[PATCH v3 3/3] hwmon: (applesmc) add charge_control_end_threshold support
From: Jordan Brough
Date: Wed Sep 30 2026 - 19:45:55 EST
Intel MacBooks keep the battery charge limit in the SMC key BCLM, and
some also have BFCL, which sets the threshold for the MagSafe LED.
Expose BCLM as charge_control_end_threshold through the power_supply
extension API, attached to the battery with the ACPI battery hooks.
Below 100%, BFCL is set 3 points below BCLM so that the LED turns green
before charging stops. On a MacBookAir6,2 the LED turns green about 0.75
points above BFCL and charging stops about 0.25 points below BCLM, so
this leaves about 2 points; other models were not measured. BFCL is only
written if the SMC has the key, and a failure to write it is only logged.
The SMC ignores BCLM values it does not accept but still reports
success, so read the value back and return -EINVAL if it was not applied.
Limits below 20 are rejected. 20 is known to work and 10 is not accepted
by the SMC on the hardware tested; the exact floor in between was not
determined.
Serialize the writes with applesmc_bat_lock, and notify userspace of
changes with power_supply_changed().
Tested-by: Michal Szpakowski <michi.szpakowski@xxxxxxxxx>
Signed-off-by: Jordan Brough <jordan@xxxxxxxxxx>
---
drivers/hwmon/Kconfig | 1 +
drivers/hwmon/applesmc.c | 146 +++++++++++++++++++++++++++++++++++++++
2 files changed, 147 insertions(+)
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index fecff8610ea8..d627b4cf11e5 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -380,6 +380,7 @@ config SENSORS_FAM15H_POWER
config SENSORS_APPLESMC
tristate "Apple SMC (Motion sensor, light sensor, keyboard backlight)"
depends on INPUT && X86
+ depends on POWER_SUPPLY || POWER_SUPPLY=n
select NEW_LEDS
select LEDS_CLASS
help
diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index d0baa10502f7..b6d9ca302730 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -33,6 +33,8 @@
#include <linux/workqueue.h>
#include <linux/err.h>
#include <linux/bits.h>
+#include <linux/power_supply.h>
+#include <acpi/battery.h>
#include <asm/barrier.h>
/* data port used by Apple SMC */
@@ -76,6 +78,20 @@
#define TEMP_SENSOR_TYPE "sp78"
+/*
+ * BCLM caps charging at a percentage. BFCL only sets when the charging LED
+ * switches from orange to green.
+ */
+#define BATTERY_CHARGE_LIMIT_KEY "BCLM" /* r/w ui8 */
+#define BATTERY_CHARGE_LIMIT_LED_KEY "BFCL" /* r/w ui8 */
+
+/*
+ * Points kept between BCLM and BFCL so the LED turns green before charging
+ * stops. Measured on a MacBookAir6,2, where a margin of 1 only just ties;
+ * 3 leaves headroom for other models.
+ */
+#define APPLESMC_BATTERY_CHARGE_LIMIT_LED_MARGIN 3
+
/* List of keys used to read/write fan speeds */
static const char *const fan_speed_fmt[] = {
"F%dAc", /* actual speed */
@@ -131,6 +147,8 @@ static struct applesmc_registers {
int num_light_sensors; /* number of light sensors */
bool has_accelerometer; /* has motion sensor */
bool has_key_backlight; /* has keyboard backlight */
+ bool has_battery_charge_limit; /* has BCLM battery charge limit */
+ bool has_battery_charge_limit_led; /* has BFCL MagSafe LED charge limit */
bool init_complete; /* true when fully initialized */
struct applesmc_entry *cache; /* cached key entries */
const char **index; /* temperature key index */
@@ -633,6 +651,12 @@ static int applesmc_init_smcreg_try(void)
if (ret)
return ret;
ret = applesmc_has_key(BACKLIGHT_KEY, &s->has_key_backlight);
+ if (ret)
+ return ret;
+ ret = applesmc_has_key(BATTERY_CHARGE_LIMIT_KEY, &s->has_battery_charge_limit);
+ if (ret)
+ return ret;
+ ret = applesmc_has_key(BATTERY_CHARGE_LIMIT_LED_KEY, &s->has_battery_charge_limit_led);
if (ret)
return ret;
@@ -724,6 +748,120 @@ static struct platform_driver applesmc_driver = {
},
};
+static DEFINE_MUTEX(applesmc_bat_lock);
+
+static int applesmc_bat_get_property(struct power_supply *psy,
+ const struct power_supply_ext *ext,
+ void *data,
+ enum power_supply_property psp,
+ union power_supply_propval *val)
+{
+ u8 limit;
+ int ret;
+
+ switch (psp) {
+ case POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD:
+ scoped_guard(mutex, &applesmc_bat_lock)
+ ret = applesmc_read_key(BATTERY_CHARGE_LIMIT_KEY, &limit, 1);
+ if (ret)
+ return ret;
+ val->intval = limit;
+ return 0;
+ default:
+ return -EINVAL;
+ }
+}
+
+static int applesmc_bat_set_property(struct power_supply *psy,
+ const struct power_supply_ext *ext,
+ void *data,
+ enum power_supply_property psp,
+ const union power_supply_propval *val)
+{
+ u8 limit, led_limit, readback;
+ int ret;
+
+ switch (psp) {
+ case POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD:
+ /* 20 is known to work and 10 is not accepted; the floor is unknown. */
+ if (val->intval < 20 || val->intval > 100)
+ return -EINVAL;
+
+ limit = val->intval;
+ /* At 100% charging is never cut short, so no margin is needed. */
+ led_limit = limit < 100 ?
