[PATCH 1/5] asm-generic: add smp_vcond_load_relaxed()

[Will Deacon]

On Tue, Nov 05, 2024 at 12:30:37PM -0600, Haris Okanovic wrote:
> Relaxed poll until desired mask/value is observed at the specified
> address or timeout.
> 
> This macro is a specialization of the generic smp_cond_load_relaxed(),
> which takes a simple mask/value condition (vcond) instead of an
> arbitrary expression. It allows architectures to better specialize the
> implementation, e.g. to enable wfe() polling of the address on arm.

This doesn't make sense to me. The existing smp_cond_load() functions
already use wfe on arm64 and I don't see why we need a special helper
just to do a mask.

> Signed-off-by: Haris Okanovic <harisokn at amazon.com>
> ---
>  include/asm-generic/barrier.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index d4f581c1e21d..112027eabbfc 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -256,6 +256,31 @@ do {									\
>  })
>  #endif
>  
> +/**
> + * smp_vcond_load_relaxed() - (Spin) wait until an expected value at address
> + * with no ordering guarantees. Spins until `(*addr & mask) == val` or
> + * `nsecs` elapse, and returns the last observed `*addr` value.
> + *
> + * @nsecs: timeout in nanoseconds
> + * @addr: pointer to an integer
> + * @mask: a bit mask applied to read values
> + * @val: Expected value with mask
> + */
> +#ifndef smp_vcond_load_relaxed

I know naming is hard, but "vcond" is especially terrible.
Perhaps smp_cond_load_timeout()?

Will