Don't use timestamp counters on ARM32 am: d955ee5d83 am: 25f11acb05

Original change: https://googleplex-android-review.googlesource.com/c/platform/art/+/24114904

Change-Id: I8dfbd25525c82b396d25a350508eee8b5c263c3e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

1 files changed, 11 insertions, 9 deletions

diff --git a/runtime/trace.cc b/runtime/trace.cc
index 429a5ae591..92f46c299c 100644
--- a/runtime/trace.cc
+++ b/runtime/trace.cc
@@ -91,10 +91,14 @@ double tsc_to_microsec_scaling_factor = -1.0;
 uint64_t GetTimestamp() {
   uint64_t t = 0;
 #if defined(__arm__)
-  // See Architecture Reference Manual ARMv7-A and ARMv7-R edition section B4.1.34
-  // Q and R specify that they should be written to lower and upper halves of 64-bit value.
-  // See: https://llvm.org/docs/LangRef.html#asm-template-argument-modifiers
-  asm volatile("mrrc p15, 1, %Q0, %R0, c14" : "=r"(t));
+  // On ARM 32 bit, we don't always have access to the timestamp counters from user space. There is
+  // no easy way to check if it is safe to read the timestamp counters. There is HWCAP_EVTSTRM which
+  // is set when generic timer is available but not necessarily from the user space. Kernel disables
+  // access to generic timer when there are known problems on the target CPUs. Sometimes access is
+  // disabled only for 32-bit processes even when 64-bit processes can accesses the timer from user
+  // space. These are not reflected in the HWCAP_EVTSTRM capability.So just fallback to
+  // clock_gettime on these processes. See b/289178149 for more discussion.
+  t = MicroTime();
 #elif defined(__aarch64__)
   // See Arm Architecture Registers  Armv8 section System Registers
   asm volatile("mrs %0, cntvct_el0" : "=r"(t));
@@ -173,11 +177,9 @@ void InitializeTimestampCounters() {
   }
 
 #if defined(__arm__)
-  double seconds_to_microseconds = 1000 * 1000;
-  uint64_t freq = 0;
-  // See Architecture Reference Manual ARMv7-A and ARMv7-R edition section B4.1.21
-  asm volatile("mrc p15, 0, %0, c14, c0, 0" : "=r"(freq));
-  tsc_to_microsec_scaling_factor = seconds_to_microseconds / static_cast<double>(freq);
+  // On ARM 32 bit, we don't always have access to the timestamp counters from
+  // user space. Seem comment in GetTimestamp for more details.
+  tsc_to_microsec_scaling_factor = 1.0;
 #elif defined(__aarch64__)
   double seconds_to_microseconds = 1000 * 1000;
   uint64_t freq = 0;