Android Hardware Composer supports alpha values applied to layers.
Enabling non-opaque layers for the STM CRTC could help offload GPU
resources for screen composition.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
---
  drivers/gpu/drm/stm/ltdc.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/stm/ltdc.c b/drivers/gpu/drm/stm/ltdc.c
index 195de30eb90c..e0fef8bacfa8 100644
--- a/drivers/gpu/drm/stm/ltdc.c
+++ b/drivers/gpu/drm/stm/ltdc.c

@@ -845,7 +845,7 @@ static void ltdc_plane_atomic_update(struct drm_plane *plane,
  			LXCFBLR_CFBLL | LXCFBLR_CFBP, val);
  
  	/* Specifies the constant alpha value */
-	val = CONSTA_MAX;
+	val = newstate->alpha >> 8;
  	reg_update_bits(ldev->regs, LTDC_L1CACR + lofs, LXCACR_CONSTA, val);
  
  	/* Specifies the blending factors */

@@ -997,6 +997,8 @@ static struct drm_plane *ltdc_plane_create(struct drm_device *ddev,
  
  	drm_plane_helper_add(plane, &ltdc_plane_helper_funcs);
  
+	drm_plane_create_alpha_property(plane);
+
  	DRM_DEBUG_DRIVER("plane:%d created\n", plane->base.id);
  
  	return plane;