Re: [RESEND PATCH v7 2/7] arm64: barrier: Support smp_cond_load_relaxed_timeout()

Arnd Bergmann
2025-10-28T08:43:12Z

On Tue, Oct 28, 2025, at 06:31, Ankur Arora wrote:
> Support waiting in smp_cond_load_relaxed_timeout() via
> __cmpwait_relaxed(). Limit this to when the event-stream is enabled,
> to ensure that we wake from WFE periodically and don't block forever
> if there are no stores to the cacheline.
>
> In the unlikely event that the event-stream is unavailable, fallback
> to spin-waiting.
>
> Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check for each
> iteration in smp_cond_load_relaxed_timeout().

After I looked at the entire series again, this one feels like
a missed opportunity. Especially on low-power systems but possibly
on any ARMv9.2+ implementation including Cortex-A320, it would
be nice to be able to both turn off the event stream and also
make this function take fewer wakeups:

> +/* Re-declared here to avoid include dependency. */
> +extern bool arch_timer_evtstrm_available(void);
> +
> +#define cpu_poll_relax(ptr, val) \
> +do { \
> + if (arch_timer_evtstrm_available()) \
> + __cmpwait_relaxed(ptr, val); \
> + else \
> + cpu_relax(); \
> +} while (0)
> +

Since the caller knows exactly how long it wants to wait for,
we should be able to fit a 'wfet' based primitive in here and
pass the timeout as another argument.

Arnd