[RFC PATCH hwc] drm_hwcomposer: set CRTC background color when available

[Stefan Schake]

Android assumes an implicit black background layer is always present
behind all layers it specifies for composition. drm_hwcomposer currently
punts responsibility for this to the kernel/DRM platform and puts layers
with per-pixel alpha content on the primary plane when requested.

On some platforms (e.g. VC4) a background color fill has a cycle cost for
the hardware composer and is not enabled by default. Instead, userland can
request a background color through a CRTC property. Use this property to
specify the implicit black background Android expects.

Signed-off-by: Stefan Schake <stschake at gmail.com>
---
Kernel changes for this (background_color) are available here:

https://github.com/stschake/linux/commits/background-upstream

Sending as RFC because I'm not entirely clear on whose responsibility
this should be, on most DRM drivers it seems to be implicit. I think
a case could also be made that VC4 should not accept alpha formats on
the lowest layer or enable background color fill when given one anyway.
On the other hand, userland control over background color seems desirable
regardless and is a feature of multiple hardware composers (i915, vc4, omap).

 drmcrtc.cpp              | 11 +++++++++++
 drmcrtc.h                |  2 ++
 drmdisplaycompositor.cpp | 15 +++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/drmcrtc.cpp b/drmcrtc.cpp
index 1b354fe..d7bcd7a 100644
--- a/drmcrtc.cpp
+++ b/drmcrtc.cpp
@@ -52,6 +52,13 @@ int DrmCrtc::Init() {
     ALOGE("Failed to get OUT_FENCE_PTR property");
     return ret;
   }
+
+  ret = drm_->GetCrtcProperty(*this, "background_color",
+                              &background_color_property_);
+  if (ret) {
+    ALOGI("Failed to get background_color property");
+    // This is an optional property
+  }
   return 0;
 }
 
@@ -86,4 +93,8 @@ const DrmProperty &DrmCrtc::mode_property() const {
 const DrmProperty &DrmCrtc::out_fence_ptr_property() const {
   return out_fence_ptr_property_;
 }
+
+const DrmProperty &DrmCrtc::background_color_property() const {
+  return background_color_property_;
+}
 }
diff --git a/drmcrtc.h b/drmcrtc.h
index c5a5599..704573a 100644
--- a/drmcrtc.h
+++ b/drmcrtc.h
@@ -46,6 +46,7 @@ class DrmCrtc {
   const DrmProperty &active_property() const;
   const DrmProperty &mode_property() const;
   const DrmProperty &out_fence_ptr_property() const;
+  const DrmProperty &background_color_property() const;
 
  private:
   DrmResources *drm_;
@@ -59,6 +60,7 @@ class DrmCrtc {
   DrmProperty active_property_;
   DrmProperty mode_property_;
   DrmProperty out_fence_ptr_property_;
+  DrmProperty background_color_property_;
 };
 }
 
diff --git a/drmdisplaycompositor.cpp b/drmdisplaycompositor.cpp
index e556e86..69c52dd 100644
--- a/drmdisplaycompositor.cpp
+++ b/drmdisplaycompositor.cpp
@@ -527,6 +527,21 @@ int DrmDisplayCompositor::CommitFrame(DrmDisplayComposition *display_comp,
       return ret;
     }
 
+    if (crtc->background_color_property().id() != 0) {
+      // Background color is specified as a 16 bit per channel RGBA value.
+      // Set a fully opaque black background as Android HWC expects.
+      const uint64_t background_color = 0xFFFF;
+
+      ret = drmModeAtomicAddProperty(pset, crtc->id(),
+                                     crtc->background_color_property().id(),
+                                     background_color) < 0;
+      if (ret) {
+        ALOGE("Failed to add CRTC background_color to pset: %d", ret);
+        drmModeAtomicFree(pset);
+        return ret;
+      }
+    }
+
     ret = drmModeAtomicAddProperty(pset, crtc->id(), crtc->mode_property().id(),
                                    mode_.blob_id) < 0 ||
           drmModeAtomicAddProperty(pset, connector->id(),
-- 
2.7.4