diff || new_diff < diff) {
+			*sys_time = tmp_sys_time;
+			*gp2 = tmp_gp2;
+			diff = new_diff;
+			IWL_DEBUG_INFO(mvm, "PTP: new times: gp2=%u sys=%lld\n",
+				       *gp2, *sys_time);
+		}
+	}
+}
+
 static int
 iwl_mvm_phc_get_crosstimestamp(struct ptp_clock_info *ptp,
 			       struct system_device_crosststamp *xtstamp)
@@ -150,8 +174,7 @@ iwl_mvm_phc_get_crosstimestamp(struct ptp_clock_info *ptp,
 		if (ret)
 			goto out;
 	} else {
-		iwl_mvm_get_sync_time(mvm, CLOCK_REALTIME, &gp2, NULL,
-				      &sys_time);
+		iwl_mvm_phc_get_crosstimestamp_loop(mvm, &sys_time, &gp2);
 	}
 
 	gp2_ns = iwl_mvm_ptp_get_adj_time(mvm, (u64)gp2 * NSEC_PER_USEC);