linux-kernel - [PATCH] backlight: pwm_bl: apply the initial backlight state with sane defaults

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Message-Id: <20250731-blpwm-v1-1-0171fd31bff9@pengutronix.de>
Date: Thu, 31 Jul 2025 10:47:18 +0200
From: Michael Grzeschik <m.grzeschik@...gutronix.de>
To: Uwe Kleine-König <ukleinek@...nel.org>, 
 Lee Jones <lee@...nel.org>, Daniel Thompson <danielt@...nel.org>, 
 Jingoo Han <jingoohan1@...il.com>, Helge Deller <deller@....de>
Cc: Pengutronix <kernel@...gutronix.de>, linux-pwm@...r.kernel.org, 
 dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Michael Grzeschik <m.grzeschik@...gutronix.de>
Subject: [PATCH] backlight: pwm_bl: apply the initial backlight state with
 sane defaults

Currently when calling pwm_apply_might_sleep in the probe routine
the pwm will be configured with an not fully defined state.

The duty_cycle is not yet set in that moment. There is a final
backlight_update_status call that will have a properly setup state.
However this change in the backlight can create a short flicker if the
backlight was already preinitialised.

We fix the flicker by moving the pwm_apply after the default duty_cycle
can be calculated.

Signed-off-by: Michael Grzeschik <m.grzeschik@...gutronix.de>
---
 drivers/video/backlight/pwm_bl.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 237d3d3f3bb1a..5924e0b9f01e7 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -518,13 +518,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	if (!state.period && (data->pwm_period_ns > 0))
 		state.period = data->pwm_period_ns;
 
-	ret = pwm_apply_might_sleep(pb->pwm, &state);
-	if (ret) {
-		dev_err_probe(&pdev->dev, ret,
-			      "failed to apply initial PWM state");
-		goto err_alloc;
-	}
-
 	memset(&props, 0, sizeof(struct backlight_properties));
 
 	if (data->levels) {
@@ -582,6 +575,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	pb->lth_brightness = data->lth_brightness * (div_u64(state.period,
 				pb->scale));
 
+	state.duty_cycle = compute_duty_cycle(pb, data->dft_brightness, &state);
+
+	ret = pwm_apply_might_sleep(pb->pwm, &state);
+	if (ret) {
+		dev_err_probe(&pdev->dev, ret,
+			      "failed to apply initial PWM state");
+		goto err_alloc;
+	}
+
 	props.type = BACKLIGHT_RAW;
 	props.max_brightness = data->max_brightness;
 	bl = backlight_device_register(dev_name(&pdev->dev), &pdev->dev, pb,

---
base-commit: 739a6c93cc755c0daf3a7e57e018a8c61047cd90
change-id: 20250731-blpwm-598ecad93c4d

Best regards,
-- 
Michael Grzeschik <m.grzeschik@...gutronix.de>