i2c: aspeed: added driver for Aspeed I2C

Added initial master support for Aspeed I2C controller. Supports
fourteen busses present in AST24XX and AST25XX BMC SoCs by Aspeed.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

3 files changed, 701 insertions, 0 deletions

diff --git a/drivers/i2c/busses/Kconfig b/drivers/i2c/busses/Kconfig
index ed7106e48ba4..ceb4df4e92ab 100644
--- a/drivers/i2c/busses/Kconfig
+++ b/drivers/i2c/busses/Kconfig
@@ -328,6 +328,16 @@ config I2C_POWERMAC
 
 comment "I2C system bus drivers (mostly embedded / system-on-chip)"
 
+config I2C_ASPEED
+	tristate "Aspeed I2C Controller"
+	depends on ARCH_ASPEED || COMPILE_TEST
+	help
+	  If you say yes to this option, support will be included for the
+	  Aspeed I2C controller.
+
+	  This driver can also be built as a module.  If so, the module
+	  will be called i2c-aspeed.
+
 config I2C_AT91
 	tristate "Atmel AT91 I2C Two-Wire interface (TWI)"
 	depends on ARCH_AT91
diff --git a/drivers/i2c/busses/Makefile b/drivers/i2c/busses/Makefile
index c89a4314c563..a43f28819a8c 100644
--- a/drivers/i2c/busses/Makefile
+++ b/drivers/i2c/busses/Makefile
@@ -29,6 +29,7 @@ obj-$(CONFIG_I2C_HYDRA)		+= i2c-hydra.o
 obj-$(CONFIG_I2C_POWERMAC)	+= i2c-powermac.o
 
 # Embedded system I2C/SMBus host controller drivers
+obj-$(CONFIG_I2C_ASPEED)	+= i2c-aspeed.o
 obj-$(CONFIG_I2C_AT91)		+= i2c-at91.o
 obj-$(CONFIG_I2C_AU1550)	+= i2c-au1550.o
 obj-$(CONFIG_I2C_AXXIA)		+= i2c-axxia.o
diff --git a/drivers/i2c/busses/i2c-aspeed.c b/drivers/i2c/busses/i2c-aspeed.c
new file mode 100644
index 000000000000..55f241f2cfcc
--- /dev/null
+++ b/drivers/i2c/busses/i2c-aspeed.c
@@ -0,0 +1,690 @@
+/*
+ *  Aspeed 24XX/25XX I2C Controller.
+ *
+ *  Copyright (C) 2012-2017 ASPEED Technology Inc.
+ *  Copyright 2017 IBM Corporation
+ *  Copyright 2017 Google, Inc.
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License version 2 as
+ *  published by the Free Software Foundation.
+ */
+
+#include <linux/clk.h>
+#include <linux/completion.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/i2c.h>
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/io.h>
+#include <linux/irq.h>
+#include <linux/irqchip/chained_irq.h>
+#include <linux/irqdomain.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of_address.h>
+#include <linux/of_irq.h>
+#include <linux/of_platform.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+/* I2C Register */
+#define ASPEED_I2C_FUN_CTRL_REG				0x00
+#define ASPEED_I2C_AC_TIMING_REG1			0x04
+#define ASPEED_I2C_AC_TIMING_REG2			0x08
+#define ASPEED_I2C_INTR_CTRL_REG			0x0c
+#define ASPEED_I2C_INTR_STS_REG				0x10
+#define ASPEED_I2C_CMD_REG				0x14
+#define ASPEED_I2C_DEV_ADDR_REG				0x18
+#define ASPEED_I2C_BYTE_BUF_REG				0x20
+
+/* Global Register Definition */
+/* 0x00 : I2C Interrupt Status Register  */
+/* 0x08 : I2C Interrupt Target Assignment  */
+
+/* Device Register Definition */
+/* 0x00 : I2CD Function Control Register  */
+#define ASPEED_I2CD_MULTI_MASTER_DIS			BIT(15)
+#define ASPEED_I2CD_SDA_DRIVE_1T_EN			BIT(8)
+#define ASPEED_I2CD_M_SDA_DRIVE_1T_EN			BIT(7)
+#define ASPEED_I2CD_M_HIGH_SPEED_EN			BIT(6)
+#define ASPEED_I2CD_MASTER_EN				BIT(0)
+
+/* 0x04 : I2CD Clock and AC Timing Control Register #1 */
+#define ASPEED_I2CD_TIME_SCL_HIGH_SHIFT			16
+#define ASPEED_I2CD_TIME_SCL_HIGH_MASK			GENMASK(19, 16)
+#define ASPEED_I2CD_TIME_SCL_LOW_SHIFT			12
+#define ASPEED_I2CD_TIME_SCL_LOW_MASK			GENMASK(15, 12)
+#define ASPEED_I2CD_TIME_BASE_DIVISOR_MASK		GENMASK(3, 0)
+#define ASPEED_I2CD_TIME_SCL_REG_MAX			GENMASK(3, 0)
+/* 0x08 : I2CD Clock and AC Timing Control Register #2 */
+#define ASPEED_NO_TIMEOUT_CTRL				0
+
+/* 0x0c : I2CD Interrupt Control Register &
+ * 0x10 : I2CD Interrupt Status Register
+ *
+ * These share bit definitions, so use the same values for the enable &
+ * status bits.
