[PATCH v7 2/2] pwm: Add clock based PWM output driver

[Nikita Travkin <nikita@trvn.ru>]

Some systems have clocks exposed to external devices. If the clock
controller supports duty-cycle configuration, such clocks can be used as
pwm outputs. In fact PWM and CLK subsystems are interfaced with in a
similar way and an "opposite" driver already exists (clk-pwm). Add a
driver that would enable pwm devices to be used via clk subsystem.

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
--

Changes in v2:
 - Address Uwe's review comments:
   - Round set clk rate up
   - Add a description with limitations of the driver
   - Disable and unprepare clock before removing pwmchip
Changes in v3:
 - Use 64bit version of div round up
 - Address Uwe's review comments:
   - Reword the limitations to avoid incorrect claims
   - Move the clk_enabled flag assignment
   - Drop unnecessary statements
Changes in v5:
 - add missed returns
Changes in v6:
 - Unprepare the clock on error
 - Drop redundant limitations points
Changes in v7:
 - Rename some variables to be in line with common naming

--
It seems like my mailserver wasn't able to send the last review
response to Uwe's so I'll repeat here that afaict clk.h has all the
methods stubbed out so compiling without HAVE_CLK is possible.
Sorry for a long delay with sending this since v6.

---
 drivers/pwm/Kconfig   |  10 +++
 drivers/pwm/Makefile  |   1 +
 drivers/pwm/pwm-clk.c | 141 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 152 insertions(+)
 create mode 100644 drivers/pwm/pwm-clk.c

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index 904de8d61828..60d13a949bc5 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -140,6 +140,16 @@ config PWM_BRCMSTB
 	  To compile this driver as a module, choose M Here: the module
 	  will be called pwm-brcmstb.c.
 
