linux-kernel - Re: [PATCH 2/3] drm/panel: ld9040: Register a backlight device

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Message-ID: <20230705173949.GA106478@ravnborg.org>
Date:   Wed, 5 Jul 2023 19:39:49 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     neil.armstrong@...aro.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/3] drm/panel: ld9040: Register a backlight device

Hi Paul,

On Wed, Jul 05, 2023 at 04:38:05PM +0200, Paul Cercueil wrote:
> Hi Neil,
> 
> Le mercredi 05 juillet 2023 à 15:57 +0200, Neil Armstrong a écrit :
> > On 03/07/2023 23:47, Paul Cercueil wrote:
> > > Register a backlight device to be able to switch between all the
> > > gamma
> > > levels.
> > > 
> > > Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> > > ---
> > >   drivers/gpu/drm/panel/panel-samsung-ld9040.c | 40
> > > ++++++++++++++++++++
> > >   1 file changed, 40 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/panel/panel-samsung-ld9040.c
> > > b/drivers/gpu/drm/panel/panel-samsung-ld9040.c
> > > index 7fd9444b42c5..b4f87d6244cb 100644
> > > --- a/drivers/gpu/drm/panel/panel-samsung-ld9040.c
> > > +++ b/drivers/gpu/drm/panel/panel-samsung-ld9040.c
> > > @@ -8,6 +8,7 @@
> > >    * Andrzej Hajda <a.hajda@...sung.com>
> > >   */
> > >   
> > > +#include <linux/backlight.h>
> > >   #include <linux/delay.h>
> > >   #include <linux/gpio/consumer.h>
> > >   #include <linux/module.h>
> > > @@ -311,8 +312,40 @@ static int ld9040_parse_dt(struct ld9040 *ctx)
> > >         return 0;
> > >   }
> > >   
> > > +static int ld9040_bl_update_status(struct backlight_device *dev)
> > > +{
> > > +       struct ld9040 *ctx = dev_get_drvdata(&dev->dev);
> > > +
> > > +       ctx->brightness = dev->props.brightness;
Use backlight_get_brightness(dev);

> > > +       ld9040_brightness_set(ctx);
> > > +
> > > +       return 0;
> > > +}
> > > +
> > > +static int ld9040_bl_get_intensity(struct backlight_device *dev)
> > > +{
> > > +       if (dev->props.fb_blank == FB_BLANK_UNBLANK &&
> > > +                       dev->props.power == FB_BLANK_UNBLANK)
> > > +               return dev->props.brightness;
> > > +
> > > +       return 0;
> > > +}
> > 
> > You can totally drop the _get_brightness.
> 
> The current behaviour is to return 0 when the framebuffer is blanked. A
> few drivers do that so I thought it was the norm; and the backlight
> core doesn't do that by default (and just uses dev->props.brightness).
> 
> It is not clear to me if that's the preferred behaviour. The
> "backlight_get_brightness" function in backlight.h seems to suggest
> that the current behaviour is correct, unless it is not supposed to be
> used in the backlight_ops.get_brightness() callback. Then in that case
> some other drivers get it wrong too.
Several drivers get it wrong.
You are supposed to provide get_brightness only when you read back a
value from the HW, which is not the case here so just drop it is the
right choice.

	Sam