[PATCH] drm: rcar-du: Add setting to PnALPHAR register on Gen3

[Biju Das]

From: LUU HOAI <hoai.luu.ub at renesas.com>

In Gen3, when Alpha blend is enabled in the PnMR register,
depending on the initial value of the PnALPHAR register,
either channel of DU might be black screen.
Therefore, this patch prevents the black screen by setting
the PnALPHAR register to all 0.

In addition, PnALPHAR register will be released in
the R-Car Gen3 Hardware Manual Rev 2.4 (Sep. 2021).

Signed-off-by: LUU HOAI <hoai.luu.ub at renesas.com>
Signed-off-by: Biju Das <biju.das.jz at bp.renesas.com>
---
This patch is based on [1]
[1] https://github.com/renesas-rcar/linux-bsp/commit/fcb34fe338cbde0a64919430733541035f20a784

Not sure this patches has to go with Fixes tag for stable??

Tested the changes on RZ/G2M board

root at hihope-rzg2m:/cip-test-scripts#  modetest -M rcar-du -w 54:alpha:55555
root at hihope-rzg2m:/cip-test-scripts# modetest -M rcar-du -s "93 at 90:1024x768 at AR24" -d -P "54 at 90:400x300+200+200 at XR24"
setting mode 1024x768-75Hz at AR24 on connectors 93, crtc 90
testing 400x300 at XR24 overlay plane 54
---
 drivers/gpu/drm/rcar-du/rcar_du_plane.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/rcar-du/rcar_du_plane.c b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
index 5c1c7bb04f3f..aff39b9253f8 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_plane.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
@@ -510,6 +510,12 @@ static void rcar_du_plane_setup_format_gen3(struct rcar_du_group *rgrp,
 
 	rcar_du_plane_write(rgrp, index, PnDDCR4,
 			    state->format->edf | PnDDCR4_CODE);
+
+	/* In Gen3, PnALPHAR register need to be set to 0
+	 * to avoid black screen issue when alpha blend is enable
+	 * on DU module
+	 */
+	rcar_du_plane_write(rgrp, index, PnALPHAR, 0x00000000);
 }
 
 static void rcar_du_plane_setup_format(struct rcar_du_group *rgrp,
-- 
2.25.1