+ */
+#define ASPEED_I2CD_INTR_SDA_DL_TIMEOUT			BIT(14)
+#define ASPEED_I2CD_INTR_BUS_RECOVER_DONE		BIT(13)
+#define ASPEED_I2CD_INTR_SCL_TIMEOUT			BIT(6)
+#define ASPEED_I2CD_INTR_ABNORMAL			BIT(5)
+#define ASPEED_I2CD_INTR_NORMAL_STOP			BIT(4)
+#define ASPEED_I2CD_INTR_ARBIT_LOSS			BIT(3)
+#define ASPEED_I2CD_INTR_RX_DONE			BIT(2)
+#define ASPEED_I2CD_INTR_TX_NAK				BIT(1)
+#define ASPEED_I2CD_INTR_TX_ACK				BIT(0)
+#define ASPEED_I2CD_INTR_ALL						       \
+		(ASPEED_I2CD_INTR_SDA_DL_TIMEOUT |			       \
+		 ASPEED_I2CD_INTR_BUS_RECOVER_DONE |			       \
+		 ASPEED_I2CD_INTR_SCL_TIMEOUT |				       \
+		 ASPEED_I2CD_INTR_ABNORMAL |				       \
+		 ASPEED_I2CD_INTR_NORMAL_STOP |				       \
+		 ASPEED_I2CD_INTR_ARBIT_LOSS |				       \
+		 ASPEED_I2CD_INTR_RX_DONE |				       \
+		 ASPEED_I2CD_INTR_TX_NAK |				       \
+		 ASPEED_I2CD_INTR_TX_ACK)
+
+/* 0x14 : I2CD Command/Status Register   */
+#define ASPEED_I2CD_SCL_LINE_STS			BIT(18)
+#define ASPEED_I2CD_SDA_LINE_STS			BIT(17)
+#define ASPEED_I2CD_BUS_BUSY_STS			BIT(16)
+#define ASPEED_I2CD_BUS_RECOVER_CMD			BIT(11)
+
+/* Command Bit */
+#define ASPEED_I2CD_M_STOP_CMD				BIT(5)
+#define ASPEED_I2CD_M_S_RX_CMD_LAST			BIT(4)
+#define ASPEED_I2CD_M_RX_CMD				BIT(3)
+#define ASPEED_I2CD_S_TX_CMD				BIT(2)
+#define ASPEED_I2CD_M_TX_CMD				BIT(1)
+#define ASPEED_I2CD_M_START_CMD				BIT(0)
+
+enum aspeed_i2c_master_state {
+	ASPEED_I2C_MASTER_START,
+	ASPEED_I2C_MASTER_TX_FIRST,
+	ASPEED_I2C_MASTER_TX,
+	ASPEED_I2C_MASTER_RX_FIRST,
+	ASPEED_I2C_MASTER_RX,
+	ASPEED_I2C_MASTER_STOP,
+	ASPEED_I2C_MASTER_INACTIVE,
+};
+
+struct aspeed_i2c_bus {
+	struct i2c_adapter		adap;
+	struct device			*dev;
+	void __iomem			*base;
+	/* Synchronizes I/O mem access to base. */
+	spinlock_t			lock;
+	struct completion		cmd_complete;
+	unsigned long			parent_clk_frequency;
+	u32				bus_frequency;
+	/* Transaction state. */
+	enum aspeed_i2c_master_state	master_state;
+	struct i2c_msg			*msgs;
+	size_t				buf_index;
+	size_t				msgs_index;
+	size_t				msgs_count;
+	bool				send_stop;
+	int				cmd_err;
+	/* Protected only by i2c_lock_bus */
+	int				master_xfer_result;
+};
+
+static int aspeed_i2c_reset(struct aspeed_i2c_bus *bus);
+
+static int aspeed_i2c_recover_bus(struct aspeed_i2c_bus *bus)
+{
+	unsigned long time_left, flags;
+	int ret = 0;
+	u32 command;
+
