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Subject[PATCH -mm 2/2] PWM LED driver
Date
From: David Brownell <david-b@pacbell.net>

This is a LED driver using the PWM on newer SOCs from Atmel; brightness
is controlled by changing the PWM duty cycle. So for example if you've
set up two leds labeled "pwm0" and "pwm1":

echo 0 > /sys/class/leds/pwm2/brightness # off (0%)
echo 80 > /sys/class/leds/pwm2/brightness
echo 255 > /sys/class/leds/pwm2/brightness # on (100%)
Note that "brightness" here isn't linear; maybe that should change.
Going from 4 to 8 probably doubles perceived brightness, while 244
to 248 is imperceptible.

This is mostly intended to be a simple example of PWM, although it's
realistic since LCD backlights are often driven with PWM to conserve
battery power (and offer brightness options).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
---
drivers/leds/Kconfig | 7 ++
drivers/leds/Makefile | 1 +
drivers/leds/leds-atmel-pwm.c | 157 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 165 insertions(+), 0 deletions(-)
create mode 100644 drivers/leds/leds-atmel-pwm.c
diff --git a/drivers/leds/Kconfig b/drivers/leds/Kconfig
index ec568fa..4e1db3b 100644
--- a/drivers/leds/Kconfig
+++ b/drivers/leds/Kconfig
@@ -18,6 +18,13 @@ config LEDS_CLASS

comment "LED drivers"

+config LEDS_ATMEL_PWM
+ tristate "LED Support using Atmel PWM outputs"
+ depends on LEDS_CLASS && ATMEL_PWM
+ help
+ This option enables support for LEDs driven using outputs
+ of the dedicated PWM controller found on newer Atmel SOCs.
+
config LEDS_CORGI
tristate "LED Support for the Sharp SL-C7x0 series"
depends on LEDS_CLASS && PXA_SHARP_C7xx
diff --git a/drivers/leds/Makefile b/drivers/leds/Makefile
index a60de1b..04bc850 100644
--- a/drivers/leds/Makefile
+++ b/drivers/leds/Makefile
@@ -5,6 +5,7 @@ obj-$(CONFIG_LEDS_CLASS) += led-class.o
obj-$(CONFIG_LEDS_TRIGGERS) += led-triggers.o

