Issue #1360 - Part 1: Simplify layers acceleration prefs.

This gets rid of platform-dependent hard-coded defaults, but keeps
build-time blocking if there is no GL provider (in which case layers
acceleration almost certainly won't work because it needs a GL
compositor and would likely crash without)

New prefs are
- layers.acceleration.enabled to enable HWA
- layers.acceleration.force to force it enabled (requires .enabled to be
  set as well)

This is the platform part of this issue. The rest will be front-end work
(Preference UI integration and pref migration)

9 files changed, 17 insertions, 74 deletions

diff --git a/gfx/thebes/gfxAndroidPlatform.h b/gfx/thebes/gfxAndroidPlatform.h
index 30e7c89ba..8975d0ab9 100644
--- a/gfx/thebes/gfxAndroidPlatform.h
+++ b/gfx/thebes/gfxAndroidPlatform.h
@@ -72,11 +72,6 @@ public:
       return true;
     }
 
-protected:
-    bool AccelerateLayersByDefault() override {
-      return true;
-    }
-
 private:
     gfxImageFormat mOffscreenFormat;
 };
diff --git a/gfx/thebes/gfxPlatform.cpp b/gfx/thebes/gfxPlatform.cpp
index 50d1fcb46..4cd044f90 100644
--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -571,40 +571,6 @@ gfxPlatform::Init()
       }
     }
 
-    // Drop a note in the crash report if we end up forcing an option that could
-    // destabilize things.  New items should be appended at the end (of an existing
-    // or in a new section), so that we don't have to know the version to interpret
-    // these cryptic strings.
-    {
-      nsAutoCString forcedPrefs;
-      // D2D prefs
-      forcedPrefs.AppendPrintf("FP(D%d%d",
-                               gfxPrefs::Direct2DDisabled(),
-                               gfxPrefs::Direct2DForceEnabled());
-      // Layers prefs
-      forcedPrefs.AppendPrintf("-L%d%d%d%d",
-                               gfxPrefs::LayersAMDSwitchableGfxEnabled(),
-                               gfxPrefs::LayersAccelerationDisabledDoNotUseDirectly(),
-                               gfxPrefs::LayersAccelerationForceEnabledDoNotUseDirectly(),
-                               gfxPrefs::LayersD3D11ForceWARP());
-      // WebGL prefs
-      forcedPrefs.AppendPrintf("-W%d%d%d%d%d%d%d%d",
-                               gfxPrefs::WebGLANGLEForceD3D11(),
-                               gfxPrefs::WebGLANGLEForceWARP(),
-                               gfxPrefs::WebGLDisabled(),
-                               gfxPrefs::WebGLDisableANGLE(),
-                               gfxPrefs::WebGLDXGLEnabled(),
-                               gfxPrefs::WebGLForceEnabled(),
-                               gfxPrefs::WebGLForceLayersReadback(),
-                               gfxPrefs::WebGLForceMSAA());
-      // Prefs that don't fit into any of the other sections
-      forcedPrefs.AppendPrintf("-T%d%d%d%d) ",
-                               gfxPrefs::AndroidRGB16Force(),
-                               gfxPrefs::CanvasAzureAccelerated(),
-                               gfxPrefs::DisableGralloc(),
-                               gfxPrefs::ForceShmemTiles());
-    }
-
     InitMoz2DLogging();
 
     gGfxPlatformPrefsLock = new Mutex("gfxPlatform::gGfxPlatformPrefsLock");
@@ -2171,7 +2137,7 @@ gfxPlatform::InitCompositorAccelerationPrefs()
                          FeatureStatus::Blocked,
                          "Acceleration blocked by platform"))
   {
-    if (gfxPrefs::LayersAccelerationDisabledDoNotUseDirectly()) {
+    if (!gfxPrefs::LayersAccelerationEnabledDoNotUseDirectly()) {
       feature.UserDisable("Disabled by pref",
                           NS_LITERAL_CSTRING("FEATURE_FAILURE_COMP_PREF"));
     } else if (acceleratedEnv && *acceleratedEnv == '0') {
@@ -2185,8 +2151,9 @@ gfxPlatform::InitCompositorAccelerationPrefs()
   }
 
