linux-kernel - RE: [PATCH] backlight: gpio_backlight: Drop output gpio direction check for initial power state

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Message-ID: <AM7PR04MB7046CF1E1561A37C7A5AC89B983FA@AM7PR04MB7046.eurprd04.prod.outlook.com>
Date:   Fri, 21 Jul 2023 05:16:57 +0000
From:   Ying Liu <victor.liu@....com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
CC:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lee@...nel.org" <lee@...nel.org>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "deller@....de" <deller@....de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Andy Shevchenko <andy@...nel.org>
Subject: RE: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state

Hi Daniel,

On Thursday, July 20, 2023 7:28 PM Daniel Thompson <daniel.thompson@...aro.org> wrote:
> 
> 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.
> 
> 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:
> 
>   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 currently
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 further.
---

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

Regards,
Liu Ying