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)
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