+ limit - APPLESMC_BATTERY_CHARGE_LIMIT_LED_MARGIN : limit;
+
+ scoped_guard(mutex, &applesmc_bat_lock) {
+ ret = applesmc_write_key(BATTERY_CHARGE_LIMIT_KEY, &limit, 1);
+ if (ret)
+ return ret;
+
+ /*
+ * The SMC silently ignores values it does not accept and
+ * still reports success, so read the limit back.
+ */
+ ret = applesmc_read_key(BATTERY_CHARGE_LIMIT_KEY, &readback, 1);
+ if (ret)
+ return ret;
+ if (readback != limit)
+ return -EINVAL;
+
+ if (smcreg.has_battery_charge_limit_led) {
+ ret = applesmc_write_key(BATTERY_CHARGE_LIMIT_LED_KEY,
+ &led_limit, 1);
+ /* BCLM is already applied, so this is not an error. */
+ if (ret)
+ pr_warn("failed to update MagSafe LED threshold (BFCL): %d\n",
+ ret);
+ }
+ }
+
+ power_supply_changed(psy);
+ return 0;
+ default:
+ return -EINVAL;
+ }
+}
+
+static int applesmc_bat_property_is_writeable(struct power_supply *psy,
+ const struct power_supply_ext *ext,
+ void *data,
+ enum power_supply_property psp)
+{
+ return psp == POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD;
+}
+
+static const enum power_supply_property applesmc_bat_props[] = {
+ POWER_SUPPLY_PROP_CHARGE_CONTROL_END_THRESHOLD,
+};
+
+static const struct power_supply_ext applesmc_bat_ext = {
+ .name = "applesmc-charge-control",
+ .properties = applesmc_bat_props,
+ .num_properties = ARRAY_SIZE(applesmc_bat_props),
+ .get_property = applesmc_bat_get_property,
+ .set_property = applesmc_bat_set_property,
+ .property_is_writeable = applesmc_bat_property_is_writeable,
+};
+
+static int applesmc_bat_add(struct power_supply *battery, struct acpi_battery_hook *hook)
+{
+ return power_supply_register_extension(battery, &applesmc_bat_ext, &pdev->dev, NULL);
+}
+
+static int applesmc_bat_remove(struct power_supply *battery, struct acpi_battery_hook *hook)
+{
+ power_supply_unregister_extension(battery, &applesmc_bat_ext);
+ return 0;
+}
+
+static struct acpi_battery_hook applesmc_bat_hook = {
+ .name = "Apple SMC Battery Charge Control",
+ .add_battery = applesmc_bat_add,
+ .remove_battery = applesmc_bat_remove,
+};
+
/*
* applesmc_calibrate - Set our "resting" values. Callers must
* hold applesmc_lock.
@@ -1563,6 +1701,10 @@ static int __init applesmc_init(void)
goto out_light_ledclass;
}
+ if (smcreg.has_battery_charge_limit &&
+ IS_REACHABLE(CONFIG_ACPI_BATTERY_HOOKS))
+ acpi_battery_hook_register(&applesmc_bat_hook);
+
return 0;
out_light_ledclass:
@@ -1589,6 +1731,10 @@ static int __init applesmc_init(void)
static void __exit applesmc_exit(void)
{
+ if (smcreg.has_battery_charge_limit &&
+ IS_REACHABLE(CONFIG_ACPI_BATTERY_HOOKS))
+ acpi_battery_hook_unregister(&applesmc_bat_hook);
+
hwmon_device_unregister(hwmon_dev);
applesmc_release_key_backlight();
applesmc_release_light_sensor();
--
2.56.0