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

From: Marc Zyngier <maz@kernel.org>
To: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Joey Gouly <joey.gouly@arm.com>,
	kernel-team@android.com
Subject: [PATCH v2 10/10] arm64: Use WFxT for __delay() when possible
Date: Tue, 19 Apr 2022 19:27:55 +0100	[thread overview]
Message-ID: <20220419182755.601427-11-maz@kernel.org> (raw)
In-Reply-To: <20220419182755.601427-1-maz@kernel.org>

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);
+	} else 	if (arch_timer_evtstrm_available()) {
 		const cycles_t timer_evt_period =
 			USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);
 
-- 
2.34.1


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