+config PWM_CLK
+	tristate "Clock based PWM support"
+	depends on HAVE_CLK || COMPILE_TEST
+	help
+	  Generic PWM framework driver for outputs that can be
+	  muxed to clocks.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called pwm-clk.
+
 config PWM_CLPS711X
 	tristate "CLPS711X PWM support"
 	depends on ARCH_CLPS711X || COMPILE_TEST
diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index 5c08bdb817b4..7bf1a29f02b8 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -10,6 +10,7 @@ obj-$(CONFIG_PWM_BCM_KONA)	+= pwm-bcm-kona.o
 obj-$(CONFIG_PWM_BCM2835)	+= pwm-bcm2835.o
 obj-$(CONFIG_PWM_BERLIN)	+= pwm-berlin.o
 obj-$(CONFIG_PWM_BRCMSTB)	+= pwm-brcmstb.o
+obj-$(CONFIG_PWM_CLK)		+= pwm-clk.o
 obj-$(CONFIG_PWM_CLPS711X)	+= pwm-clps711x.o
 obj-$(CONFIG_PWM_CRC)		+= pwm-crc.o
 obj-$(CONFIG_PWM_CROS_EC)	+= pwm-cros-ec.o
diff --git a/drivers/pwm/pwm-clk.c b/drivers/pwm/pwm-clk.c
new file mode 100644
index 000000000000..357d0c50dedd
--- /dev/null
+++ b/drivers/pwm/pwm-clk.c
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Clock based PWM controller
+ *
+ * Copyright (c) 2021 Nikita Travkin <nikita@trvn.ru>
+ *
+ * This is an "adapter" driver that allows PWM consumers to use
+ * system clocks with duty cycle control as PWM outputs.
+ *
+ * Limitations:
+ * - Due to the fact that exact behavior depends on the underlying
+ *   clock driver, various limitations are possible.
+ * - Underlying clock may not be able to give 0% or 100% duty cycle
+ *   (constant off or on), exact behavior will depend on the clock.
+ * - When the PWM is disabled, the clock will be disabled as well,
+ *   line state will depend on the clock.
+ * - The clk API doesn't expose the necessary calls to implement
+ *   .get_state().
+ */
+
+#include <linux/kernel.h>
+#include <linux/math64.h>
+#include <linux/err.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/clk.h>
+#include <linux/pwm.h>
+
+struct pwm_clk_chip {
+	struct pwm_chip chip;
+	struct clk *clk;
+	bool clk_enabled;
+};
+
+#define to_pwm_clk_chip(_chip) container_of(_chip, struct pwm_clk_chip, chip)
+
+static int pwm_clk_apply(struct pwm_chip *chip, struct pwm_device *pwm,
+			 const struct pwm_state *state)
+{
+	struct pwm_clk_chip *pcchip = to_pwm_clk_chip(chip);
+	int ret;
+	u32 rate;
+	u64 period = state->period;
+	u64 duty_cycle = state->duty_cycle;
+
+	if (!state->enabled) {
+		if (pwm->state.enabled) {
+			clk_disable(pcchip->clk);
+			pcchip->clk_enabled = false;
+		}
+		return 0;
+	} else if (!pwm->state.enabled) {
+		ret = clk_enable(pcchip->clk);
+		if (ret)
+			return ret;
+		pcchip->clk_enabled = true;
+	}
+
+	rate = DIV64_U64_ROUND_UP(NSEC_PER_SEC, period);
+	ret = clk_set_rate(pcchip->clk, rate);
+	if (ret)
+		return ret;
+
+	if (state->polarity == PWM_POLARITY_INVERSED)
+		duty_cycle = period - duty_cycle;
+
+	return clk_set_duty_cycle(pcchip->clk, duty_cycle, period);
+}
+
+static const struct pwm_ops pwm_clk_ops = {
+	.apply = pwm_clk_apply,
+	.owner = THIS_MODULE,
+};
+
+static int pwm_clk_probe(struct platform_device *pdev)
+{
+	struct pwm_clk_chip *pcchip;
+	int ret;
+
+	pcchip = devm_kzalloc(&pdev->dev, sizeof(*pcchip), GFP_KERNEL);
+	if (!pcchip)
+		return -ENOMEM;
+
+	pcchip->clk = devm_clk_get(&pdev->dev, NULL);
+	if (IS_ERR(pcchip->clk))
+		return dev_err_probe(&pdev->dev, PTR_ERR(pcchip->clk),
+				     "Failed to get clock\n");
+
+	pcchip->chip.dev = &pdev->dev;
+	pcchip->chip.ops = &pwm_clk_ops;
+	pcchip->chip.npwm = 1;
+
+	ret = clk_prepare(pcchip->clk);
+	if (ret < 0)
+		return dev_err_probe(&pdev->dev, ret, "Failed to prepare clock\n");
+
+	ret = pwmchip_add(&pcchip->chip);
+	if (ret < 0) {
+		clk_unprepare(pcchip->clk);
+		return dev_err_probe(&pdev->dev, ret, "Failed to add pwm chip\n");
+	}
+
+	platform_set_drvdata(pdev, pcchip);
+	return 0;
+}
+
+static int pwm_clk_remove(struct platform_device *pdev)
+{
+	struct pwm_clk_chip *pcchip = platform_get_drvdata(pdev);
+
+	pwmchip_remove(&pcchip->chip);
+
+	if (pcchip->clk_enabled)
+		clk_disable(pcchip->clk);
+
+	clk_unprepare(pcchip->clk);
+
+	return 0;
+}
+
+static const struct of_device_id pwm_clk_dt_ids[] = {
+	{ .compatible = "clk-pwm", },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, pwm_clk_dt_ids);
+
+static struct platform_driver pwm_clk_driver = {
+	.driver = {
+		.name = "pwm-clk",
+		.of_match_table = pwm_clk_dt_ids,
+	},
+	.probe = pwm_clk_probe,
+	.remove = pwm_clk_remove,
+};
+module_platform_driver(pwm_clk_driver);
+
+MODULE_ALIAS("platform:pwm-clk");
+MODULE_AUTHOR("Nikita Travkin <nikita@trvn.ru>");
+MODULE_DESCRIPTION("Clock based PWM driver");
+MODULE_LICENSE("GPL");
-- 
2.35.3