diff --git a/drivers/mfd/axp20x.c b/drivers/mfd/axp20x.c
index 20b95af..25115fe 100644
--- a/drivers/mfd/axp20x.c
+++ b/drivers/mfd/axp20x.c
@@ -31,6 +31,8 @@
 
 #define AXP20X_OFF	0x80
 
+#define AXP806_REG_ADDR_EXT_ADDR_SLAVE_MODE	BIT(4)
+
 static const char * const axp20x_model_names[] = {
 	"AXP152",
 	"AXP202",
@@ -861,6 +863,30 @@
 {
 	int ret;
 
+	/*
+	 * The AXP806 supports either master/standalone or slave mode.
+	 * Slave mode allows sharing the serial bus, even with multiple
+	 * AXP806 which all have the same hardware address.
+	 *
+	 * This is done with extra "serial interface address extension",
+	 * or AXP806_BUS_ADDR_EXT, and "register address extension", or
+	 * AXP806_REG_ADDR_EXT, registers. The former is read-only, with
+	 * 1 bit customizable at the factory, and 1 bit depending on the
+	 * state of an external pin. The latter is writable. The device
+	 * will only respond to operations to its other registers when
+	 * the these device addressing bits (in the upper 4 bits of the
+	 * registers) match.
+	 *
+	 * Since we only support an AXP806 chained to an AXP809 in slave
+	 * mode, and there isn't any existing hardware which uses AXP806
+	 * in master mode, or has 2 AXP806s in the same system, we can
+	 * just program the register address extension to the slave mode
+	 * address.
+	 */
+	if (axp20x->variant == AXP806_ID)
+		regmap_write(axp20x->regmap, AXP806_REG_ADDR_EXT,
+			     AXP806_REG_ADDR_EXT_ADDR_SLAVE_MODE);
+
 	ret = regmap_add_irq_chip(axp20x->regmap, axp20x->irq,
 			  IRQF_ONESHOT | IRQF_SHARED | axp20x->irq_flags,
 			   -1, axp20x->regmap_irq_chip, &axp20x->regmap_irqc);