[PATCH] backlight: ktd253: Bring up in a known state

[PATCH] backlight: ktd253: Bring up in a known state

[Linus Walleij]

The KTD253 backlight might already be on when the driver
is probed: then we don't really know what the current
ratio is and all light intensity settings will be off
relative to what it was at boot.

To fix this, bring up the backlight OFF then move it to
the default backlight from there so we know the state.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/video/backlight/ktd253-backlight.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/drivers/video/backlight/ktd253-backlight.c b/drivers/video/backlight/ktd253-backlight.c
index e3fee3f1f582..d7b287cffd5c 100644
--- a/drivers/video/backlight/ktd253-backlight.c
+++ b/drivers/video/backlight/ktd253-backlight.c
@@ -137,15 +137,7 @@ static int ktd253_backlight_probe(struct platform_device *pdev)
 		brightness = max_brightness;
 	}
 
-	if (brightness)
-		/* This will be the default ratio when the KTD253 is enabled */
-		ktd253->ratio = KTD253_MAX_RATIO;
-	else
-		ktd253->ratio = 0;
-
-	ktd253->gpiod = devm_gpiod_get(dev, "enable",
-				       brightness ? GPIOD_OUT_HIGH :
-				       GPIOD_OUT_LOW);
+	ktd253->gpiod = devm_gpiod_get(dev, "enable", GPIOD_OUT_LOW);
 	if (IS_ERR(ktd253->gpiod)) {
 		ret = PTR_ERR(ktd253->gpiod);
 		if (ret != -EPROBE_DEFER)
@@ -153,6 +145,8 @@ static int ktd253_backlight_probe(struct platform_device *pdev)
 		return ret;
 	}
 	gpiod_set_consumer_name(ktd253->gpiod, dev_name(dev));
+	/* Bring backlight to a known off state */
+	msleep(KTD253_T_OFF_MS);
 
 	bl = devm_backlight_device_register(dev, dev_name(dev), dev, ktd253,
 					    &ktd253_backlight_ops, NULL);
-- 
2.29.2

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Re: [PATCH] backlight: ktd253: Bring up in a known state

[Daniel Thompson]

On Tue, Jan 26, 2021 at 10:32:00PM +0100, Linus Walleij wrote:
> The KTD253 backlight might already be on when the driver
> is probed: then we don't really know what the current
> ratio is and all light intensity settings will be off
> relative to what it was at boot.
> 
> To fix this, bring up the backlight OFF then move it to
> the default backlight from there so we know the state.
> 
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Hmnnn... horid, since the backlight will flicker during boot,
but I recall that this bit of hardware is pretty horid anyway
so it is not easily avoided.

Therefore, slightly grudgingly,
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>


Daniel.
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Re: [PATCH] backlight: ktd253: Bring up in a known state

[Linus Walleij]

On Fri, Feb 5, 2021 at 6:27 PM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
> On Tue, Jan 26, 2021 at 10:32:00PM +0100, Linus Walleij wrote:
> > The KTD253 backlight might already be on when the driver
> > is probed: then we don't really know what the current
> > ratio is and all light intensity settings will be off
> > relative to what it was at boot.
> >
> > To fix this, bring up the backlight OFF then move it to
> > the default backlight from there so we know the state.
> >
> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
>
> Hmnnn... horid, since the backlight will flicker during boot,
> but I recall that this bit of hardware is pretty horid anyway
> so it is not easily avoided.
>
> Therefore, slightly grudgingly,
> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>

Thanks, and I agree, this makes things better than before though.

The actual problem translates to the generic problem of bringing
misc boot-time conditions over to the kernel, and handing over
any type of hardware in a half-initialized state. We don't really have
a solution to that yet, many have tried.

For this specific case, one can imagine a "kinetic,boot-time-brightness"
(or similar) DT property, with the idea that the boot loader or
DT author knows what to put in there. I put that on my TODO
(the code in this patch will still needed if that property isn't provided).

Yours,
Linus Walleij
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Re: [PATCH] backlight: ktd253: Bring up in a known state

[Lee Jones]

On Tue, 26 Jan 2021, Linus Walleij wrote:

> The KTD253 backlight might already be on when the driver
> is probed: then we don't really know what the current
> ratio is and all light intensity settings will be off
> relative to what it was at boot.
> 
> To fix this, bring up the backlight OFF then move it to
> the default backlight from there so we know the state.
> 
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  drivers/video/backlight/ktd253-backlight.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)

Grudgingly applied, thanks.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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