+	spin_lock_irqsave(&bus->lock, flags);
+	command = readl(bus->base + ASPEED_I2C_CMD_REG);
+
+	if (command & ASPEED_I2CD_SDA_LINE_STS) {
+		/* Bus is idle: no recovery needed. */
+		if (command & ASPEED_I2CD_SCL_LINE_STS)
+			goto out;
+		dev_dbg(bus->dev, "SCL hung (state %x), attempting recovery\n",
+			command);
+
+		reinit_completion(&bus->cmd_complete);
+		writel(ASPEED_I2CD_M_STOP_CMD, bus->base + ASPEED_I2C_CMD_REG);
+		spin_unlock_irqrestore(&bus->lock, flags);
+
+		time_left = wait_for_completion_timeout(
+				&bus->cmd_complete, bus->adap.timeout);
+
+		spin_lock_irqsave(&bus->lock, flags);
+		if (time_left == 0)
+			goto reset_out;
+		else if (bus->cmd_err)
+			goto reset_out;
+		/* Recovery failed. */
+		else if (!(readl(bus->base + ASPEED_I2C_CMD_REG) &
+			   ASPEED_I2CD_SCL_LINE_STS))
+			goto reset_out;
+	/* Bus error. */
+	} else {
+		dev_dbg(bus->dev, "SDA hung (state %x), attempting recovery\n",
+			command);
+
+		reinit_completion(&bus->cmd_complete);
+		/* Writes 1 to 8 SCL clock cycles until SDA is released. */
+		writel(ASPEED_I2CD_BUS_RECOVER_CMD,
+		       bus->base + ASPEED_I2C_CMD_REG);
+		spin_unlock_irqrestore(&bus->lock, flags);
+
+		time_left = wait_for_completion_timeout(
+				&bus->cmd_complete, bus->adap.timeout);
+
+		spin_lock_irqsave(&bus->lock, flags);
+		if (time_left == 0)
+			goto reset_out;
+		else if (bus->cmd_err)
+			goto reset_out;
+		/* Recovery failed. */
+		else if (!(readl(bus->base + ASPEED_I2C_CMD_REG) &
+			   ASPEED_I2CD_SDA_LINE_STS))
+			goto reset_out;
+	}
+
+out:
+	spin_unlock_irqrestore(&bus->lock, flags);
+
+	return ret;
+
+reset_out:
+	spin_unlock_irqrestore(&bus->lock, flags);
+
+	return aspeed_i2c_reset(bus);
+}
+
+/* precondition: bus.lock has been acquired. */
+static void aspeed_i2c_do_start(struct aspeed_i2c_bus *bus)
+{
+	u32 command = ASPEED_I2CD_M_START_CMD | ASPEED_I2CD_M_TX_CMD;
+	struct i2c_msg *msg = &bus->msgs[bus->msgs_index];
+	u8 slave_addr = msg->addr << 1;
+
+	bus->master_state = ASPEED_I2C_MASTER_START;
+	bus->buf_index = 0;
+
+	if (msg->flags & I2C_M_RD) {
+		slave_addr |= 1;
+		command |= ASPEED_I2CD_M_RX_CMD;
+		/* Need to let the hardware know to NACK after RX. */
+		if (msg->len == 1 && !(msg->flags & I2C_M_RECV_LEN))
+			command |= ASPEED_I2CD_M_S_RX_CMD_LAST;
+	}
+
+	writel(slave_addr, bus->base + ASPEED_I2C_BYTE_BUF_REG);
