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

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Message-ID: <20230720131029.GB2525277@aspen.lan>
Date:   Thu, 20 Jul 2023 14:10:29 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Ying Liu <victor.liu@....com>,
        "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>,
        Andy Shevchenko <andy@...nel.org>
Subject: Re: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state

On Thu, Jul 20, 2023 at 02:56:32PM +0200, Bartosz Golaszewski wrote:
> On Thu, Jul 20, 2023 at 1:27 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 ...
> >
> > > So, let's drop output gpio
> > > direction check and only check gpio value to set the initial power state.
> >
> > This check was specifically added by Bartosz so I'd be interested in his
> > opinion of this change (especially since he is now a GPIO maintainer)!
> >
> > What motivates (or motivated) the need to check the direction rather
> > than just read that current logic level on the pin?
> >
> >
> > Daniel.
> > [I'm done but since Bartosz and Linus were not on copy of the original
> > thread I've left the rest of the patch below as a convenience ;-) ]
> >
>
> This was done in commit: 706dc68102bc ("backlight: gpio: Explicitly
> set the direction of the GPIO").
>
> Let me quote myself from it:
> --
> The GPIO backlight driver currently requests the line 'as is', without
> actively setting its direction. This can lead to problems: if the line
> is in input mode by default, we won't be able to drive it later when
> updating the status and also reading its initial value doesn't make
> sense for backlight setting.
> --

You are perhaps quoting the wrong bit here ;-). The currently proposed
patch leaves the code to put the pin into output mode unmodified. However
there was an extra line at the bottom of your commit message:
--
Also: check the current direction and only read the value if it's output.
--

This was the bit I wanted to check on, since the proposed patch
literally reverses this!

However...


> I agree with Thomas that it's highly unlikely the bootloader "hints"
> at any specific backlight settings. That being said, the change itself
> looks correct to me. The other branch of that if will always unblank
> the backlight if the GPIO is in input mode which may not be desirable.

... if you're happy the proposed change is OK then I'm happy too!
I came to the same conclusion after reviewing the GPIO code this morning,
however I copied you in because I was worried I might have overlooked
something.


> I don't see any obvious problem with this change, just make sure the
> commit message makes more sense.

Agreed.


Daniel.