Re: [PATCH v2 10/10] arm64: Use WFxT for __delay() when possible

Catalin Marinas
2022-04-20T17:44:12Z

On Tue, Apr 19, 2022 at 07:27:55PM +0100, Marc Zyngier wrote:
> Marginally optimise __delay() by using a WFIT/WFET sequence.
> It probably is a win if no interrupt fires during the delay.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/lib/delay.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
> index 1688af0a4c97..5b7890139bc2 100644
> --- a/arch/arm64/lib/delay.c
> +++ b/arch/arm64/lib/delay.c
> @@ -27,7 +27,17 @@ void __delay(unsigned long cycles)
> {
> cycles_t start = get_cycles();
>
> - if (arch_timer_evtstrm_available()) {
> + if (cpus_have_const_cap(ARM64_HAS_WFXT)) {
> + u64 end = start + cycles;
> +
> + /*
> + * Start with WFIT. If an interrupt makes us resume
> + * early, use a WFET loop to complete the delay.
> + */
> + wfit(end);
> + while ((get_cycles() - start) < cycles)
> + wfet(end);

Do you use WFET here as a pending interrupt would cause WFIT to complete
immediately?

> + } else if (arch_timer_evtstrm_available()) {

Nit: two spaces between else and if ;).

Acked-by: Catalin Marinas <catalin.marinas@arm.com>