power: pm8921-bms: limit voltage correction

The "adjust_soc" algorithm where we change the open circuit voltage
(ocv) in steps so that the state of charge (soc) starts approaching
the estimated soc, causes nonlinearity in the soc curves if an incorrect
resistance value is fed to the algorithm.

As battery ages and temperature changes, it is hard to estimate the
exact battery resistance.

So to fix the nonlinearity, limit the amount by which the ocv is
changed. Make it proportional to the current, i.e. change ocv by
small amounts in light load and let it change by proportionally
large amounts in heavy load situations.

Also, make the point where the soc is adjusted configurable via platform
data instead of forcing it to 25%.

Change-Id: Idc141e6bf3172dab278afe1900f5a1f9cdd624dd
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>

arch/arm/mach-msm/board-8064-pmic.c

@@ -394,6 +394,7 @@ apq8064_pm8921_bms_pdata __devinitdata = {
 	.v_cutoff		= 3400,
 	.max_voltage_uv		= MAX_VOLTAGE_MV * 1000,
 	.shutdown_soc_valid_limit = 20,
+	.adjust_soc_low_threshold = 25,
 };
 
 static struct pm8921_platform_data

arch/arm/mach-msm/board-8930-pmic.c

@@ -345,6 +345,7 @@ static struct pm8921_bms_platform_data pm8921_bms_pdata __devinitdata = {
 	.v_cutoff		= 3400,
 	.max_voltage_uv		= MAX_VOLTAGE_MV * 1000,
 	.shutdown_soc_valid_limit = 20,
+	.adjust_soc_low_threshold = 25,
 };
 
 static struct pm8038_platform_data pm8038_platform_data __devinitdata = {

arch/arm/mach-msm/board-8960-pmic.c

@@ -426,6 +426,7 @@ static struct pm8921_bms_platform_data pm8921_bms_pdata __devinitdata = {
 	.max_voltage_uv		= MAX_VOLTAGE_MV * 1000,
 	.rconn_mohm		= 18,
 	.shutdown_soc_valid_limit = 20,
+	.adjust_soc_low_threshold = 25,
 };
 
 #define	PM8921_LC_LED_MAX_CURRENT	4	/* I = 4mA */

drivers/power/pm8921-bms.c

@@ -142,6 +142,7 @@ struct pm8921_bms_chip {
 	int			ignore_shutdown_soc;
 	int			prev_iavg_ua;
 	int			prev_uuc_iavg_ma;
+	int			adjust_soc_low_threshold;
 };
 
 /*
@@ -1440,6 +1441,7 @@ static int adjust_soc(struct pm8921_bms_chip *chip, int soc, int batt_temp,
 	int soc_new = 0;
 	int m = 0;
 	int rc = 0;
+	int delta_ocv_uv_limit = 0;
 
 	rc = pm8921_bms_get_simultaneous_battery_voltage_and_current(
 							&ibat_ua,
@@ -1452,6 +1454,9 @@ static int adjust_soc(struct pm8921_bms_chip *chip, int soc, int batt_temp,
 
 	if (ibat_ua < 0)
 		goto out;
+
+	delta_ocv_uv_limit = DIV_ROUND_CLOSEST(ibat_ua, 1000);
+
 	ocv_est_uv = vbat_uv + (ibat_ua * rbatt)/1000;
 	pc_est = calculate_pc(chip, ocv_est_uv, batt_temp, last_chargecycles);
 	soc_est = div_s64((s64)fcc_uah * pc_est - uuc_uah*100,
@@ -1469,7 +1474,8 @@ static int adjust_soc(struct pm8921_bms_chip *chip, int soc, int batt_temp,
 	 * and  cause a bad user experience
 	 */
 	if (soc_est == soc
-		|| (is_between(45, 25, soc_est) && is_between(50, 20, soc))
+		|| (is_between(45, chip->adjust_soc_low_threshold, soc_est)
+		&& is_between(50, chip->adjust_soc_low_threshold - 5, soc))
 		|| soc >= 90)
 		goto out;
 
@@ -1504,6 +1510,18 @@ static int adjust_soc(struct pm8921_bms_chip *chip, int soc, int batt_temp,
 
 	delta_ocv_uv = div_s64((soc - soc_est) * (s64)m * 1000,
 							n * (pc - pc_new));
+
+	if (abs(delta_ocv_uv) > delta_ocv_uv_limit) {
+		pr_debug("limiting delta ocv %d limit = %d\n", delta_ocv_uv,
+				delta_ocv_uv_limit);
+
+		if (delta_ocv_uv > 0)
+			delta_ocv_uv = delta_ocv_uv_limit;
+		else
+			delta_ocv_uv = -1 * delta_ocv_uv_limit;
+		pr_debug("new delta ocv = %d\n", delta_ocv_uv);
+	}
+
 	chip->last_ocv_uv -= delta_ocv_uv;
 
 	if (chip->last_ocv_uv >= chip->max_voltage_uv)
@@ -3038,6 +3056,10 @@ static int __devinit pm8921_bms_probe(struct platform_device *pdev)
 	chip->start_percent = -EINVAL;
 	chip->end_percent = -EINVAL;
 	chip->shutdown_soc_valid_limit = pdata->shutdown_soc_valid_limit;
+	chip->adjust_soc_low_threshold = pdata->adjust_soc_low_threshold;
+	if (chip->adjust_soc_low_threshold >= 45)
+		chip->adjust_soc_low_threshold = 45;
+
 	chip->ignore_shutdown_soc = pdata->ignore_shutdown_soc;
 	rc = set_battery_data(chip);
 	if (rc) {

include/linux/mfd/pm8xxx/pm8921-bms.h

@@ -132,6 +132,7 @@ struct pm8921_bms_platform_data {
 	int				enable_fcc_learning;
 	int				shutdown_soc_valid_limit;
 	int				ignore_shutdown_soc;
+	int				adjust_soc_low_threshold;
 };
 
 #if defined(CONFIG_PM8921_BMS) || defined(CONFIG_PM8921_BMS_MODULE)