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

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Date:   Thu, 9 Jun 2022 19:45:11 +0200
From:   Stephen Kitt <steve@....org>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.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 Sam, Daniel,

On Thu, 9 Jun 2022 19:30:57 +0200, Sam Ravnborg <sam@...nborg.org> wrote:
> thanks for taking care of all these backlight simplifications - this
> really helps to make the code simpler and more readable.

You’re welcome! I noticed fb_blank was deprecated and near enough unused, and
started digging...

> 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.

Thanks for the reviews!

I’ve prepared a v2 without the (brightness < 0) logic, I’m about to submit
it.

Regards,

Stephen

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