power: supply: ab8500: Standardize BTI resistance

The Battery Type Indicator (BTI) resistor is a resistor mounted
between a special terminal on the battery and ground. By sending
a fixed current (such as 7mA) through this resistor and measuring
the voltage over it, the resistance can be determined, and this
verifies the battery type.

Typical side view of the battery:

  o     o     o
 GND   BTI   +3.8V

Typical example of the electrical layout:

  +3.8 V   BTI
    |       |
    | +     |
 _______   [ ] 7kOhm
   ___      |
    |       |
    |       |
   GND     GND

By verifying this resistance before attempting to charge the
battery we add an additional level of security.

In some systems this is used for plug-and-play of batteries with
different capacity. In other cases, this is merely used to verify
that the right type of battery is connected, if several batteries
have the same physical shape and can be plugged into the same
slot. Sometimes this is just a surplus security mechanism.

Nokia and Samsung among many other vendors are known to use these
BTI resistors.

Add the BTI properties to struct power_supply_battery_info and
switch the AB8500 charger code over to using it.

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

1 files changed, 25 insertions, 1 deletions

diff --git a/drivers/power/supply/power_supply_core.c b/drivers/power/supply/power_supply_core.c
index e3d6d3ff492a..3d5047d3fe99 100644
--- a/drivers/power/supply/power_supply_core.c
+++ b/drivers/power/supply/power_supply_core.c
@@ -607,7 +607,9 @@ int power_supply_get_battery_info(struct power_supply *psy,
 	info->temp_min                       = INT_MIN;
 	info->temp_max                       = INT_MAX;
 	info->factory_internal_resistance_uohm  = -EINVAL;
-	info->resist_table = NULL;
+	info->resist_table                   = NULL;
+	info->bti_resistance_ohm             = -EINVAL;
+	info->bti_resistance_tolerance       = -EINVAL;
 
 	for (index = 0; index < POWER_SUPPLY_OCV_TEMP_MAX; index++) {
 		info->ocv_table[index]       = NULL;
@@ -938,6 +940,28 @@ int power_supply_batinfo_ocv2cap(struct power_supply_battery_info *info,
 }
 EXPORT_SYMBOL_GPL(power_supply_batinfo_ocv2cap);
 
+bool power_supply_battery_bti_in_range(struct power_supply_battery_info *info,
+				       int resistance)
+{
+	int low, high;
+
+	/* Nothing like this can be checked */
+	if (info->bti_resistance_ohm <= 0)
+		return false;
+
+	/* This will be extremely strict and unlikely to work */
+	if (info->bti_resistance_tolerance <= 0)
+		return (info->bti_resistance_ohm == resistance);
+
+	low = info->bti_resistance_ohm -
+		(info->bti_resistance_ohm * info->bti_resistance_tolerance) / 100;
+	high = info->bti_resistance_ohm +
+		(info->bti_resistance_ohm * info->bti_resistance_tolerance) / 100;
+
+	return ((resistance >= low) && (resistance <= high));
+}
+EXPORT_SYMBOL_GPL(power_supply_battery_bti_in_range);
+
 int power_supply_get_property(struct power_supply *psy,
 			    enum power_supply_property psp,
 			    union power_supply_propval *val)