[FFmpeg-devel] [PATCH] aarch64: Use cntvct_el0 as timer register on Android

[Martin Storsjö]

The default timer register pmccntr_el0 usually requires enabling
access with e.g. a kernel module.
---
cntvct_el0 has significantly better resolution than
av_gettime_relative (while the unscaled nanosecond output of
clock_gettime is much higher resolution).

In one tested case, the cntvct_el0 timer has a frequency of 25 MHz
(readable via the register cntfrq_el0).
---
 libavutil/aarch64/timer.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/libavutil/aarch64/timer.h b/libavutil/aarch64/timer.h
index fadc9568f8..966f17081a 100644
--- a/libavutil/aarch64/timer.h
+++ b/libavutil/aarch64/timer.h
@@ -33,7 +33,16 @@ static inline uint64_t read_time(void)
     uint64_t cycle_counter;
     __asm__ volatile(
         "isb                   \t\n"
+#if defined(__ANDROID__)
+        // cntvct_el0 has lower resolution than pmccntr_el0, but is usually
+        // accessible from user space by default.
+        "mrs %0, cntvct_el0        "
+#else
+        // pmccntr_el0 has higher resolution, but is usually not accessible
+        // from user space by default (but access can be enabled with a custom
+        // kernel module).
         "mrs %0, pmccntr_el0       "
+#endif
         : "=r"(cycle_counter) :: "memory" );
 
     return cycle_counter;
-- 
2.39.3 (Apple Git-146)