+	writel(command, bus->base + ASPEED_I2C_CMD_REG);
+}
+
+/* precondition: bus.lock has been acquired. */
+static void aspeed_i2c_do_stop(struct aspeed_i2c_bus *bus)
+{
+	bus->master_state = ASPEED_I2C_MASTER_STOP;
+	writel(ASPEED_I2CD_M_STOP_CMD, bus->base + ASPEED_I2C_CMD_REG);
+}
+
+/* precondition: bus.lock has been acquired. */
+static void aspeed_i2c_next_msg_or_stop(struct aspeed_i2c_bus *bus)
+{
+	if (bus->msgs_index + 1 < bus->msgs_count) {
+		bus->msgs_index++;
+		aspeed_i2c_do_start(bus);
+	} else {
+		aspeed_i2c_do_stop(bus);
+	}
+}
+
+static int aspeed_i2c_is_irq_error(u32 irq_status)
+{
+	if (irq_status & ASPEED_I2CD_INTR_ARBIT_LOSS)
+		return -EAGAIN;
+	if (irq_status & (ASPEED_I2CD_INTR_SDA_DL_TIMEOUT |
+			  ASPEED_I2CD_INTR_SCL_TIMEOUT))
+		return -EBUSY;
+	if (irq_status & (ASPEED_I2CD_INTR_ABNORMAL))
+		return -EPROTO;
+
+	return 0;
+}
+
+static bool aspeed_i2c_master_irq(struct aspeed_i2c_bus *bus)
+{
+	u32 irq_status, status_ack = 0, command = 0;
+	struct i2c_msg *msg;
+	u8 recv_byte;
+	int ret;
+
+	spin_lock(&bus->lock);
+	irq_status = readl(bus->base + ASPEED_I2C_INTR_STS_REG);
+	/* Ack all interrupt bits. */
+	writel(irq_status, bus->base + ASPEED_I2C_INTR_STS_REG);
+
+	if (irq_status & ASPEED_I2CD_INTR_BUS_RECOVER_DONE) {
+		bus->master_state = ASPEED_I2C_MASTER_INACTIVE;
+		status_ack |= ASPEED_I2CD_INTR_BUS_RECOVER_DONE;
+		goto out_complete;
+	}
+
+	/*
+	 * We encountered an interrupt that reports an error: the hardware
+	 * should clear the command queue effectively taking us back to the
+	 * INACTIVE state.
+	 */
+	ret = aspeed_i2c_is_irq_error(irq_status);
+	if (ret < 0) {
+		dev_dbg(bus->dev, "received error interrupt: 0x%08x",
+			irq_status);
+		bus->cmd_err = ret;
+		bus->master_state = ASPEED_I2C_MASTER_INACTIVE;
+		goto out_complete;
+	}
+
+	/* We are in an invalid state; reset bus to a known state. */
+	if (!bus->msgs && bus->master_state != ASPEED_I2C_MASTER_STOP) {
+		dev_err(bus->dev, "bus in unknown state");
+		bus->cmd_err = -EIO;
+		aspeed_i2c_do_stop(bus);
+		goto out_no_complete;
+	}
+	msg = &bus->msgs[bus->msgs_index];
+
+	/*
+	 * START is a special case because we still have to handle a subsequent
+	 * TX or RX immediately after we handle it, so we handle it here and
+	 * then update the state and handle the new state below.
