CVS commit: src/sys/arch/arm/cortex

["Rin Okuyama" <rin%netbsd.org@localhost>]

Module Name:    src
Committed By:   rin
Date:           Thu Jan  9 06:55:25 UTC 2025

Modified Files:
        src/sys/arch/arm/cortex: gtmr.c

Log Message:
gtmr_delay: Put SPINLOCK_BACKOFF_HOOK in busy loop

It is expanded as a `yield` instruction for aarch64.

This slightly improves peak performance for some drivers on
MP environments, as tested by msaitoh@ (thanks!).

XXX
This is what OpenBSD does, while FreeBSD just busy-spins.

However, unfortunately, benefits from `yield` should be limited,
as we still read count register quite frequently.

For long-duration delays, Linux uses `wfit` and `wfet` insns if
provided, and falls back to `wfe` otherwise:

https://github.com/torvalds/linux/blob/master/arch/arm64/lib/delay.c#L26-L51

Event streams should be configured for this purpose.


To generate a diff of this commit:
cvs rdiff -u -r1.49 -r1.50 src/sys/arch/arm/cortex/gtmr.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.