[PATCH v2 08/11] serial: mvebu-uart: implement UART clock driver for configuring UART base clock

[Pali Rohár]

This patch implements a new device driver for controlling UART clocks on
Marvell Armada 3700 SoC. This device driver is loaded for devices which
match compatible string "marvell,armada-3700-uart-clock".

There are more pitfalls related to UART clocks. Both UARTs use same base
clock source. Also divisors for TBG base clock are shared between both
UARTs and are configured only from UART1 address space. Clocks can be
enabled / disabled separately for UART1 and UART2, but they are controlled
only from UART1 address space. Moreover Marvell Armada 3700 Functional
Specifications has swapped bits for enabling/disabling UART1 and UART2
clocks.

So driver for controlling UART2 needs to have access to UART1 address space
as UART1 address space contains some bits exclusively used by UART2 and
also bits which are shared for both UART1 and UART2.

For changing UART base clock (which controls both UARTs) during boot when
UART driver is not ready and only early console is active, is not simple
operation as it is required to also recalculate divisors to not change UART
baudrate used by early console. So for this operation UART1 clock driver
needs to access also into address space of UART2 where are registers for
UART2 divisors.

For these reasons, this new device driver for UART clocks does not use
ioremap_resource(), but only ioremap() to prevent resource conflicts
between UART clock driver and UART driver.

Shared between drivers are only two 4-bytes registers: UART Clock Control
and UART 2 Baud Rate Divisor. Access to these two registers are protected
by one spinlock to prevent any conflicts. Access is required only during
probe time, changing baudrate and during suspend/resume.

Hardware can be configured to use one of following clocks as UART base
clock: TBG-A-P, TBG-B-P, TBG-A-S, TBG-B-S, xtal. Not every clock is usable
for higher buadrates. In DT node can be specified any subset and kernel
choose the best one, which still supports required baudrate 9600. For
smooth boot log output it is needed to specify clock used by early console
otherwise garbage would be put on UART during probing for UART clock driver
and transitioning from early console to normal console.

This change is required to enable and configure TBG clock as a base clock
for UART. TBG clock is required to achieve higher baudrates than 230400.

Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
---
 drivers/tty/serial/Kconfig      |   1 +
 drivers/tty/serial/mvebu-uart.c | 519 +++++++++++++++++++++++++++++++-
 2 files changed, 518 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 0c4cd4a348f4..b3726797a0f5 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -1441,6 +1441,7 @@ config SERIAL_STM32_CONSOLE
 config SERIAL_MVEBU_UART
 	bool "Marvell EBU serial port support"
 	depends on ARCH_MVEBU || COMPILE_TEST
+	depends on COMMON_CLK
 	select SERIAL_CORE
 	help
 	  This driver is for Marvell EBU SoC's UART. If you have a machine
diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index 414e92064ac6..a31235add99f 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -8,12 +8,14 @@
 */
 
 #include <linux/clk.h>
+#include <linux/clk-provider.h>
 #include <linux/console.h>
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/init.h>
 #include <linux/io.h>
 #include <linux/iopoll.h>
+#include <linux/math64.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_device.h>
@@ -68,8 +70,31 @@
 #define  STAT_BRK_ERR		(STAT_BRK_DET | STAT_FRM_ERR \
 				 | STAT_PAR_ERR | STAT_OVR_ERR)
 
+/*
+ * Marvell Armada 3700 Functional Specifications describes that bit 21 of UART
+ * Clock Control register controls UART1 and bit 20 controls UART2. But in
+ * reality bit 21 controls UART2 and bit 20 controls UART1. This seems to be a
+ * bug in Marvell documentation. Hence following CLK_DIS macros are swapped.
+ */
+
 #define UART_BRDV		0x10
+/* These bits are located in UART1 address space and control UART2 */
+#define  UART2_CLK_DIS		BIT(21)
+/* These bits are located in UART1 address space and control UART1 */
+#define  UART1_CLK_DIS		BIT(20)
+/* These bits are located in UART1 address space and control both UARTs */
+#define  CLK_NO_XTAL		BIT(19)
+#define  CLK_TBG_DIV1_SHIFT	15
+#define  CLK_TBG_DIV1_MASK	0x7
+#define  CLK_TBG_DIV1_MAX	6
+#define  CLK_TBG_DIV2_SHIFT	12
+#define  CLK_TBG_DIV2_MASK	0x7
+#define  CLK_TBG_DIV2_MAX	6
+#define  CLK_TBG_SEL_SHIFT	10
+#define  CLK_TBG_SEL_MASK	0x3
+/* These bits are located in both UARTs address space */
 #define  BRDV_BAUD_MASK         0x3FF
+#define  BRDV_BAUD_MAX		BRDV_BAUD_MASK
 
