From: joshua stein <jcs@jcs.org>
Subject: acpiec: try a short busy-wait first
To: tech@openbsd.org
Date: Mon, 10 Nov 2025 21:12:47 -0600

A single execution of acpisbs_read() does 1027 EC reads, 299 writes, 
and takes about 5980 milliseconds.  It has to do this on the ACPI 
task queue thread which means anything else behind it has to wait up 
to nearly 6 seconds to run.

If we busy-wait for a wee bit before falling back on tsleep, the 
same acpisbs function with the same number of EC reads/writes 
finishes in 125 milliseconds.


diff --git sys/dev/acpi/acpiec.c sys/dev/acpi/acpiec.c
index e6add9e7ef0..fc490ecbd33 100644
--- sys/dev/acpi/acpiec.c
+++ sys/dev/acpi/acpiec.c
@@ -91,7 +91,8 @@ void
 acpiec_wait(struct acpiec_softc *sc, uint8_t mask, uint8_t val)
 {
 	static int acpiecnowait;
-	uint8_t		stat;
+	int tries = 0;
+	uint8_t	stat;
 
 	dnprintf(40, "%s: EC wait_ns for: %b == %02x\n",
 	    DEVNAME(sc), (int)mask,
@@ -100,7 +101,7 @@ acpiec_wait(struct acpiec_softc *sc, uint8_t mask, uint8_t val)
 	while (((stat = acpiec_status(sc)) & mask) != val) {
 		if (stat & EC_STAT_SCI_EVT)
 			sc->sc_gotsci = 1;
-		if (cold || (stat & EC_STAT_BURST))
+		if (cold || (stat & EC_STAT_BURST) || tries++ < 300)
 			delay(1);
 		else
 			tsleep(&acpiecnowait, PWAIT, "acpiec", 1);