   // This has specific meaning elsewhere, so we always record it.
-  if (gfxPrefs::LayersAccelerationForceEnabledDoNotUseDirectly()) {
-    feature.UserForceEnable("Force-enabled by pref");
+  if (gfxPrefs::LayersAccelerationEnabledDoNotUseDirectly() &&
+      gfxPrefs::LayersAccelerationForceEnabledDoNotUseDirectly()) {
+    feature.UserForceEnable("Force-enabled by prefs");
   }
 
   // Safe mode trumps everything.
diff --git a/gfx/thebes/gfxPlatform.h b/gfx/thebes/gfxPlatform.h
index 68bb99ea4..642cf909f 100644
--- a/gfx/thebes/gfxPlatform.h
+++ b/gfx/thebes/gfxPlatform.h
@@ -698,7 +698,7 @@ protected:
     virtual already_AddRefed<mozilla::gfx::VsyncSource> CreateHardwareVsyncSource();
 
     // Returns whether or not layers should be accelerated by default on this platform.
-    virtual bool AccelerateLayersByDefault();
+    bool AccelerateLayersByDefault();
 
     // Returns a prioritized list of available compositor backends for acceleration.
     virtual void GetAcceleratedCompositorBackends(nsTArray<mozilla::layers::LayersBackend>& aBackends);
diff --git a/gfx/thebes/gfxPlatformGtk.h b/gfx/thebes/gfxPlatformGtk.h
index 9959c0e13..22ed4b08f 100644
--- a/gfx/thebes/gfxPlatformGtk.h
+++ b/gfx/thebes/gfxPlatformGtk.h
@@ -132,10 +132,6 @@ public:
       return true;
     }
 
-    bool AccelerateLayersByDefault() override {
-      return true;
-    }
-
 #ifdef GL_PROVIDER_GLX
     already_AddRefed<mozilla::gfx::VsyncSource> CreateHardwareVsyncSource() override;
 #endif
diff --git a/gfx/thebes/gfxPlatformMac.cpp b/gfx/thebes/gfxPlatformMac.cpp
index 79684dd19..75c5236a8 100644
--- a/gfx/thebes/gfxPlatformMac.cpp
+++ b/gfx/thebes/gfxPlatformMac.cpp
@@ -372,12 +372,6 @@ gfxPlatformMac::ReadAntiAliasingThreshold()
     return threshold;
 }
 
-bool
-gfxPlatformMac::AccelerateLayersByDefault()
-{
-  return true;
-}
-
 // This is the renderer output callback function, called on the vsync thread
 static CVReturn VsyncCallback(CVDisplayLinkRef aDisplayLink,
                               const CVTimeStamp* aNow,
diff --git a/gfx/thebes/gfxPlatformMac.h b/gfx/thebes/gfxPlatformMac.h
index 0807614f6..ea4c1a101 100644
--- a/gfx/thebes/gfxPlatformMac.h
+++ b/gfx/thebes/gfxPlatformMac.h
@@ -81,9 +81,6 @@ public:
     // lower threshold on font anti-aliasing
     uint32_t GetAntiAliasingThreshold() { return mFontAntiAliasingThreshold; }
 
-protected:
-    bool AccelerateLayersByDefault() override;
-
 private:
     virtual void GetPlatformCMSOutputProfile(void* &mem, size_t &size) override;
 
diff --git a/gfx/thebes/gfxPrefs.h b/gfx/thebes/gfxPrefs.h
index 1253fdb48..d02f15699 100644
--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -448,11 +448,11 @@ private:
   DECL_GFX_PREF(Once, "image.multithreaded_decoding.limit",    ImageMTDecodingLimit, int32_t, -1);
   DECL_GFX_PREF(Live, "image.webp.enabled",                    ImageWebPEnabled, bool, true);
 