 #define UART_OSAMP		0x14
 #define  OSAMP_DEFAULT_DIVISOR	16
@@ -153,6 +178,8 @@ static struct mvebu_uart *to_mvuart(struct uart_port *port)
 
 static struct uart_port mvebu_uart_ports[MVEBU_NR_UARTS];
 
+static DEFINE_SPINLOCK(mvebu_uart_lock);
+
 /* Core UART Driver Operations */
 static unsigned int mvebu_uart_tx_empty(struct uart_port *port)
 {
@@ -445,6 +472,7 @@ static void mvebu_uart_shutdown(struct uart_port *port)
 static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
 {
 	unsigned int d_divisor, m_divisor;
+	unsigned long flags;
 	u32 brdv, osamp;
 
 	if (!port->uartclk)
@@ -463,10 +491,12 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
 	m_divisor = OSAMP_DEFAULT_DIVISOR;
 	d_divisor = DIV_ROUND_CLOSEST(port->uartclk, baud * m_divisor);
 
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
 	brdv = readl(port->membase + UART_BRDV);
 	brdv &= ~BRDV_BAUD_MASK;
 	brdv |= d_divisor;
 	writel(brdv, port->membase + UART_BRDV);
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
 
 	osamp = readl(port->membase + UART_OSAMP);
 	osamp &= ~OSAMP_DIVISORS_MASK;
@@ -762,6 +792,7 @@ static int mvebu_uart_suspend(struct device *dev)
 {
 	struct mvebu_uart *mvuart = dev_get_drvdata(dev);
 	struct uart_port *port = mvuart->port;
+	unsigned long flags;
 
 	uart_suspend_port(&mvebu_uart_driver, port);
 
@@ -770,7 +801,9 @@ static int mvebu_uart_suspend(struct device *dev)
 	mvuart->pm_regs.ctrl = readl(port->membase + UART_CTRL(port));
 	mvuart->pm_regs.intr = readl(port->membase + UART_INTR(port));
 	mvuart->pm_regs.stat = readl(port->membase + UART_STAT);
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
 	mvuart->pm_regs.brdv = readl(port->membase + UART_BRDV);
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
 	mvuart->pm_regs.osamp = readl(port->membase + UART_OSAMP);
 
 	device_set_wakeup_enable(dev, true);
@@ -782,13 +815,16 @@ static int mvebu_uart_resume(struct device *dev)
 {
 	struct mvebu_uart *mvuart = dev_get_drvdata(dev);
 	struct uart_port *port = mvuart->port;
+	unsigned long flags;
 
 	writel(mvuart->pm_regs.rbr, port->membase + UART_RBR(port));
 	writel(mvuart->pm_regs.tsh, port->membase + UART_TSH(port));
 	writel(mvuart->pm_regs.ctrl, port->membase + UART_CTRL(port));
 	writel(mvuart->pm_regs.intr, port->membase + UART_INTR(port));
 	writel(mvuart->pm_regs.stat, port->membase + UART_STAT);
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
 	writel(mvuart->pm_regs.brdv, port->membase + UART_BRDV);
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
 	writel(mvuart->pm_regs.osamp, port->membase + UART_OSAMP);
 
 	uart_resume_port(&mvebu_uart_driver, port);
@@ -975,6 +1011,476 @@ static struct platform_driver mvebu_uart_platform_driver = {
 	},
 };
 
