linux-kernel - Re: [PATCH] fbdev: atmel_lcdfb: Rework backlight status updates

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Message-ID: <20220609095412.fccofr2e2kpzhw4t@maple.lan>
Date:   Thu, 9 Jun 2022 10:54:12 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Stephen Kitt <steve@....org>
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Helge Deller <deller@....de>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbdev: atmel_lcdfb: Rework backlight status updates

On Wed, Jun 08, 2022 at 10:56:23PM +0200, Stephen Kitt wrote:
> Instead of checking the state of various backlight_properties fields
> against the memorised state in atmel_lcdfb_info.bl_power,
> atmel_bl_update_status() should retrieve the desired state using
> backlight_get_brightness (which takes into account the power state,
> blanking etc.). This means the explicit checks using props.fb_blank
> and props.power can be dropped.
> 
> Then brightness can only be negative if the backlight is on but
> props.brightness is negative, so the test before reading the
> brightness value from the hardware can be simplified to
> (brightness < 0).

props.brightness should always be in the interval 0..max_brightness.

This is enforced by the main backlight code (and APIs to set the
brightness use unsigned values). Thus props.brightness could only be
negative is the driver explicitly sets a negative value as some kind of
placeholder (which this driver does not do).

I don't think there is any need to keep this logic.


Daniel.


> diff --git a/drivers/video/fbdev/atmel_lcdfb.c b/drivers/video/fbdev/atmel_lcdfb.c
> index 1fc8de4ecbeb..06159a4da293 100644
> --- a/drivers/video/fbdev/atmel_lcdfb.c
> +++ b/drivers/video/fbdev/atmel_lcdfb.c
> @@ -109,22 +108,10 @@ static u32 contrast_ctr = ATMEL_LCDC_PS_DIV8
>  static int atmel_bl_update_status(struct backlight_device *bl)
>  {
>  	struct atmel_lcdfb_info *sinfo = bl_get_data(bl);
> -	int			power = sinfo->bl_power;
> -	int			brightness = bl->props.brightness;
> +	int			brightness = backlight_get_brightness(bl);
>  
> -	/* REVISIT there may be a meaningful difference between
> -	 * fb_blank and power ... there seem to be some cases
> -	 * this doesn't handle correctly.
> -	 */
> -	if (bl->props.fb_blank != sinfo->bl_power)
> -		power = bl->props.fb_blank;
> -	else if (bl->props.power != sinfo->bl_power)
> -		power = bl->props.power;
> -
> -	if (brightness < 0 && power == FB_BLANK_UNBLANK)
> +	if (brightness < 0)
>  		brightness = lcdc_readl(sinfo, ATMEL_LCDC_CONTRAST_VAL);
> -	else if (power != FB_BLANK_UNBLANK)
> -		brightness = 0;
>  
>  	lcdc_writel(sinfo, ATMEL_LCDC_CONTRAST_VAL, brightness);
>  	if (contrast_ctr & ATMEL_LCDC_POL_POSITIVE)