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

[Martin Storsjö]

On Fri, 7 Jun 2024, Martin Storsjö wrote:

> 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" );

Zhao, does this implementation seem useful to you? Does it give you better 
(more accurate, less noisy?) benchmarking numbers on Android, than the 
fallback based on clock_gettime?

// Martin