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

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Date:   Thu, 9 Jun 2022 19:30:57 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Stephen Kitt <steve@....org>, linux-fbdev@...r.kernel.org,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Helge Deller <deller@....de>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] fbdev: atmel_lcdfb: Rework backlight status updates

Hi Stephen,

thanks for taking care of all these backlight simplifications - this
really helps to make the code simpler and more readable.

On Thu, Jun 09, 2022 at 10:54:12AM +0100, Daniel Thompson wrote:
> 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 is right - please drop the "if (brightness < 0)" logic.
I have looked a bit on the datasheet in my attempt to do a drm version
of this driver - something that I am yet to succeed and the backlight
core avoid any negative values.

	Sam