+	 */
+	if (bus->master_state == ASPEED_I2C_MASTER_START) {
+		if (unlikely(!(irq_status & ASPEED_I2CD_INTR_TX_ACK))) {
+			pr_devel("no slave present at %02x", msg->addr);
+			status_ack |= ASPEED_I2CD_INTR_TX_NAK;
+			bus->cmd_err = -ENXIO;
+			aspeed_i2c_do_stop(bus);
+			goto out_no_complete;
+		}
+		status_ack |= ASPEED_I2CD_INTR_TX_ACK;
+		if (msg->len == 0) { /* SMBUS_QUICK */
+			aspeed_i2c_do_stop(bus);
+			goto out_no_complete;
+		}
+		if (msg->flags & I2C_M_RD)
+			bus->master_state = ASPEED_I2C_MASTER_RX_FIRST;
+		else
+			bus->master_state = ASPEED_I2C_MASTER_TX_FIRST;
+	}
+
+	switch (bus->master_state) {
+	case ASPEED_I2C_MASTER_TX:
+		if (unlikely(irq_status & ASPEED_I2CD_INTR_TX_NAK)) {
+			dev_dbg(bus->dev, "slave NACKed TX");
+			status_ack |= ASPEED_I2CD_INTR_TX_NAK;
+			goto error_and_stop;
+		} else if (unlikely(!(irq_status & ASPEED_I2CD_INTR_TX_ACK))) {
+			dev_err(bus->dev, "slave failed to ACK TX");
+			goto error_and_stop;
+		}
+		status_ack |= ASPEED_I2CD_INTR_TX_ACK;
+		/* fallthrough intended */
+	case ASPEED_I2C_MASTER_TX_FIRST:
+		if (bus->buf_index < msg->len) {
+			bus->master_state = ASPEED_I2C_MASTER_TX;
+			writel(msg->buf[bus->buf_index++],
+			       bus->base + ASPEED_I2C_BYTE_BUF_REG);
+			writel(ASPEED_I2CD_M_TX_CMD,
+			       bus->base + ASPEED_I2C_CMD_REG);
+		} else {
+			aspeed_i2c_next_msg_or_stop(bus);
+		}
+		goto out_no_complete;
+	case ASPEED_I2C_MASTER_RX_FIRST:
+		/* RX may not have completed yet (only address cycle) */
+		if (!(irq_status & ASPEED_I2CD_INTR_RX_DONE))
+			goto out_no_complete;
+		/* fallthrough intended */
+	case ASPEED_I2C_MASTER_RX:
+		if (unlikely(!(irq_status & ASPEED_I2CD_INTR_RX_DONE))) {
+			dev_err(bus->dev, "master failed to RX");
+			goto error_and_stop;
+		}
+		status_ack |= ASPEED_I2CD_INTR_RX_DONE;
+
+		recv_byte = readl(bus->base + ASPEED_I2C_BYTE_BUF_REG) >> 8;
+		msg->buf[bus->buf_index++] = recv_byte;
+
+		if (msg->flags & I2C_M_RECV_LEN) {
+			if (unlikely(recv_byte > I2C_SMBUS_BLOCK_MAX)) {
+				bus->cmd_err = -EPROTO;
+				aspeed_i2c_do_stop(bus);
+				goto out_no_complete;
+			}
+			msg->len = recv_byte +
+					((msg->flags & I2C_CLIENT_PEC) ? 2 : 1);
+			msg->flags &= ~I2C_M_RECV_LEN;
+		}
+
+		if (bus->buf_index < msg->len) {
+			bus->master_state = ASPEED_I2C_MASTER_RX;
+			command = ASPEED_I2CD_M_RX_CMD;
+			if (bus->buf_index + 1 == msg->len)
+				command |= ASPEED_I2CD_M_S_RX_CMD_LAST;
+			writel(command, bus->base + ASPEED_I2C_CMD_REG);
+		} else {
+			aspeed_i2c_next_msg_or_stop(bus);
+		}
+		goto out_no_complete;
+	case ASPEED_I2C_MASTER_STOP:
+		if (unlikely(!(irq_status & ASPEED_I2CD_INTR_NORMAL_STOP))) {
+			dev_err(bus->dev, "master failed to STOP");
+			bus->cmd_err = -EIO;
+			/* Do not STOP as we have already tried. */
+		} else {
+			status_ack |= ASPEED_I2CD_INTR_NORMAL_STOP;
+		}
+
+		bus->master_state = ASPEED_I2C_MASTER_INACTIVE;