-  DECL_GFX_PREF(Once, "layers.acceleration.disabled",          LayersAccelerationDisabledDoNotUseDirectly, bool, false);
+  DECL_GFX_PREF(Once, "layers.acceleration.enabled",           LayersAccelerationEnabledDoNotUseDirectly, bool, true);
   DECL_GFX_PREF(Live, "layers.acceleration.draw-fps",          LayersDrawFPS, bool, false);
   DECL_GFX_PREF(Live, "layers.acceleration.draw-fps.print-histogram",  FPSPrintHistogram, bool, false);
   DECL_GFX_PREF(Live, "layers.acceleration.draw-fps.write-to-file", WriteFPSToFile, bool, false);
-  DECL_GFX_PREF(Once, "layers.acceleration.force-enabled",     LayersAccelerationForceEnabledDoNotUseDirectly, bool, false);
+  DECL_GFX_PREF(Once, "layers.acceleration.force",             LayersAccelerationForceEnabledDoNotUseDirectly, bool, false);
   DECL_GFX_PREF(Once, "layers.allow-d3d9-fallback",            LayersAllowD3D9Fallback, bool, false);
   DECL_GFX_PREF(Once, "layers.amd-switchable-gfx.enabled",     LayersAMDSwitchableGfxEnabled, bool, false);
   DECL_GFX_PREF(Once, "layers.async-pan-zoom.enabled",         AsyncPanZoomEnabledDoNotUseDirectly, bool, true);
diff --git a/gfx/thebes/gfxWindowsPlatform.h b/gfx/thebes/gfxWindowsPlatform.h
index 129365f82..b56dab338 100644
--- a/gfx/thebes/gfxWindowsPlatform.h
+++ b/gfx/thebes/gfxWindowsPlatform.h
@@ -226,9 +226,6 @@ public:
     bool SupportsPluginDirectDXGIDrawing();
 
 protected:
-    bool AccelerateLayersByDefault() override {
-      return true;
-    }
     void GetAcceleratedCompositorBackends(nsTArray<mozilla::layers::LayersBackend>& aBackends) override;
     virtual void GetPlatformCMSOutputProfile(void* &mem, size_t &size) override;
 
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index c5daf9213..ada4dbc5f 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4497,7 +4497,16 @@ pref("network.tcp.keepalive.probe_count", 4);
 #endif
 
 // Whether to disable acceleration for all widgets.
-pref("layers.acceleration.disabled", false);
+#if defined(XP_UNIX) && !defined(XP_MACOSX)
+// On Linux this is disabled by default for known issues with "free" drivers
+pref("layers.acceleration.enabled", false);
+#else
+pref("layers.acceleration.enabled", true);
+#endif
+// Whether to force acceleration on, ignoring blacklists.
+// This requires layers.acceleration.enabled to be set to true
+pref("layers.acceleration.force", false);
+
 // Preference that when switched at runtime will run a series of benchmarks
 // and output the result to stderr.
 pref("layers.bench.enabled", false);
@@ -4506,18 +4515,6 @@ pref("layers.bench.enabled", false);
 pref("layers.gpu-process.dev.enabled", true);
 #endif
 
-// Whether to force acceleration on, ignoring blacklists.
-#ifdef ANDROID
-// bug 838603 -- on Android, accidentally blacklisting OpenGL layers
-// means a startup crash for everyone.
-// Temporarily force-enable GL compositing.  This is default-disabled
-// deep within the bowels of the widgetry system.  Remove me when GL
-// compositing isn't default disabled in widget/android.
-pref("layers.acceleration.force-enabled", true);
-#else
-pref("layers.acceleration.force-enabled", false);
-#endif
-
 pref("layers.acceleration.draw-fps", false);
 
 // Enable DEAA antialiasing for transformed layers in the compositor