# LED Platform Drivers
+obj-$(CONFIG_LEDS_ATMEL_PWM) += leds-atmel-pwm.o
obj-$(CONFIG_LEDS_CORGI) += leds-corgi.o
obj-$(CONFIG_LEDS_LOCOMO) += leds-locomo.o
obj-$(CONFIG_LEDS_SPITZ) += leds-spitz.o
diff --git a/drivers/leds/leds-atmel-pwm.c b/drivers/leds/leds-atmel-pwm.c
new file mode 100644
index 0000000..af61f55
--- /dev/null
+++ b/drivers/leds/leds-atmel-pwm.c
@@ -0,0 +1,157 @@
+#include <linux/kernel.h>
+#include <linux/platform_device.h>
+#include <linux/leds.h>
+#include <linux/io.h>
+#include <linux/atmel_pwm.h>
+
+
+struct pwmled {
+ struct led_classdev cdev;
+ struct pwm_channel pwmc;
+ struct gpio_led *desc;
+ u32 mult;
+ u8 active_low;
+};
+
+
+/*
+ * For simplicity, we use "brightness" as if it were a linear function
+ * of PWM duty cycle. However, a logarithmic function of duty cycle is
+ * probably a better match for perceived brightness: two is half as bright
+ * as four, four is half as bright as eight, etc
+ */
+static void pwmled_brightness(struct led_classdev *cdev, enum led_brightness b)
+{
+ struct pwmled *led;
+
+ /* update the duty cycle for the *next* period */
+ led = container_of(cdev, struct pwmled, cdev);
+ pwm_channel_writel(&led->pwmc, PWM_CUPD, led->mult * (unsigned) b);
+}
+
+/*
+ * NOTE: we reuse the platform_data structure of GPIO leds,
+ * but repurpose its "gpio" number as a PWM channel number.
+ */
+static int __init pwmled_probe(struct platform_device *pdev)
+{
+ const struct gpio_led_platform_data *pdata;
+ struct pwmled *leds;
+ unsigned i;
+ int status;
+
+ pdata = pdev->dev.platform_data;
+ if (!pdata || pdata->num_leds < 1)
+ return -ENODEV;
+
+ leds = kcalloc(pdata->num_leds, sizeof(*leds), GFP_KERNEL);
+ if (!leds)
+ return -ENOMEM;
+
+ for (i = 0; i < pdata->num_leds; i++) {
+ struct pwmled *led = leds + i;
+ const struct gpio_led *dat = pdata->leds + i;
+ u32 tmp;
+
+ led->cdev.name = dat->name;
+ led->cdev.brightness = LED_OFF;
+ led->cdev.brightness_set = pwmled_brightness;
+ led->cdev.default_trigger = dat->default_trigger;
+
+ led->active_low = dat->active_low;
+
+ status = pwm_channel_alloc(dat->gpio, &led->pwmc);
+ if (status < 0)
+ goto err;
+
+ /*
+ * Prescale clock by 2^x, so PWM counts in low MHz.
+ * Start each cycle with the LED active, so increasing
+ * the duty cycle gives us more time on (== brighter).
+ */
+ tmp = 5;
+ if (!led->active_low)
+ tmp |= PWM_CPR_CPOL;
+ pwm_channel_writel(&led->pwmc, PWM_CMR, tmp);
+
+ /*
+ * Pick a period so PWM cycles at 100+ Hz; and a multiplier
+ * for scaling duty cycle: brightness * mult.
+ */
+ tmp = (led->pwmc.mck / (1 << 5)) / 100;
+ tmp /= 255;
+ led->mult = tmp;
+ pwm_channel_writel(&led->pwmc, PWM_CDTY,
+ led->cdev.brightness * 255);
+ pwm_channel_writel(&led->pwmc, PWM_CPRD,
+ LED_FULL * tmp);
+
+ pwm_channel_enable(&led->pwmc);
+
+ /* Hand it over to the LED framework */
+ status = led_classdev_register(&pdev->dev, &led->cdev);
+ if (status < 0) {
+ pwm_channel_free(&led->pwmc);
+ goto err;
+ }
+ }
+
+ platform_set_drvdata(pdev, leds);
+ return 0;
+
+err:
+ if (i > 0) {
+ for (i = i - 1; i >= 0; i--) {
+ led_classdev_unregister(&leds[i].cdev);
+ pwm_channel_free(&leds[i].pwmc);
+ }
+ }
+ kfree(leds);
+
+ return status;
+}
+
+static int __exit pwmled_remove(struct platform_device *pdev)
+{
+ const struct gpio_led_platform_data *pdata;
+ struct pwmled *leds;
+ unsigned i;
+
+ pdata = pdev->dev.platform_data;
+ leds = platform_get_drvdata(pdev);
+
+ for (i = 0; i < pdata->num_leds; i++) {
+ struct pwmled *led = leds + i;
+
+ led_classdev_unregister(&led->cdev);
+ pwm_channel_free(&led->pwmc);
+ }
+
+ kfree(leds);
+ platform_set_drvdata(pdev, NULL);
+ return 0;
+}
+
+static struct platform_driver pwmled_driver = {
+ .driver = {
+ .name = "leds-atmel-pwm",
+ .owner = THIS_MODULE,
+ },
+ /* REVISIT add suspend() and resume() methods */
+ .remove = __exit_p(pwmled_remove),
+};
+
+static int __init modinit(void)
+{
+ return platform_driver_probe(&pwmled_driver, pwmled_probe);
+}
+module_init(modinit);
+
+static void __exit modexit(void)
+{
+ platform_driver_unregister(&pwmled_driver);
+}
+module_exit(modexit);
+
+MODULE_DESCRIPTION("Driver for LEDs with PWM-controlled brightness");
+MODULE_LICENSE("GPL");
--
1.5.3.8


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