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From:   Ying Liu <victor.liu@nxp.com>
To:     Daniel Thompson <daniel.thompson@linaro.org>
CC:     "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "lee@kernel.org" <lee@kernel.org>,
        "jingoohan1@gmail.com" <jingoohan1@gmail.com>,
        "deller@gmx.de" <deller@gmx.de>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Andy Shevchenko <andy@kernel.org>
Subject: RE: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state
Thread-Topic: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state
Thread-Index: Adm60FHbZeccRsgE2UutMj3nHgFTFQALOCUAACT+sfA=
Date:   Fri, 21 Jul 2023 05:16:57 +0000
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References: <20230720061105.154821-1-victor.liu@nxp.com>
 <20230720112742.GA2525277@aspen.lan>
In-Reply-To: <20230720112742.GA2525277@aspen.lan>
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Hi Daniel,

On Thursday, July 20, 2023 7:28 PM Daniel Thompson <daniel.thompson@linaro.=
org> wrote:
>=20
> On Thu, Jul 20, 2023 at 06:06:27AM +0000, Ying Liu wrote:
> > Bootloader may leave gpio direction as input and gpio value as logical =
low.
> > It hints that initial backlight power state should be
> FB_BLANK_POWERDOWN
> > since the gpio value is literally logical low.
>=20
> To be honest this probably "hints" that the bootloader simply didn't
> consider the backlight at all :-) . I'd rather the patch description
> focus on what circumstances lead to the current code making a bad
> decision. More like:
>=20
>   If the GPIO pin is in the input state but the backlight is currently
>   off due to default pull downs then ...

How about this patch description?

---------------------------------8<----------------------------------------=
---
Without this patch, if gpio pin is in input state but backlight is currentl=
y
off due to default pull downs, then initial power state is set to
FB_BLANK_UNBLANK in DT boot mode with phandle link and the backlight is
effectively turned on in gpio_backlight_probe(), which is undesirable
according to patch description of commit ec665b756e6f ("backlight:
gpio-backlight: Correct initial power state handling").

Quote:
---
If in DT boot we have phandle link then leave the GPIO in a state which the
bootloader left it and let the user of the backlight to configure it furthe=
r.
---

So, let's drop output gpio direction check and only check gpio value to set
the initial power state.
---------------------------------8<----------------------------------------=
---

Regards,
Liu Ying