zero-sugar: Power off if battery capacity <= 5%

Verify that the battery capacity is more than 5% before
booting the device.

board/reMarkable/zero-sugar/max77818_battery.c

@@ -10,6 +10,8 @@
 #define MAX77818_FG_CONFIG__FGCC_ENABLED				0x0800
 #define MAX77818_FG_CONFIG__FGCC_DISABLED				0x0000
 
+#define MAX77818_REG_FG_REPSOC 0x06
+
 static struct udevice *fgDev = NULL;
 
 int max77818_init_fg_device(void)
@@ -124,3 +126,48 @@ int max77818_restore_fgcc(bool restore_state)
 		return max77818_set_fgcc_state(false, NULL);
 	}
 }
+
+int max77818_get_battery_capacity(u8 *capacity)
+{
+	int ret;
+	u16 value;
+
+	if (!fgDev) {
+		ret = max77818_init_fg_device();
+		if (ret)
+			return ret;
+	}
+
+	ret = max77818_i2c_reg_read16(fgDev,
+				      MAX77818_REG_FG_REPSOC,
+				      &value);
+	if (ret)
+		return ret;
+
+	*capacity = (u8)(value >> 8);
+
+	return 0;
+}
+
+static int do_max77818_get_battery_capacity(cmd_tbl_t *cmdtp,
+					   int flag,
+					   int argc,
+					   char * const argv[])
+{
+	int ret;
+	u8 capacity;
+
+	ret = max77818_get_battery_capacity(&capacity);
+	if (ret)
+		return ret;
+
+	printf("Capacity: %u%%\n", capacity);
+
+	return 0;
+}
+
+U_BOOT_CMD(
+	max77818_get_battery_capacity, 1, 1, do_max77818_get_battery_capacity,
+	"Get battery state of charge",
+	""
+);

board/reMarkable/zero-sugar/max77818_battery.h

@@ -8,4 +8,6 @@ int max77818_read_fgcc_state(bool *state);
 int max77818_set_fgcc_state(bool enabled, bool *restore_state);
 int max77818_restore_fgcc(bool restore_state);
 
+int max77818_get_battery_capacity(u8 *capacity);
+
 #endif /* FG_INIT_H */

board/reMarkable/zero-sugar/zero-sugar.c

@@ -14,6 +14,7 @@
 #include "digitizer_init.h"
 #include "max77818.h"
 #include "max77818_charger.h"
+#include "max77818_battery.h"
 #include "serial_download_trap.h"
 
 #include <asm/arch/clock.h>
@@ -47,7 +48,6 @@
 #include <asm/setup.h>
 #include <asm/bootm.h>
 
-#define SNVS_BASE_ADDR 0x30370000
 #define SNVS_REG_LPCR SNVS_BASE_ADDR + 0x38
 #define SNVS_MASK_POWEROFF (BIT(5) | BIT(6))
 
@@ -102,6 +102,7 @@ static void power_perfs(void)
 static void init_charger(void)
 {
 	int ret;
+	u8 capacity;
 
 	printf("Enabling SAFEOUT1\n");
 	ret = max77818_enable_safeout1();
@@ -116,6 +117,19 @@ static void init_charger(void)
 		printf("%s: Failed to set charger config: %d\n",
 		       __func__, ret);
 	}
+
+	printf("Reading remaining battery capacity\n");
+	ret = max77818_get_battery_capacity(&capacity);
+	if (ret) {
+		printf("%s: Failed to read battery capacity: %d\n", __func__, ret);
+	}
+	else if (capacity < 5) {
+		printf("Battery too low (%u%% < 5%%), turning off\n", capacity);
+		snvs_poweroff();
+	}
+	else {
+		printf("Battery currently at %u%%\n", capacity);
+	}
 }
 
 static void save_serial(void)