video: backlight: Support PWMs without a known period_ns

The PWM device provided by Chrome OS EC doesn't really support anything
other than setting a relative duty cycle. To support it as a backlight,
this patch makes the PWM period optional in the device tree and pretends
the valid brightness range is its period_ns.

Also adds a sandbox test for a PWM channel that has a fixed period,
checking that the resulting duty_cycle matches on a set_config() even if
the requested period_ns can't be set.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

4 files changed, 42 insertions, 8 deletions

diff --git a/drivers/pwm/sandbox_pwm.c b/drivers/pwm/sandbox_pwm.c
index 318dce7..4df15f0 100644
--- a/drivers/pwm/sandbox_pwm.c
+++ b/drivers/pwm/sandbox_pwm.c
@@ -59,8 +59,15 @@ static int sandbox_pwm_set_config(struct udevice *dev, uint channel,
 	if (channel >= NUM_CHANNELS)
 		return -ENOSPC;
 	chan = &priv->chan[channel];
-	chan->period_ns = period_ns;
-	chan->duty_ns = duty_ns;
+
+	if (channel == 2) {
+		/* Pretend to have some fixed period */
+		chan->period_ns = 4096;
+		chan->duty_ns =  duty_ns * 4096 / period_ns;
+	} else {
+		chan->period_ns = period_ns;
+		chan->duty_ns = duty_ns;
+	}
 
 	return 0;
 }
diff --git a/drivers/video/pwm_backlight.c b/drivers/video/pwm_backlight.c
index 9e32bc4..4c86215 100644
--- a/drivers/video/pwm_backlight.c
+++ b/drivers/video/pwm_backlight.c
@@ -62,10 +62,17 @@ static int set_pwm(struct pwm_backlight_priv *priv)
 	uint duty_cycle;
 	int ret;
 
-	duty_cycle = priv->period_ns * (priv->cur_level - priv->min_level) /
-		(priv->max_level - priv->min_level);
-	ret = pwm_set_config(priv->pwm, priv->channel, priv->period_ns,
-			     duty_cycle);
+	if (priv->period_ns) {
+		duty_cycle = priv->period_ns * (priv->cur_level - priv->min_level) /
+			(priv->max_level - priv->min_level);
+		ret = pwm_set_config(priv->pwm, priv->channel, priv->period_ns,
+				     duty_cycle);
+	} else {
+		/* PWM driver will internally scale these like the above. */
+		ret = pwm_set_config(priv->pwm, priv->channel,
+				     priv->max_level - priv->min_level,
+				     priv->cur_level - priv->min_level);
+	}
 	if (ret)
 		return log_ret(ret);
 
@@ -213,10 +220,11 @@ static int pwm_backlight_of_to_plat(struct udevice *dev)
 		log_debug("Cannot get PWM: ret=%d\n", ret);
 		return log_ret(ret);
 	}
-	if (args.args_count < 2)
+	if (args.args_count < 1)
 		return log_msg_ret("Not enough arguments to pwm\n", -EINVAL);
 	priv->channel = args.args[0];
-	priv->period_ns = args.args[1];
+	if (args.args_count > 1)
+		priv->period_ns = args.args[1];
 	if (args.args_count > 2)
 		priv->polarity = args.args[2];
 
diff --git a/include/pwm.h b/include/pwm.h
index f995970..668551e 100644
--- a/include/pwm.h
+++ b/include/pwm.h
@@ -17,6 +17,10 @@ struct pwm_ops {
 	/**
 	 * set_config() - Set the PWM configuration
 	 *
+	 * Change both the PWM device's period and it's duty period if
+	 * possible. Otherwise, set an appropriate duty period that best
+	 * matches the given period_ns / duty_ns ratio for the device.
+	 *
 	 * @dev:	PWM device to update
 	 * @channel:	PWM channel to update
 	 * @period_ns:	PWM period in nanoseconds
@@ -51,6 +55,10 @@ struct pwm_ops {
 /**
  * pwm_set_config() - Set the PWM configuration
  *
+ * Change both the PWM device's period and it's duty period if
+ * possible. Otherwise, set an appropriate duty period that best
+ * matches the given period_ns / duty_ns ratio for the device.
+ *
  * @dev:	PWM device to update
  * @channel:	PWM channel to update
  * @period_ns:	PWM period in nanoseconds
diff --git a/test/dm/pwm.c b/test/dm/pwm.c
index 0de6dba..b624cf3 100644
--- a/test/dm/pwm.c
+++ b/test/dm/pwm.c
@@ -6,6 +6,7 @@
 #include <common.h>
 #include <dm.h>
 #include <pwm.h>
+#include <asm/test.h>
 #include <dm/test.h>
 #include <test/test.h>
 #include <test/ut.h>
@@ -14,6 +15,10 @@
 static int dm_test_pwm_base(struct unit_test_state *uts)
 {
 	struct udevice *dev;
+	uint period_ns;
+	uint duty_ns;
+	bool enable;
+	bool polarity;
 
 	ut_assertok(uclass_get_device(UCLASS_PWM, 0, &dev));
 	ut_assertnonnull(dev);
@@ -24,6 +29,12 @@ static int dm_test_pwm_base(struct unit_test_state *uts)
 	ut_asserteq(-ENOSPC, pwm_set_enable(dev, 3, true));
 	ut_assertok(pwm_set_invert(dev, 0, true));
 
+	ut_assertok(pwm_set_config(dev, 2, 100, 50));
+	ut_assertok(sandbox_pwm_get_config(dev, 2, &period_ns, &duty_ns,
+					   &enable, &polarity));
+	ut_asserteq(period_ns, 4096);
+	ut_asserteq(duty_ns, 50 * 4096 / 100);
+
 	ut_assertok(uclass_get_device(UCLASS_PWM, 1, &dev));
 	ut_asserteq(-ENODEV, uclass_get_device(UCLASS_PWM, 2, &dev));