+		goto out_complete;
+	case ASPEED_I2C_MASTER_INACTIVE:
+		dev_err(bus->dev,
+			"master received interrupt 0x%08x, but is inactive",
+			irq_status);
+		bus->cmd_err = -EIO;
+		/* Do not STOP as we should be inactive. */
+		goto out_complete;
+	default:
+		WARN(1, "unknown master state\n");
+		bus->master_state = ASPEED_I2C_MASTER_INACTIVE;
+		bus->cmd_err = -EINVAL;
+		goto out_complete;
+	}
+error_and_stop:
+	bus->cmd_err = -EIO;
+	aspeed_i2c_do_stop(bus);
+	goto out_no_complete;
+out_complete:
+	bus->msgs = NULL;
+	if (bus->cmd_err)
+		bus->master_xfer_result = bus->cmd_err;
+	else
+		bus->master_xfer_result = bus->msgs_index + 1;
+	complete(&bus->cmd_complete);
+out_no_complete:
+	if (irq_status != status_ack)
+		dev_err(bus->dev,
+			"irq handled != irq. expected 0x%08x, but was 0x%08x\n",
+			irq_status, status_ack);
+	spin_unlock(&bus->lock);
+	return !!irq_status;
+}
+
+static irqreturn_t aspeed_i2c_bus_irq(int irq, void *dev_id)
+{
+	struct aspeed_i2c_bus *bus = dev_id;
+
+	return aspeed_i2c_master_irq(bus) ? IRQ_HANDLED : IRQ_NONE;
+}
+
+static int aspeed_i2c_master_xfer(struct i2c_adapter *adap,
+				  struct i2c_msg *msgs, int num)
+{
+	struct aspeed_i2c_bus *bus = i2c_get_adapdata(adap);
+	unsigned long time_left, flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&bus->lock, flags);
+	bus->cmd_err = 0;
+
+	/* If bus is busy, attempt recovery. We assume a single master
+	 * environment.
+	 */
+	if (readl(bus->base + ASPEED_I2C_CMD_REG) & ASPEED_I2CD_BUS_BUSY_STS) {
+		spin_unlock_irqrestore(&bus->lock, flags);
+		ret = aspeed_i2c_recover_bus(bus);
+		if (ret)
+			return ret;
+		spin_lock_irqsave(&bus->lock, flags);
+	}
+
+	bus->cmd_err = 0;
+	bus->msgs = msgs;
+	bus->msgs_index = 0;
+	bus->msgs_count = num;
+
+	reinit_completion(&bus->cmd_complete);
+	aspeed_i2c_do_start(bus);
+	spin_unlock_irqrestore(&bus->lock, flags);
+
+	time_left = wait_for_completion_timeout(&bus->cmd_complete,
+						bus->adap.timeout);
+
+	if (time_left == 0)
+		return -ETIMEDOUT;
+	else
+		return bus->master_xfer_result;
+}
+
+static u32 aspeed_i2c_functionality(struct i2c_adapter *adap)
+{
+	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL | I2C_FUNC_SMBUS_BLOCK_DATA;
+}
+
+static const struct i2c_algorithm aspeed_i2c_algo = {
+	.master_xfer	= aspeed_i2c_master_xfer,
+	.functionality	= aspeed_i2c_functionality,
+};
+
+static u32 aspeed_i2c_get_clk_reg_val(u32 divisor)
+{
+	u32 base_clk, clk_high, clk_low, tmp;
+
+	/*
+	 * The actual clock frequency of SCL is:
+	 *	SCL_freq = APB_freq / (base_freq * (SCL_high + SCL_low))
+	 *		 = APB_freq / divisor
+	 * where base_freq is a programmable clock divider; its value is
+	 *	base_freq = 1 << base_clk
+	 * SCL_high is the number of base_freq clock cycles that SCL stays high
+	 * and SCL_low is the number of base_freq clock cycles that SCL stays
+	 * low for a period of SCL.
