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 <val@packett.cool>
Tested-by: Abel Vesa <abel.vesa@linaro.org>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250706204446.8918-1-val@packett.cool
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index 1ecc3df7e3167d13636e194c4aab44ee8979aa11..4aaeae4fa03c36cf8697d819fe352e5ff5623f8e 100644 (file)
--- 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",