+/* This code is based on clk-fixed-factor.c driver and modified. */
+
+struct mvebu_uart_clock {
+	struct clk_hw clk_hw;
+	int clock_idx;
+	u32 pm_context_reg1;
+	u32 pm_context_reg2;
+};
+
+struct mvebu_uart_clock_base {
+	struct mvebu_uart_clock clocks[2];
+	unsigned int parent_rates[5];
+	int parent_idx;
+	unsigned int div;
+	void __iomem *reg1;
+	void __iomem *reg2;
+	bool configured;
+};
+
+#define PARENT_CLOCK_XTAL 4
+
+#define to_uart_clock(hw) container_of(hw, struct mvebu_uart_clock, clk_hw)
+#define to_uart_clock_base(uart_clock) container_of(uart_clock, \
+	struct mvebu_uart_clock_base, clocks[uart_clock->clock_idx])
+
+static int mvebu_uart_clock_prepare(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	unsigned int prev_clock_idx, prev_clock_rate, prev_d1d2;
+	unsigned int parent_clock_idx, parent_clock_rate;
+	unsigned long flags;
+	unsigned int d1, d2;
+	u64 divisor;
+	u32 val;
+
+	/*
+	 * This function just reconfigures UART Clock Control register (located
+	 * in UART1 address space which controls both UART1 and UART2) to
+	 * selected UART base clock and recalculate current UART1/UART2 divisors
+	 * in their address spaces, so final baudrate will not be changed by
+	 * switching UART base clock. This is required otherwise kernel boot log
+	 * stops working. It is needed to ensure that UART baudrate does not
+	 * change during this setup. It is one time operation, so based on
+	 * "configured" member this function is skipped on second call. Because
+	 * this UART Clock Control register (UART_BRDV) is shared between UART1
+	 * baudrate function, UART1 clock selector and UART2 clock selector,
+	 * every access to UART_BRDV (reg1) needs to be protected by lock.
+	 */
+
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
+
+	if (uart_clock_base->configured) {
+		spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+		return 0;
+	}
+
+	parent_clock_idx = uart_clock_base->parent_idx;
+	parent_clock_rate = uart_clock_base->parent_rates[parent_clock_idx];
+
+	val = readl(uart_clock_base->reg1);
+
+	if (uart_clock_base->div > CLK_TBG_DIV1_MAX) {
+		d1 = CLK_TBG_DIV1_MAX;
+		d2 = uart_clock_base->div / CLK_TBG_DIV1_MAX;
+	} else {
+		d1 = uart_clock_base->div;
+		d2 = 1;
+	}
+
+	if (val & CLK_NO_XTAL) {
+		prev_clock_idx = (val >> CLK_TBG_SEL_SHIFT) & CLK_TBG_SEL_MASK;
+		prev_d1d2 = ((val >> CLK_TBG_DIV1_SHIFT) & CLK_TBG_DIV1_MASK)
+			  * ((val >> CLK_TBG_DIV2_SHIFT) & CLK_TBG_DIV2_MASK);
+	} else {
+		prev_clock_idx = PARENT_CLOCK_XTAL;
+		prev_d1d2 = 1;
+	}
+
+	/* Note that uart_clock_base->parent_rates[i] may not be available */
+	prev_clock_rate = uart_clock_base->parent_rates[prev_clock_idx];
+
+	/* Recalculate UART1 divisor so UART1 baudrate does not change */
+	if (prev_clock_rate) {
+		divisor = DIV_U64_ROUND_CLOSEST((u64)(val & BRDV_BAUD_MASK) *
+						parent_clock_rate * prev_d1d2,
+						prev_clock_rate * d1 * d2);
+		if (divisor < 1)
+			divisor = 1;
+		else if (divisor > BRDV_BAUD_MAX)
+			divisor = BRDV_BAUD_MAX;
+		val = (val & ~BRDV_BAUD_MASK) | divisor;
+	}
+
+	if (parent_clock_idx != PARENT_CLOCK_XTAL) {
+		/* Do not use XTAL, select TBG clock and TBG d1 * d2 divisors */
+		val |= CLK_NO_XTAL;
+		val &= ~(CLK_TBG_DIV1_MASK << CLK_TBG_DIV1_SHIFT);
+		val |= d1 << CLK_TBG_DIV1_SHIFT;
+		val &= ~(CLK_TBG_DIV2_MASK << CLK_TBG_DIV2_SHIFT);
+		val |= d2 << CLK_TBG_DIV2_SHIFT;
+		val &= ~(CLK_TBG_SEL_MASK << CLK_TBG_SEL_SHIFT);
+		val |= parent_clock_idx << CLK_TBG_SEL_SHIFT;
+	} else {
