[PATCH] drm/dp: drm_edp_backlight_set_level: do not always send 3-byte commands

[Val Packett <val@packett.cool>]

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>
---

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