backlight: pwm: Reject legacy PWM request for device defined in DT

Platform PWM backlight data provided by board's device tree should be
complete enough to successfully request a pwm device using pwm_get()
API. This change fixes a bug, when an arbitrary (first found) PWM is
connected to a "pwm-backlight" compatible device, when explicit PWM
device reference is not given.

Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
already describes "pwms" as a required property, instead of blind
selection of a potentially wrong PWM reject legacy PWM device
registration request, leave legacy API only for non-dt cases.

Based on initial implementation done by Dmitry Eremin-Solenikov.

Reported-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/pwm_bl.c

@@ -271,19 +271,18 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	}
 
 	pb->pwm = devm_pwm_get(&pdev->dev, NULL);
-	if (IS_ERR(pb->pwm)) {
-		ret = PTR_ERR(pb->pwm);
-		if (ret == -EPROBE_DEFER)
-			goto err_alloc;
-
+	if (IS_ERR(pb->pwm) && PTR_ERR(pb->pwm) != -EPROBE_DEFER
+	    && !pdev->dev.of_node) {
 		dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
 		pb->legacy = true;
 		pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
-		if (IS_ERR(pb->pwm)) {
-			dev_err(&pdev->dev, "unable to request legacy PWM\n");
-			ret = PTR_ERR(pb->pwm);
-			goto err_alloc;
-		}
+	}
+
+	if (IS_ERR(pb->pwm)) {
+		ret = PTR_ERR(pb->pwm);
+		if (ret != -EPROBE_DEFER)
+			dev_err(&pdev->dev, "unable to request PWM\n");
+		goto err_alloc;
 	}
 
 	dev_dbg(&pdev->dev, "got pwm for backlight\n");