Re: [PATCH] backlight: led_bl: fix initial power state

Daniel Thompson
2023-07-05T14:07:40Z

On Tue, Jul 04, 2023 at 07:07:31PM +0200, Sam Ravnborg wrote:
> Hi Daniel,
>
> > > @@ -200,8 +200,8 @@ static int led_bl_probe(struct platform_device *pdev)
> > > props.type = BACKLIGHT_RAW;
> > > props.max_brightness = priv->max_brightness;
> > > props.brightness = priv->default_brightness;
> > > - props.power = (priv->default_brightness > 0) ? FB_BLANK_POWERDOWN :
> > > - FB_BLANK_UNBLANK;
> > > + props.power = (priv->default_brightness > 0) ? FB_BLANK_UNBLANK :
> > > + FB_BLANK_POWERDOWN;
> >
> > The logic was wrong before but I think will still be wrong after the
> > change too (e.g. the bogus logic is probably avoiding backlight flicker
> > in some use cases).
> >
> > The logic here needs to be similar to what pwm_bl.c implements in
> > pwm_backlight_initial_power_state(). Whilst it might be better
> > to implement this in led_bl_get_leds() let me show what I mean
> > in code that fits in the current line:
> >
> > /*
> > * Activate the backlight if the LEDs are already lit *or*
> > * there is no phandle link (meaning the backlight power
> > * state cannot be synced with the display state).
> > */
> > props.power = (active_at_boot || !dev->node->phandle) ?
> > FB_BLANK_UNBLANK : FB_BLANK_POWERDOWN;
> >
> The following code does the same using helpers:
>
> if (active_at_boot || !dev->node->phandle))
> backlight_enable(bd);
> else
> backlight_disable(bd);
>
> The code needs to execute after backlight_device_register() so maybe not
> so great an idea?!?

It would introduce a need for a bunch of new locks since userspace
could get in between device creation and the call to the helpers.

Additionally, it is even correct for a driver to forcefully set
fb_blank to powerdown during the probe? All current drivers set
fb_blank to unblank during the probe.

Daniel.