+	 * The actual register has a minimum SCL_high and SCL_low minimum of 1;
+	 * thus, they start counting at zero. So
+	 *	SCL_high = clk_high + 1
+	 *	SCL_low	 = clk_low + 1
+	 * Thus,
+	 *	SCL_freq = APB_freq /
+	 *		((1 << base_clk) * (clk_high + 1 + clk_low + 1))
+	 * The documentation recommends clk_high >= 8 and clk_low >= 7 when
+	 * possible; this last constraint gives us the following solution:
+	 */
+	base_clk = divisor > 33 ? ilog2((divisor - 1) / 32) + 1 : 0;
+	tmp = divisor / (1 << base_clk);
+	clk_high = tmp / 2 + tmp % 2;
+	clk_low = tmp - clk_high;
+
+	clk_high -= 1;
+	clk_low -= 1;
+
+	return ((clk_high << ASPEED_I2CD_TIME_SCL_HIGH_SHIFT)
+		& ASPEED_I2CD_TIME_SCL_HIGH_MASK)
+			| ((clk_low << ASPEED_I2CD_TIME_SCL_LOW_SHIFT)
+			   & ASPEED_I2CD_TIME_SCL_LOW_MASK)
+			| (base_clk & ASPEED_I2CD_TIME_BASE_DIVISOR_MASK);
+}
+
+/* precondition: bus.lock has been acquired. */
+static int aspeed_i2c_init_clk(struct aspeed_i2c_bus *bus)
+{
+	u32 divisor, clk_reg_val;
+
+	divisor = bus->parent_clk_frequency / bus->bus_frequency;
+	clk_reg_val = aspeed_i2c_get_clk_reg_val(divisor);
+	writel(clk_reg_val, bus->base + ASPEED_I2C_AC_TIMING_REG1);
+	writel(ASPEED_NO_TIMEOUT_CTRL, bus->base + ASPEED_I2C_AC_TIMING_REG2);
+
+	return 0;
+}
+
+/* precondition: bus.lock has been acquired. */
+static int aspeed_i2c_init(struct aspeed_i2c_bus *bus,
+			     struct platform_device *pdev)
+{
+	u32 fun_ctrl_reg = ASPEED_I2CD_MASTER_EN;
+	int ret;
+
+	/* Disable everything. */
+	writel(0, bus->base + ASPEED_I2C_FUN_CTRL_REG);
+
+	ret = aspeed_i2c_init_clk(bus);
+	if (ret < 0)
+		return ret;
+
+	if (!of_property_read_bool(pdev->dev.of_node, "multi-master"))
+		fun_ctrl_reg |= ASPEED_I2CD_MULTI_MASTER_DIS;
+
+	/* Enable Master Mode */
+	writel(readl(bus->base + ASPEED_I2C_FUN_CTRL_REG) | fun_ctrl_reg,
+	       bus->base + ASPEED_I2C_FUN_CTRL_REG);
+
+	/* Set interrupt generation of I2C controller */
+	writel(ASPEED_I2CD_INTR_ALL, bus->base + ASPEED_I2C_INTR_CTRL_REG);
+
+	return 0;
+}
+
+static int aspeed_i2c_reset(struct aspeed_i2c_bus *bus)
+{
+	struct platform_device *pdev = to_platform_device(bus->dev);
+	unsigned long flags;
+	int ret;
+
+	spin_lock_irqsave(&bus->lock, flags);
+
+	/* Disable and ack all interrupts. */
+	writel(0, bus->base + ASPEED_I2C_INTR_CTRL_REG);
+	writel(0xffffffff, bus->base + ASPEED_I2C_INTR_STS_REG);
+
+	ret = aspeed_i2c_init(bus, pdev);
+
+	spin_unlock_irqrestore(&bus->lock, flags);
+
+	return ret;
+}
+
+static int aspeed_i2c_probe_bus(struct platform_device *pdev)
+{
+	struct aspeed_i2c_bus *bus;
+	struct clk *parent_clk;
+	struct resource *res;
+	int irq, ret;
+
+	bus = devm_kzalloc(&pdev->dev, sizeof(*bus), GFP_KERNEL);
+	if (!bus)
+		return -ENOMEM;
+
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	bus->base = devm_ioremap_resource(&pdev->dev, res);
