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>

1 files changed, 157 insertions, 0 deletions

diff --git a/drivers/leds/leds-atmel-pwm.c b/drivers/leds/leds-atmel-pwm.c
new file mode 100644
index 00000000000..af61f55571f
--- /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");