+		/* Use XTAL, TBG bits are then ignored */
+		val &= ~CLK_NO_XTAL;
+	}
+
+	writel(val, uart_clock_base->reg1);
+
+	/* Recalculate UART2 divisor so UART2 baudrate does not change */
+	if (prev_clock_rate) {
+		val = readl(uart_clock_base->reg2);
+		divisor = DIV_U64_ROUND_CLOSEST((u64)(val & BRDV_BAUD_MASK) *
+						parent_clock_rate * prev_d1d2,
+						prev_clock_rate * d1 * d2);
+		if (divisor < 1)
+			divisor = 1;
+		else if (divisor > BRDV_BAUD_MAX)
+			divisor = BRDV_BAUD_MAX;
+		val = (val & ~BRDV_BAUD_MASK) | divisor;
+		writel(val, uart_clock_base->reg2);
+	}
+
+	uart_clock_base->configured = true;
+
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+
+	return 0;
+}
+
+static int mvebu_uart_clock_enable(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	unsigned long flags;
+	u32 val;
+
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
+
+	val = readl(uart_clock_base->reg1);
+
+	if (uart_clock->clock_idx == 0)
+		val &= ~UART1_CLK_DIS;
+	else
+		val &= ~UART2_CLK_DIS;
+
+	writel(val, uart_clock_base->reg1);
+
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+
+	return 0;
+}
+
+static void mvebu_uart_clock_disable(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	unsigned long flags;
+	u32 val;
+
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
+
+	val = readl(uart_clock_base->reg1);
+
+	if (uart_clock->clock_idx == 0)
+		val |= UART1_CLK_DIS;
+	else
+		val |= UART2_CLK_DIS;
+
+	writel(val, uart_clock_base->reg1);
+
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+}
+
+static int mvebu_uart_clock_is_enabled(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	u32 val;
+
+	val = readl(uart_clock_base->reg1);
+
+	if (uart_clock->clock_idx == 0)
+		return !(val & UART1_CLK_DIS);
+	else
+		return !(val & UART2_CLK_DIS);
+}
+
+static int mvebu_uart_clock_save_context(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
+	uart_clock->pm_context_reg1 = readl(uart_clock_base->reg1);
+	uart_clock->pm_context_reg2 = readl(uart_clock_base->reg2);
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+
+	return 0;
+}
+
+static void mvebu_uart_clock_restore_context(struct clk_hw *hw)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&mvebu_uart_lock, flags);
+	writel(uart_clock->pm_context_reg1, uart_clock_base->reg1);
+	writel(uart_clock->pm_context_reg2, uart_clock_base->reg2);
+	spin_unlock_irqrestore(&mvebu_uart_lock, flags);
+}
+
+static unsigned long mvebu_uart_clock_recalc_rate(struct clk_hw *hw,
+						  unsigned long parent_rate)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+
+	return parent_rate / uart_clock_base->div;
+}
+
+static long mvebu_uart_clock_round_rate(struct clk_hw *hw, unsigned long rate,
+					unsigned long *parent_rate)
+{
+	struct mvebu_uart_clock *uart_clock = to_uart_clock(hw);
+	struct mvebu_uart_clock_base *uart_clock_base =
+						to_uart_clock_base(uart_clock);
+
+	return *parent_rate / uart_clock_base->div;
+}
+
+static int mvebu_uart_clock_set_rate(struct clk_hw *hw, unsigned long rate,
+				     unsigned long parent_rate)
+{
+	/*
+	 * We must report success but we can do so unconditionally because
+	 * mvebu_uart_clock_round_rate returns values that ensure this call is a
+	 * nop.
+	 */
+
+	return 0;
+}
+
+static const struct clk_ops mvebu_uart_clock_ops = {
+	.prepare = mvebu_uart_clock_prepare,
+	.enable = mvebu_uart_clock_enable,
+	.disable = mvebu_uart_clock_disable,
+	.is_enabled = mvebu_uart_clock_is_enabled,
+	.save_context = mvebu_uart_clock_save_context,