+	if (IS_ERR(bus->base))
+		return PTR_ERR(bus->base);
+
+	parent_clk = devm_clk_get(&pdev->dev, NULL);
+	if (IS_ERR(parent_clk))
+		return PTR_ERR(parent_clk);
+	bus->parent_clk_frequency = clk_get_rate(parent_clk);
+	/* We just need the clock rate, we don't actually use the clk object. */
+	devm_clk_put(&pdev->dev, parent_clk);
+
+	ret = of_property_read_u32(pdev->dev.of_node,
+				   "bus-frequency", &bus->bus_frequency);
+	if (ret < 0) {
+		dev_err(&pdev->dev,
+			"Could not read bus-frequency property\n");
+		bus->bus_frequency = 100000;
+	}
+
+	/* Initialize the I2C adapter */
+	spin_lock_init(&bus->lock);
+	init_completion(&bus->cmd_complete);
+	bus->adap.owner = THIS_MODULE;
+	bus->adap.retries = 0;
+	bus->adap.timeout = 5 * HZ;
+	bus->adap.algo = &aspeed_i2c_algo;
+	bus->adap.dev.parent = &pdev->dev;
+	bus->adap.dev.of_node = pdev->dev.of_node;
+	strlcpy(bus->adap.name, pdev->name, sizeof(bus->adap.name));
+	i2c_set_adapdata(&bus->adap, bus);
+
+	bus->dev = &pdev->dev;
+
+	/* Clean up any left over interrupt state. */
+	writel(0, bus->base + ASPEED_I2C_INTR_CTRL_REG);
+	writel(0xffffffff, bus->base + ASPEED_I2C_INTR_STS_REG);
+	/*
+	 * bus.lock does not need to be held because the interrupt handler has
+	 * not been enabled yet.
+	 */
+	ret = aspeed_i2c_init(bus, pdev);
+	if (ret < 0)
+		return ret;
+
+	irq = irq_of_parse_and_map(pdev->dev.of_node, 0);
+	ret = devm_request_irq(&pdev->dev, irq, aspeed_i2c_bus_irq,
+			       0, dev_name(&pdev->dev), bus);
+	if (ret < 0)
+		return ret;
+
+	ret = i2c_add_adapter(&bus->adap);
+	if (ret < 0)
+		return ret;
+
+	platform_set_drvdata(pdev, bus);
+
+	dev_info(bus->dev, "i2c bus %d registered, irq %d\n",
+		 bus->adap.nr, irq);
+
+	return 0;
+}
+
+static int aspeed_i2c_remove_bus(struct platform_device *pdev)
+{
+	struct aspeed_i2c_bus *bus = platform_get_drvdata(pdev);
+	unsigned long flags;
+
+	spin_lock_irqsave(&bus->lock, flags);
+
+	/* Disable everything. */
+	writel(0, bus->base + ASPEED_I2C_FUN_CTRL_REG);
+	writel(0, bus->base + ASPEED_I2C_INTR_CTRL_REG);
+
+	spin_unlock_irqrestore(&bus->lock, flags);
+
+	i2c_del_adapter(&bus->adap);
+
+	return 0;
+}
+
+static const struct of_device_id aspeed_i2c_bus_of_table[] = {
+	{ .compatible = "aspeed,ast2400-i2c-bus", },
+	{ .compatible = "aspeed,ast2500-i2c-bus", },
+	{ },
+};
+MODULE_DEVICE_TABLE(of, aspeed_i2c_bus_of_table);
+
+static struct platform_driver aspeed_i2c_bus_driver = {
+	.probe		= aspeed_i2c_probe_bus,
+	.remove		= aspeed_i2c_remove_bus,
+	.driver		= {
+		.name		= "aspeed-i2c-bus",
+		.of_match_table	= aspeed_i2c_bus_of_table,
+	},
+};
+module_platform_driver(aspeed_i2c_bus_driver);
+
+MODULE_AUTHOR("Brendan Higgins <brendanhiggins@google.com>");
+MODULE_DESCRIPTION("Aspeed I2C Bus Driver");
+MODULE_LICENSE("GPL v2");