linux-kernel - Re: [PATCH 1/2] drm/dp: Add drm_edp_backlight_get

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Message-ID: <CAJ6xRxW0jb+UfKOpp7W9eAorPVpuaTFu+4cy31ydHDy6naSduw@mail.gmail.com>
Date: Fri, 26 Sep 2025 17:27:07 +0800
From: Aaron Ma <aaron.ma@...onical.com>
To: "Kandpal, Suraj" <suraj.kandpal@...el.com>
Cc: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>, 
	"mripard@...nel.org" <mripard@...nel.org>, "tzimmermann@...e.de" <tzimmermann@...e.de>, 
	"airlied@...il.com" <airlied@...il.com>, "simona@...ll.ch" <simona@...ll.ch>, 
	"jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>, "Vivi, Rodrigo" <rodrigo.vivi@...el.com>, 
	"Deak, Imre" <imre.deak@...el.com>, 
	"joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>
Subject: Re: [PATCH 1/2] drm/dp: Add drm_edp_backlight_get_level

On Fri, Sep 26, 2025 at 5:23 PM Kandpal, Suraj <suraj.kandpal@...el.com> wrote:
>
> > Subject: [PATCH 1/2] drm/dp: Add drm_edp_backlight_get_level
> >
> > Implement drm_edp_backlight_get_level() to read the current backlight
> > brightness level from eDP DPCD registers via AUX channel.
> >
> > Signed-off-by: Aaron Ma <aaron.ma@...onical.com>
> > ---
> >  drivers/gpu/drm/display/drm_dp_helper.c | 52 +++++++++++++++++++++++++
> >  include/drm/display/drm_dp_helper.h     |  1 +
> >  2 files changed, 53 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
> > b/drivers/gpu/drm/display/drm_dp_helper.c
> > index 1ecc3df7e3167..0cfb357ebd9e2 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -3945,6 +3945,58 @@ int drm_dp_pcon_convert_rgb_to_ycbcr(struct
> > drm_dp_aux *aux, u8 color_spc)  }
> > EXPORT_SYMBOL(drm_dp_pcon_convert_rgb_to_ycbcr);
> >
> > +/**
> > + * drm_edp_backlight_get_level - Get the backlight level of eDP DPCD
> > +via AUX
> > + * @aux: The DP aux device
> > + * @bl: Backlight capability info from the panel
> > + *
> > + * Reads the current backlight brightness level from luminance mode
> > + * (24-bit value in nits) or DPCD AUX mode(16-bit and 8-bit modes).
> > + *
> > + * Returns: Current backlight level.
> > + */
> > +u32 drm_edp_backlight_get_level(struct drm_dp_aux *aux, const struct
> > +drm_edp_backlight_info *bl) {
> > +     int ret;
> > +     u8 buf[3] = { 0 };
> > +     u32 level = 0;
> > +
> > +     if (!(bl->aux_set || bl->luminance_set))
> > +             return 0;
> > +
> > +     if (bl->luminance_set) {
> > +             ret = drm_dp_dpcd_read(aux,
> > DP_EDP_PANEL_TARGET_LUMINANCE_VALUE, buf, sizeof(buf));
> > +             if (ret < 0) {
> > +                     DRM_DEV_ERROR(aux->drm_dev->dev,
> > +                                   "%s: Failed to read luminance value: %d\n",
> > +                                   aux->name, ret);
> > +                     return 0;
> > +             }
> > +             level = (buf[2] << 16 | buf[1] << 8 | buf[0]) / 1000;
> > +     } else if (bl->lsb_reg_used) {
> > +             ret = drm_dp_dpcd_read(aux,
> > DP_EDP_BACKLIGHT_BRIGHTNESS_MSB, buf, 2);
> > +             if (ret < 0) {
> > +                     DRM_DEV_ERROR(aux->drm_dev->dev,
>
> Use drm_err here ditto for same where ever this has been used
>

I will send v2.
Thanks,
Aaron


> Regards,
> Suraj Kandpal
>
> > +                                   "%s: Failed to read backlight level: %d\n",
> > +                                   aux->name, ret);
> > +                     return 0;
> > +             }
> > +             level = buf[0] << 8 | buf[1];
> > +     } else {
> > +             ret = drm_dp_dpcd_read(aux,
> > DP_EDP_BACKLIGHT_BRIGHTNESS_MSB, buf, 1);
> > +             if (ret < 0) {
> > +                     DRM_DEV_ERROR(aux->drm_dev->dev,
> > +                                   "%s: Failed to read backlight level: %d\n",
> > +                                   aux->name, ret);
> > +                     return 0;
> > +             }
> > +             level = buf[0];
> > +     }
> > +
> > +     return level;
> > +}
> > +EXPORT_SYMBOL(drm_edp_backlight_get_level);
> > +
> >  /**
> >   * drm_edp_backlight_set_level() - Set the backlight level of an eDP panel via
> > AUX
> >   * @aux: The DP AUX channel to use
> > diff --git a/include/drm/display/drm_dp_helper.h
> > b/include/drm/display/drm_dp_helper.h
> > index 87caa4f1fdb86..0b045a47ae573 100644
> > --- a/include/drm/display/drm_dp_helper.h
> > +++ b/include/drm/display/drm_dp_helper.h
> > @@ -864,6 +864,7 @@ drm_edp_backlight_init(struct drm_dp_aux *aux,
> > struct drm_edp_backlight_info *bl
> >                      u32 max_luminance,
> >                      u16 driver_pwm_freq_hz, const u8
> > edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE],
> >                      u32 *current_level, u8 *current_mode, bool
> > need_luminance);
> > +u32 drm_edp_backlight_get_level(struct drm_dp_aux *aux, const struct
> > +drm_edp_backlight_info *bl);
> >  int drm_edp_backlight_set_level(struct drm_dp_aux *aux, const struct
> > drm_edp_backlight_info *bl,
> >                               u32 level);
> >  int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct
> > drm_edp_backlight_info *bl,
> > --
> > 2.43.0
>