+	.restore_context = mvebu_uart_clock_restore_context,
+	.round_rate = mvebu_uart_clock_round_rate,
+	.set_rate = mvebu_uart_clock_set_rate,
+	.recalc_rate = mvebu_uart_clock_recalc_rate,
+};
+
+static int mvebu_uart_clock_register(struct device *dev,
+				     struct mvebu_uart_clock *uart_clock,
+				     const char *name,
+				     const char *parent_name)
+{
+	struct clk_init_data init = { };
+
+	uart_clock->clk_hw.init = &init;
+
+	init.name = name;
+	init.ops = &mvebu_uart_clock_ops;
+	init.flags = 0;
+	init.num_parents = 1;
+	init.parent_names = &parent_name;
+
+	return devm_clk_hw_register(dev, &uart_clock->clk_hw);
+}
+
+static int mvebu_uart_clock_probe(struct platform_device *pdev)
+{
+	static const char *const uart_clk_names[] = { "uart_1", "uart_2" };
+	static const char *const parent_clk_names[] = { "TBG-A-P", "TBG-B-P",
+							"TBG-A-S", "TBG-B-S",
+							"xtal" };
+	struct clk *parent_clks[ARRAY_SIZE(parent_clk_names)];
+	struct mvebu_uart_clock_base *uart_clock_base;
+	struct clk_hw_onecell_data *hw_clk_data;
+	struct device *dev = &pdev->dev;
+	int i, parent_clk_idx, ret;
+	unsigned long div, rate;
+	struct resource *res;
+	unsigned int d1, d2;
+
+	BUILD_BUG_ON(ARRAY_SIZE(uart_clk_names) !=
+		     ARRAY_SIZE(uart_clock_base->clocks));
+	BUILD_BUG_ON(ARRAY_SIZE(parent_clk_names) !=
+		     ARRAY_SIZE(uart_clock_base->parent_rates));
+
+	uart_clock_base = devm_kzalloc(dev,
+				       sizeof(*uart_clock_base),
+				       GFP_KERNEL);
+	if (!uart_clock_base)
+		return -ENOMEM;
+
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!res) {
+		dev_err(dev, "Couldn't get first register\n");
+		return -ENOENT;
+	}
+
+	/*
+	 * UART Clock Control register (reg1 / UART_BRDV) is in address range
+	 * of UART1 (standard UART variant), controls clock source and dividers
+	 * for both UART1 and UART2 and is supplied via DT as first resource.
+	 * Therefore use ioremap() function rather than ioremap_resource() to
+	 * avoid conflicts with UART1 driver. Access to UART_BRDV is protected
+	 * by lock shared between clock and UART driver.
+	 */
+	uart_clock_base->reg1 = devm_ioremap(dev, res->start,
+					     resource_size(res));
+	if (IS_ERR(uart_clock_base->reg1))
+		return PTR_ERR(uart_clock_base->reg1);
+
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 1);
+	if (!res) {
+		dev_err(dev, "Couldn't get second register\n");
+		return -ENOENT;
+	}
+
+	/*
+	 * UART 2 Baud Rate Divisor register (reg2 / UART_BRDV) is in address
+	 * range of UART2 (extended UART variant), controls only one UART2
+	 * specific divider and is supplied via DT as second resource.
+	 * Therefore use ioremap() function rather than ioremap_resource() to
+	 * avoid conflicts with UART2 driver. Access to UART_BRDV is protected
+	 * by lock shared between clock and UART driver.
+	 */
+	uart_clock_base->reg2 = devm_ioremap(dev, res->start,
+					     resource_size(res));
+	if (IS_ERR(uart_clock_base->reg2))
+		return PTR_ERR(uart_clock_base->reg2);
+
+	hw_clk_data = devm_kzalloc(dev,
+				   struct_size(hw_clk_data, hws,
+					       ARRAY_SIZE(uart_clk_names)),
+				   GFP_KERNEL);
+	if (!hw_clk_data)
+		return -ENOMEM;
+
+	hw_clk_data->num = ARRAY_SIZE(uart_clk_names);
+	for (i = 0; i < ARRAY_SIZE(uart_clk_names); i++) {
+		hw_clk_data->hws[i] = &uart_clock_base->clocks[i].clk_hw;
+		uart_clock_base->clocks[i].clock_idx = i;
+	}
+
+	parent_clk_idx = -1;
+
+	for (i = 0; i < ARRAY_SIZE(parent_clk_names); i++) {
+		parent_clks[i] = devm_clk_get(dev, parent_clk_names[i]);
