PWM LED driver

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>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/leds/Kconfig b/drivers/leds/Kconfig
index 851a3b0..859814f 100644 (file)
--- 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 bc6afc8..84ced3b 100644 (file)
--- 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 (file)
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");