[PATCH v9 1/5] drm/mediatek: ovl: Fix XRGB format breakage for blend_modes unsupported SoCs

[Jason-JH.Lin]

If the constant alpha always enable, the SoCs that is not supported the
ignore pixel alpha bit will still use constant alpha. That will break
the original constant alpha setting of XRGB foramt for blend_modes
unsupported SoCs, such as MT8173.

Note that ignore pixel alpha bit is suppored if the SoC support the
blend_modes.
Make the constatnt alpha only enable when having a vliad blend_mode or
setting the has_alpha to fix the downgrade issue.

Fixes: bc46eb5d5d77 ("drm/mediatek: Support DRM plane alpha in OVL")
Signed-off-by: Jason-JH.Lin <jason-jh.lin at mediatek.com>
---
 drivers/gpu/drm/mediatek/mtk_disp_ovl.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/mediatek/mtk_disp_ovl.c b/drivers/gpu/drm/mediatek/mtk_disp_ovl.c
index 89b439dcf3a6..8453a72f9e59 100644
--- a/drivers/gpu/drm/mediatek/mtk_disp_ovl.c
+++ b/drivers/gpu/drm/mediatek/mtk_disp_ovl.c
@@ -473,8 +473,14 @@ void mtk_ovl_layer_config(struct device *dev, unsigned int idx,
 
 	con = ovl_fmt_convert(ovl, fmt, blend_mode);
 	if (state->base.fb) {
-		con |= OVL_CON_AEN;
 		con |= state->base.alpha & OVL_CON_ALPHA;
+
+		/*
+		 * For blend_modes supported SoCs, always enable constant alpha.
+		 * For blend_modes unsupported SoCs, enable constant alpha when has_alpha is set.
+		 */
+		if (blend_mode || state->base.fb->format->has_alpha)
+			con |= OVL_CON_AEN;
 	}
 
 	/* CONST_BLD must be enabled for XRGB formats although the alpha channel
-- 
2.43.0