linux-kernel - Re: [PATCH 1/2] platform/loongarch: laptop: Get brightness setting from EC on probe

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Message-ID: <CAAhV-H7pvaz5N0-EfvhDNHAXJtR13p9Xi5hfgDxOpeXi9zMbTQ@mail.gmail.com>
Date: Tue, 3 Jun 2025 12:11:48 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Yao Zi <ziyao@...root.org>
Cc: Jianmin Lv <lvjianmin@...ngson.cn>, WANG Xuerui <kernel@...0n.name>, 
	linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev, 
	Mingcong Bai <jeffbai@...c.io>, Kexy Biscuit <kexybiscuit@...c.io>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] platform/loongarch: laptop: Get brightness setting
 from EC on probe

On Sat, May 31, 2025 at 7:39 PM Yao Zi <ziyao@...root.org> wrote:
>
> Previously 1 is unconditionally taken as current brightness value. This
> causes problems since it's required to restore brightness settings on
> resumption, and a value that doesn't match EC's state before suspension
> will cause surprising changes of screen brightness.
laptop_backlight_register() isn't called at resuming, so I think your
problem has nothing to do with suspend (S3).

But there is really a problem about hibernation (S4): the brightness
is 1 during booting, but when switching to the target kernel, the
brightness may jump to the old value.

If the above case is what you meet, please update the commit message.

Huacai

>
> Let's get brightness from EC and take it as the current brightness on
> probe of the laptop driver to avoid the surprising behavior. Tested on
> TongFang L860-T2 3A5000 laptop.
>
> Cc: stable@...r.kernel.org
> Fixes: 6246ed09111f ("LoongArch: Add ACPI-based generic laptop driver")
> Signed-off-by: Yao Zi <ziyao@...root.org>
> ---
>  drivers/platform/loongarch/loongson-laptop.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/platform/loongarch/loongson-laptop.c b/drivers/platform/loongarch/loongson-laptop.c
> index 99203584949d..828bd62e3596 100644
> --- a/drivers/platform/loongarch/loongson-laptop.c
> +++ b/drivers/platform/loongarch/loongson-laptop.c
> @@ -392,7 +392,7 @@ static int laptop_backlight_register(void)
>         if (!acpi_evalf(hotkey_handle, &status, "ECLL", "d"))
>                 return -EIO;
>
> -       props.brightness = 1;
> +       props.brightness = ec_get_brightness();
>         props.max_brightness = status;
>         props.type = BACKLIGHT_PLATFORM;
>
> --
> 2.49.0
>
>