[PATCH] backlight: pwm_bl: Avoid backlight flicker applying initial PWM state

[Philipp Zabel]

The initial PWM state returned by pwm_init_state() has a duty cycle
of 0 ns. To avoid backlight flicker when taking over an enabled
display from the bootloader, skip the initial pwm_apply_state()
and leave the PWM be until backlight_update_state() will apply the
state with the desired brightness.

Signed-off-by: Philipp Zabel <p.za...@pengutronix.de>
---
With a PWM driver that allows to inherit PWM state from the bootloader,
postponing the initial pwm_apply_state() with 0 ns duty cycle allows to
set the desired duty cycle before the PWM is set, avoiding a short flicker
if the backlight was previously enabled and will be enabled again.
---
 drivers/video/backlight/pwm_bl.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index fce412234d10..47a917038f58 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -531,12 +531,10 @@ 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_state(pb->pwm, &state);
-       if (ret) {
-               dev_err(&pdev->dev, "failed to apply initial PWM state: %d\n",
-                       ret);
-               goto err_alloc;
-       }
+       /*
+        * No need to apply initial state, except in the error path.
+        * State will be applied by backlight_update_status() on success.
+        */
 
        memset(&props, 0, sizeof(struct backlight_properties));
 
@@ -573,7 +571,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
                if (ret < 0) {
                        dev_err(&pdev->dev,
                                "failed to setup default brightness table\n");
-                       goto err_alloc;
+                       goto err_apply;
                }
 
                for (i = 0; i <= data->max_brightness; i++) {
@@ -602,7 +600,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
        if (IS_ERR(bl)) {
                dev_err(&pdev->dev, "failed to register backlight\n");
                ret = PTR_ERR(bl);
-               goto err_alloc;
+               goto err_apply;
        }
 
        if (data->dft_brightness > data->max_brightness) {
@@ -619,6 +617,8 @@ static int pwm_backlight_probe(struct platform_device *pdev)
        platform_set_drvdata(pdev, bl);
        return 0;
 
+err_apply:
+       pwm_apply_state(pb->pwm, &state);
 err_alloc:
        if (data->exit)
                data->exit(&pdev->dev);

---
base-commit: ac9a78681b921877518763ba0e89202254349d1b
change-id: 20230608-backlight-pwm-avoid-flicker-d2dd8d12f826

Best regards,
-- 
Philipp Zabel <p.za...@pengutronix.de>