Hello Philipp,
On Thu, Jun 08, 2023 at 04:11:14PM +0200, Philipp Zabel wrote:
> The initial PWM state returned by pwm_init_state() has a duty cycle
> of 0 ns.
This is only true for drivers without a .get_state() callback, isn't it?
> 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.zabel@xxxxxxxxxxxxxx>
> ---
> 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.
Why do you want to modify the PWM in the error path? I would have
expected not touching it at all in .probe() is fine?!
> + * State will be applied by backlight_update_status() on success.
> + */
>
> memset(&props, 0, sizeof(struct backlight_properties));
>
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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