[PATCH v2] drm: rcar-du: Fix Alpha blending issue on Gen3

[Biju Das <biju.das.jz@bp.renesas.com>]

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

As per R-Car-Gen3_Common_OPC_Customer_Notifications_V30.1.pdf,
unexpected image output(such as incorrect colors or planes being
invisible) can happen on the below conditions, as PnALPHAR register
is not initialized by reset.

When alpha blending (PpMRm.PpSPIM=0b101) is performed and:
	•two Planes are enabled on any DUn (n=0,1,2,3)
		oDSPRn= 0x0000 0031 or 0x0000 0013
	•or DU0 and DU1 is used for display at the same time
		oDSPR0= 0x0000 0001 and DSPR1= 0x0000 0003
		oDSPR0= 0x0000 0003 and DSPR1= 0x0000 0001
	•or DU2 and DU3(H3 Only) is used for display at the same time
		oDSPR2= 0x0000 0001 and DSPR3= 0x0000 0003
		oDSPR2= 0x0000 0003 and DSPR3= 0x0000 0001

This patch set PnALPHAR register to 0 to avoid this issue.

Signed-off-by: LUU HOAI <hoai.luu.ub@renesas.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
---
v1->v2:
 * Updated commit description
 * Updated the code comments
---
 drivers/gpu/drm/rcar-du/rcar_du_plane.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/rcar-du/rcar_du_plane.c b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
index f214a8b6cfd3..aa80c44dd8d7 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_plane.c
+++ b/drivers/gpu/drm/rcar-du/rcar_du_plane.c
@@ -510,6 +510,18 @@ 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);
+
+	/*
+	 * On Gen3, some DU channels have two planes, each being wired to a
+	 * separate VSPD instance. The DU can then blend two planes. While
+	 * this feature isn't used by the driver, issues related to alpha
+	 * blending (such as incorrect colors or planes being invisible) may
+	 * still occur if the PnALPHAR register has a stale value. Set the
+	 * register to 0 to avoid this.
+	 */
+
+	/* TODO: Check if alpha-blending should be disabled in PnMR. */
+	rcar_du_plane_write(rgrp, index, PnALPHAR, 0);
 }
 
 static void rcar_du_plane_setup_format(struct rcar_du_group *rgrp,
-- 
2.25.1