drm/mediatek: ovl: Fix XRGB format breakage for blend_modes unsupported SoCs

OVL_CON_AEN is for alpha blending enable.
For the SoC that is supported the blend_modes, OVL_CON_AEN will always
enabled to use constant alpha and then use the ignore_pixel_alpha bit
to do the alpha blending for XRGB8888 format.
Note that ignore pixel alpha bit is not supported if the SoC is not
supported the blend_modes.
So it will break the original setting of XRGB8888 format for the
blend_modes unsupported SoCs, such as MT8173.

To fix the downgrade issue, enable alpha blending only when a valid
blend_mode or has_alpha is set.

Fixes: bc46eb5 ("drm/mediatek: Support DRM plane alpha in OVL")
Signed-off-by: Jason-JH.Lin <[email protected]>
Reviewed-by: CK Hu <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[email protected]/
Signed-off-by: Chun-Kuang Hu <[email protected]>

Author: Jason-JH.Lin

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 alpha blending.
+		 * For blend_modes unsupported SoCs, enable alpha blending 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