power: supply: ab8500: Exit maintenance if too low voltage

The maintenance charging is supposedly designed such that the
maintenance current compensates for the battery discharge curve,
and as the charging progress from CC/CV -> maintenance A ->
maintenance B states, we end up on a reasonable voltage to
restart ordinary CC/CV charging after the safety timer at the
maintenance B state exits.

However: old batteries discharge quicker, and in an old
battery we might not get to the expiration of the maintenance B
timer before the battery is completely depleted and the system
powers off with an empty battery.

This is hardly the desire of anyone leaving their phone in the
charger for a few days!

Introduce a second clause in both maintenance states such that
we exit the state and return to ordinary CC/CV charging if
the voltage drops below charge_restart_voltage_uv or 95%
if this is not defined for the battery.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/supply/ab8500_chargalg.c

@@ -1514,6 +1514,14 @@ static void ab8500_chargalg_algorithm(struct ab8500_chargalg *di)
 			ab8500_chargalg_stop_maintenance_timer(di);
 			ab8500_chargalg_state_to(di, STATE_MAINTENANCE_B_INIT);
 		}
+		/*
+		 * This happens if the voltage drops too quickly during
+		 * maintenance charging, especially in older batteries.
+		 */
+		if (ab8500_chargalg_time_to_restart(di)) {
+			ab8500_chargalg_state_to(di, STATE_NORMAL_INIT);
+			dev_info(di->dev, "restarted charging from maintenance state A - battery getting old?\n");
+		}
 		break;
 
 	case STATE_MAINTENANCE_B_INIT:
@@ -1538,6 +1546,14 @@ static void ab8500_chargalg_algorithm(struct ab8500_chargalg *di)
 			ab8500_chargalg_stop_maintenance_timer(di);
 			ab8500_chargalg_state_to(di, STATE_NORMAL_INIT);
 		}
+		/*
+		 * This happens if the voltage drops too quickly during
+		 * maintenance charging, especially in older batteries.
+		 */
+		if (ab8500_chargalg_time_to_restart(di)) {
+			ab8500_chargalg_state_to(di, STATE_NORMAL_INIT);
+			dev_info(di->dev, "restarted charging from maintenance state B - battery getting old?\n");
+		}
 		break;
 
 	case STATE_TEMP_LOWHIGH_INIT: