clk: Add PWM clock driver

Some board designers, when running out of clock output pads, decide to
(mis)use PWM output pads to provide a clock to external components.
This driver supports this practice by providing an adapter between the
PWM and clock bindings in the device tree. As the PWM bindings specify
the period in the device tree, this is a fixed clock.
Tested-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Turquette <mturquette@linaro.org>

Documentation/devicetree/bindings/clock/pwm-clock.txt

+Binding for an external clock signal driven by a PWM pin.
+
+This binding uses the common clock binding[1] and the common PWM binding[2].
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+[2] Documentation/devicetree/bindings/pwm/pwm.txt
+
+Required properties:
+- compatible : shall be "pwm-clock".
+- #clock-cells : from common clock binding; shall be set to 0.
+- pwms : from common PWM binding; this determines the clock frequency
+  via the period given in the PWM specifier.
+
+Optional properties:
+- clock-output-names : From common clock binding.
+- clock-frequency : Exact output frequency, in case the PWM period
+  is not exact but was rounded to nanoseconds.
+
+Example:
+	clock {
+		compatible = "pwm-clock";
+		#clock-cells = <0>;
+		clock-frequency = <25000000>;
+		clock-output-names = "mipi_mclk";
+		pwms = <&pwm2 0 40>; /* 1 / 40 ns = 25 MHz */
+	};

drivers/clk/Kconfig

@@ -130,6 +130,13 @@ config COMMON_CLK_PALMAS
 	  This driver supports TI Palmas devices 32KHz output KG and KG_AUDIO
 	  using common clock framework.
 
+config COMMON_CLK_PWM
+	tristate "Clock driver for PWMs used as clock outputs"
+	depends on PWM
+	---help---
+	  Adapter driver so that any PWM output can be (mis)used as clock signal
+	  at 50% duty cycle.
+
 config COMMON_CLK_PXA
 	def_bool COMMON_CLK && ARCH_PXA
 	---help---

drivers/clk/Makefile

@@ -43,6 +43,7 @@ obj-$(CONFIG_ARCH_U300)			+= clk-u300.o
 obj-$(CONFIG_ARCH_VT8500)		+= clk-vt8500.o
 obj-$(CONFIG_COMMON_CLK_WM831X)		+= clk-wm831x.o
 obj-$(CONFIG_COMMON_CLK_XGENE)		+= clk-xgene.o
+obj-$(CONFIG_COMMON_CLK_PWM)		+= clk-pwm.o
 obj-$(CONFIG_COMMON_CLK_AT91)		+= at91/
 obj-$(CONFIG_ARCH_BCM_MOBILE)		+= bcm/
 obj-$(CONFIG_ARCH_BERLIN)		+= berlin/

drivers/clk/clk-pwm.c

+/*
+ * Copyright (C) 2014 Philipp Zabel, Pengutronix
+ *
+ * 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.
+ *
+ * PWM (mis)used as clock output
+ */
+#include <linux/clk-provider.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/pwm.h>
+
+struct clk_pwm {
+	struct clk_hw hw;
+	struct pwm_device *pwm;
+	u32 fixed_rate;
+};
+
+static inline struct clk_pwm *to_clk_pwm(struct clk_hw *hw)
+{
+	return container_of(hw, struct clk_pwm, hw);
+}
+
+static int clk_pwm_prepare(struct clk_hw *hw)
+{
+	struct clk_pwm *clk_pwm = to_clk_pwm(hw);
+
+	return pwm_enable(clk_pwm->pwm);
+}
+
+static void clk_pwm_unprepare(struct clk_hw *hw)
+{
+	struct clk_pwm *clk_pwm = to_clk_pwm(hw);
+
+	pwm_disable(clk_pwm->pwm);
+}
+
+static unsigned long clk_pwm_recalc_rate(struct clk_hw *hw,
+					 unsigned long parent_rate)
+{
+	struct clk_pwm *clk_pwm = to_clk_pwm(hw);
+
+	return clk_pwm->fixed_rate;
+}
+
+static const struct clk_ops clk_pwm_ops = {
+	.prepare = clk_pwm_prepare,
+	.unprepare = clk_pwm_unprepare,
+	.recalc_rate = clk_pwm_recalc_rate,
+};
+
+static int clk_pwm_probe(struct platform_device *pdev)
+{
+	struct device_node *node = pdev->dev.of_node;
+	struct clk_init_data init;
+	struct clk_pwm *clk_pwm;
+	struct pwm_device *pwm;
+	const char *clk_name;
+	struct clk *clk;
+	int ret;
+
+	clk_pwm = devm_kzalloc(&pdev->dev, sizeof(*clk_pwm), GFP_KERNEL);
+	if (!clk_pwm)
+		return -ENOMEM;
+
+	pwm = devm_pwm_get(&pdev->dev, NULL);
+	if (IS_ERR(pwm))
+		return PTR_ERR(pwm);
+
+	if (!pwm->period) {
+		dev_err(&pdev->dev, "invalid PWM period\n");
+		return -EINVAL;
+	}
+
+	if (of_property_read_u32(node, "clock-frequency", &clk_pwm->fixed_rate))
+		clk_pwm->fixed_rate = NSEC_PER_SEC / pwm->period;
+
+	if (pwm->period != NSEC_PER_SEC / clk_pwm->fixed_rate &&
+	    pwm->period != DIV_ROUND_UP(NSEC_PER_SEC, clk_pwm->fixed_rate)) {
+		dev_err(&pdev->dev,
+			"clock-frequency does not match PWM period\n");
+		return -EINVAL;
+	}
+
+	ret = pwm_config(pwm, (pwm->period + 1) >> 1, pwm->period);
+	if (ret < 0)
+		return ret;
+
+	clk_name = node->name;
+	of_property_read_string(node, "clock-output-names", &clk_name);
+
+	init.name = clk_name;
+	init.ops = &clk_pwm_ops;
+	init.flags = CLK_IS_BASIC | CLK_IS_ROOT;
+	init.num_parents = 0;
+
+	clk_pwm->pwm = pwm;
+	clk_pwm->hw.init = &init;
+	clk = devm_clk_register(&pdev->dev, &clk_pwm->hw);
+	if (IS_ERR(clk))
+		return PTR_ERR(clk);
+
+	return of_clk_add_provider(node, of_clk_src_simple_get, clk);
+}
+
+static int clk_pwm_remove(struct platform_device *pdev)
+{
+	of_clk_del_provider(pdev->dev.of_node);
+
+	return 0;
+}
+
+static const struct of_device_id clk_pwm_dt_ids[] = {
+	{ .compatible = "pwm-clock" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, clk_pwm_dt_ids);
+
+static struct platform_driver clk_pwm_driver = {
+	.probe = clk_pwm_probe,
+	.remove = clk_pwm_remove,
+	.driver = {
+		.name = "pwm-clock",
+		.of_match_table = of_match_ptr(clk_pwm_dt_ids),
+	},
+};
+
+module_platform_driver(clk_pwm_driver);
+
+MODULE_AUTHOR("Philipp Zabel <p.zabel@pengutronix.de>");
+MODULE_DESCRIPTION("PWM clock driver");
+MODULE_LICENSE("GPL");