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

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Message-ID: <8e81dd5d9f62139e740ea123aa2087cb1bbcb04b.camel@crapouillou.net>
Date:   Wed, 05 Jul 2023 16:38:05 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     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>,
        Sam Ravnborg <sam@...nborg.org>
Cc:     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 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;
> > +       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.

Cheers,
-Paul

> Neil
> 
> > +
> > +static const struct backlight_ops ld9040_bl_ops = {
> > +       .get_brightness = ld9040_bl_get_intensity,
> > +       .update_status  = ld9040_bl_update_status,
> > +};
> > +
> > +static const struct backlight_properties ld9040_bl_props = {
> > +       .type = BACKLIGHT_RAW,
> > +       .scale = BACKLIGHT_SCALE_NON_LINEAR,
> > +       .max_brightness = ARRAY_SIZE(ld9040_gammas) - 1,
> > +       .brightness = ARRAY_SIZE(ld9040_gammas) - 1,
> > +};
> > +
> >   static int ld9040_probe(struct spi_device *spi)
> >   {
> > +       struct backlight_device *bldev;
> >         struct device *dev = &spi->dev;
> >         struct ld9040 *ctx;
> >         int ret;
> > @@ -354,6 +387,13 @@ static int ld9040_probe(struct spi_device
> > *spi)
> >         drm_panel_init(&ctx->panel, dev, &ld9040_drm_funcs,
> >                        DRM_MODE_CONNECTOR_DPI);
> >   
> > +
> > +       bldev = devm_backlight_device_register(dev, dev_name(dev),
> > dev,
> > +                                              ctx, &ld9040_bl_ops,
> > +                                              &ld9040_bl_props);
> > +       if (IS_ERR(bldev))
> > +               return PTR_ERR(bldev);
> > +
> >         drm_panel_add(&ctx->panel);
> >   
> >         return 0;
>