linux-kernel - [PATCH v4 11/17] clocksource/arm_arch_timer: Drop unnecessary ISB on CVAL programming

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Date:   Sun, 17 Oct 2021 13:42:19 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Will Deacon <will@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Shier <pshier@...gle.com>,
        Raghavendra Rao Ananta <rananta@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        kernel-team@...roid.com
Subject: [PATCH v4 11/17] clocksource/arm_arch_timer: Drop unnecessary ISB on CVAL programming

Switching from TVAL to CVAL has a small drawback: we need an ISB
before reading the counter. We cannot get rid of it, but we can
instead remove the one that comes just after writing to CVAL.

This reduces the number of ISBs from 3 to 2 when programming
the timer.

Signed-off-by: Marc Zyngier <maz@...nel.org>
---
 arch/arm/include/asm/arch_timer.h   | 4 ++--
 arch/arm64/include/asm/arch_timer.h | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm/include/asm/arch_timer.h b/arch/arm/include/asm/arch_timer.h
index 9f4b895b78f7..bb129b6d2366 100644
--- a/arch/arm/include/asm/arch_timer.h
+++ b/arch/arm/include/asm/arch_timer.h
@@ -31,6 +31,7 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
 			asm volatile("mcr p15, 0, %0, c14, c2, 1" : : "r" ((u32)val));
+			isb();
 			break;
 		case ARCH_TIMER_REG_CVAL:
 			asm volatile("mcrr p15, 2, %Q0, %R0, c14" : : "r" (val));
@@ -42,6 +43,7 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
 			asm volatile("mcr p15, 0, %0, c14, c3, 1" : : "r" ((u32)val));
+			isb();
 			break;
 		case ARCH_TIMER_REG_CVAL:
 			asm volatile("mcrr p15, 3, %Q0, %R0, c14" : : "r" (val));
@@ -52,8 +54,6 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 	} else {
 		BUILD_BUG();
 	}
-
-	isb();
 }
 
 static __always_inline
diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 4f4aa13dd01e..b8000ef71a2c 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -95,6 +95,7 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
 			write_sysreg(val, cntp_ctl_el0);
+			isb();
 			break;
 		case ARCH_TIMER_REG_CVAL:
 			write_sysreg(val, cntp_cval_el0);
@@ -106,6 +107,7 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
 			write_sysreg(val, cntv_ctl_el0);
+			isb();
 			break;
 		case ARCH_TIMER_REG_CVAL:
 			write_sysreg(val, cntv_cval_el0);
@@ -116,8 +118,6 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u64 val)
 	} else {
 		BUILD_BUG();
 	}
-
-	isb();
 }
 
 static __always_inline
-- 
2.30.2