> -----Original Message-----
> From: Val Packett <v...@packett.cool>
> Sent: Monday, July 7, 2025 2:12 AM
> To: Maarten Lankhorst <maarten.lankho...@linux.intel.com>; Maxime Ripard
> <mrip...@kernel.org>; Thomas Zimmermann <tzimmerm...@suse.de>;
> David Airlie <airl...@gmail.com>; Simona Vetter <sim...@ffwll.ch>; Kandpal,
> Suraj <suraj.kand...@intel.com>; Murthy, Arun R <arun.r.mur...@intel.com>
> Cc: Val Packett <v...@packett.cool>; Nautiyal, Ankit K
> <ankit.k.nauti...@intel.com>; dri-devel@lists.freedesktop.org; linux-
> ker...@vger.kernel.org
> Subject: [PATCH] drm/dp: drm_edp_backlight_set_level: do not always send
> 3-byte commands
>
> At least some panels using the LSB register are not happy with the
> unconditional increase of the command buffer to 3 bytes.
>
> With the BOE NE14QDM in my Dell Latitude 7455, the recent patches for
> luminance based brightness have introduced a regression: the brightness
> range stopped being contiguous and became nonsensical (it probably was
> interpreting the last 2 bytes of the buffer and not the first 2).
>
> Change from using a fixed sizeof() to a length variable that's only set to 3
> when luminance is used. Let's leave the default as 2 even for the single-byte
> version, since that's how it worked before.
>
> Fixes: f2db78e37fe7 ("drm/dp: Modify drm_edp_backlight_set_level")
> Signed-off-by: Val Packett <v...@packett.cool>
LGTM,
Reviewed-by: Suraj Kandpal <suraj.kand...@intel.com>
> ---
>
> Video evidence (haha): https://files.catbox.moe/sp1g9v.mp4
>
> As this fix is tiny, if you prefer to fix it differently somehow, feel free
> to just
> redo it yourselves without waiting for me to respin it :)
>
> drivers/gpu/drm/display/drm_dp_helper.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c
> b/drivers/gpu/drm/display/drm_dp_helper.c
> index db7896c7edb8..7eaa118d78c3 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -3962,6 +3962,7 @@ int drm_edp_backlight_set_level(struct
> drm_dp_aux *aux, const struct drm_edp_bac
> int ret;
> unsigned int offset = DP_EDP_BACKLIGHT_BRIGHTNESS_MSB;
> u8 buf[3] = { 0 };
> + size_t len = 2;
>
> /* The panel uses the PWM for controlling brightness levels */
> if (!(bl->aux_set || bl->luminance_set)) @@ -3974,6 +3975,7 @@ int
> drm_edp_backlight_set_level(struct drm_dp_aux *aux, const struct
> drm_edp_bac
> buf[1] = (level & 0x00ff00) >> 8;
> buf[2] = (level & 0xff0000) >> 16;
> offset = DP_EDP_PANEL_TARGET_LUMINANCE_VALUE;
> + len = 3;
> } else if (bl->lsb_reg_used) {
> buf[0] = (level & 0xff00) >> 8;
> buf[1] = (level & 0x00ff);
> @@ -3981,7 +3983,7 @@ int drm_edp_backlight_set_level(struct
> drm_dp_aux *aux, const struct drm_edp_bac
> buf[0] = level;
> }
>
> - ret = drm_dp_dpcd_write_data(aux, offset, buf, sizeof(buf));
> + ret = drm_dp_dpcd_write_data(aux, offset, buf, len);
> if (ret < 0) {
> drm_err(aux->drm_dev,
> "%s: Failed to write aux backlight level: %d\n",
> --
> 2.49.0
