Bug 1692609 - Force 60hz for RFP and use that as the time-atom r=jgilbert

Differential Revision: https://phabricator.services.mozilla.com/D148122

gfx/thebes/gfxPlatform.cpp

@@ -29,6 +29,7 @@
 #include "mozilla/StaticPrefs_layout.h"
 #include "mozilla/StaticPrefs_layers.h"
 #include "mozilla/StaticPrefs_media.h"
+#include "mozilla/StaticPrefs_privacy.h"
 #include "mozilla/StaticPrefs_webgl.h"
 #include "mozilla/StaticPrefs_widget.h"
 #include "mozilla/Telemetry.h"
@@ -942,6 +943,10 @@ void gfxPlatform::Init() {
     Preferences::RegisterCallback(
         gfxPlatform::ReInitFrameRate,
         nsDependentCString(StaticPrefs::GetPrefName_layout_frame_rate()));
+    Preferences::RegisterCallback(
+        gfxPlatform::ReInitFrameRate,
+        nsDependentCString(
+            StaticPrefs::GetPrefName_privacy_resistFingerprinting()));
   }
 
   // Create the sRGB to output display profile transforms. They can be accessed
@@ -3024,17 +3029,26 @@ bool gfxPlatform::IsInLayoutAsapMode() {
   // the second is that the compositor goes ASAP and the refresh driver
   // goes at whatever the configurated rate is. This only checks the version
   // talos uses, which is the refresh driver and compositor are in lockstep.
+  // Ignore privacy_resistFingerprinting to preserve ASAP mode there.
   return StaticPrefs::layout_frame_rate() == 0;
 }
 
+static int LayoutFrameRateFromPrefs() {
+  auto val = StaticPrefs::layout_frame_rate();
+  if (StaticPrefs::privacy_resistFingerprinting()) {
+    val = 60;
+  }
+  return val;
+}
+
 /* static */
 bool gfxPlatform::ForceSoftwareVsync() {
-  return StaticPrefs::layout_frame_rate() > 0;
+  return LayoutFrameRateFromPrefs() > 0;
 }
 
 /* static */
 int gfxPlatform::GetSoftwareVsyncRate() {
-  int preferenceRate = StaticPrefs::layout_frame_rate();
+  int preferenceRate = LayoutFrameRateFromPrefs();
   if (preferenceRate <= 0) {
     return gfxPlatform::GetDefaultFrameRate();
   }

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -156,12 +156,26 @@ nsRFPService* nsRFPService::GetOrCreate() {
   return sRFPService;
 }
 
+constexpr double RFP_TIME_ATOM_MS = 16.667;  // 60Hz, 1000/60 but rounded.
+/*
+In RFP RAF always runs at 60Hz, so we're ~0.02% off of 1000/60 here.
+```js
+extra_frames_per_frame = 16.667 / (1000/60) - 1 // 0.00028
+sec_per_extra_frame = 1 / (extra_frames_per_frame * 60) // 833.33
+min_per_extra_frame = sec_per_extra_frame / 60 // 13.89
+```
+We expect an extra frame every ~14 minutes, which is enough to be smooth.
+16.67 would be ~1.4 minutes, which is OK, but is more noticable.
+Put another way, if this is the only unacceptable hitch you have across 14
+minutes, I'm impressed, and we might revisit this.
+*/
+
 /* static */
 double nsRFPService::TimerResolution() {
   double prefValue = StaticPrefs::
       privacy_resistFingerprinting_reduceTimerPrecision_microseconds();
   if (StaticPrefs::privacy_resistFingerprinting()) {
-    return std::max(100000.0, prefValue);
+    return std::max(RFP_TIME_ATOM_MS * 1000.0, prefValue);
   }
   return prefValue;
 }