+		if (IS_ERR(parent_clks[i])) {
+			if (PTR_ERR(parent_clks[i]) == -EPROBE_DEFER)
+				return -EPROBE_DEFER;
+			dev_warn(dev, "Couldn't get the parent clock %s: %ld\n",
+				parent_clk_names[i], PTR_ERR(parent_clks[i]));
+			continue;
+		}
+
+		ret = clk_prepare_enable(parent_clks[i]);
+		if (ret) {
+			dev_warn(dev, "Couldn't enable parent clock %s: %d\n",
+				parent_clk_names[i], ret);
+			continue;
+		}
+		rate = clk_get_rate(parent_clks[i]);
+		uart_clock_base->parent_rates[i] = rate;
+
+		if (i != PARENT_CLOCK_XTAL) {
+			/*
+			 * Calculate the smallest TBG d1 and d2 divisors that
+			 * still can provide 9600 baudrate.
+			 */
+			d1 = DIV_ROUND_UP(rate, 9600 * OSAMP_DEFAULT_DIVISOR *
+						BRDV_BAUD_MAX);
+			if (d1 < 1)
+				d1 = 1;
+			else if (d1 > CLK_TBG_DIV1_MAX)
+				d1 = CLK_TBG_DIV1_MAX;
+
+			d2 = DIV_ROUND_UP(rate, 9600 * OSAMP_DEFAULT_DIVISOR *
+						BRDV_BAUD_MAX * d1);
+			if (d2 < 1)
+				d2 = 1;
+			else if (d2 > CLK_TBG_DIV2_MAX)
+				d2 = CLK_TBG_DIV2_MAX;
+		} else {
+			/*
+			 * When UART clock uses XTAL clock as a source then it
+			 * is not possible to use d1 and d2 divisors.
+			 */
+			d1 = d2 = 1;
+		}
+
+		/* Skip clock source which cannot provide 9600 baudrate */
+		if (rate > 9600 * OSAMP_DEFAULT_DIVISOR * BRDV_BAUD_MAX * d1 * d2)
+			continue;
+
+		/*
+		 * Choose TBG clock source with the smallest divisors. Use XTAL
+		 * clock source only in case TBG is not available as XTAL cannot
+		 * be used for baudrates higher than 230400.
+		 */
+		if (parent_clk_idx == -1 ||
+		    (i != PARENT_CLOCK_XTAL && div > d1 * d2)) {
+			parent_clk_idx = i;
+			div = d1 * d2;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(parent_clk_names); i++) {
+		if (i == parent_clk_idx || IS_ERR(parent_clks[i]))
+			continue;
+		clk_disable_unprepare(parent_clks[i]);
+		devm_clk_put(dev, parent_clks[i]);
+	}
+
+	if (parent_clk_idx == -1) {
+		dev_err(dev, "No usable parent clock\n");
+		return -ENOENT;
+	}
+
+	uart_clock_base->parent_idx = parent_clk_idx;
+	uart_clock_base->div = div;
+
+	dev_notice(dev, "Using parent clock %s as base UART clock\n",
+		   __clk_get_name(parent_clks[parent_clk_idx]));
+
+	for (i = 0; i < ARRAY_SIZE(uart_clk_names); i++) {
+		ret = mvebu_uart_clock_register(dev,
+				&uart_clock_base->clocks[i],
+				uart_clk_names[i],
+				__clk_get_name(parent_clks[parent_clk_idx]));
+		if (ret) {
+			dev_err(dev, "Can't register UART clock %d: %d\n",
+				i, ret);
+			return ret;
+		}
+	}
+
+	return devm_of_clk_add_hw_provider(dev, of_clk_hw_onecell_get,
+					   hw_clk_data);
+}
+
+static const struct of_device_id mvebu_uart_clock_of_match[] = {
+	{ .compatible = "marvell,armada-3700-uart-clock", },
+	{ }
+};
+
+static struct platform_driver mvebu_uart_clock_platform_driver = {
+	.probe = mvebu_uart_clock_probe,
+	.driver		= {
+		.name	= "mvebu-uart-clock",
+		.of_match_table = mvebu_uart_clock_of_match,
+	},
+};
+
 static int __init mvebu_uart_init(void)
 {
 	int ret;
@@ -983,10 +1489,19 @@ static int __init mvebu_uart_init(void)
 	if (ret)
 		return ret;
 
+	ret = platform_driver_register(&mvebu_uart_clock_platform_driver);
+	if (ret) {
+		uart_unregister_driver(&mvebu_uart_driver);
+		return ret;
+	}
+
 	ret = platform_driver_register(&mvebu_uart_platform_driver);
-	if (ret)
+	if (ret) {
+		platform_driver_unregister(&mvebu_uart_clock_platform_driver);
 		uart_unregister_driver(&mvebu_uart_driver);
+		return ret;
+	}
 
-	return ret;
+	return 0;
 }
 arch_initcall(mvebu_uart_init);
-- 
2.20.1