[PATCH 2/2] ARM: mmp: fix timer_read delay

From: Doug Brown <doug@schmorgal.com>
To: Russell King <linux@armlinux.org.uk>, Lubomir Rintel <lkundrak@v3.sk>
Cc: soc@kernel.org, Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
	Doug Brown <doug@schmorgal.com>
Subject: [PATCH 2/2] ARM: mmp: fix timer_read delay
Date: Sat,  3 Dec 2022 16:51:17 -0800	[thread overview]
Message-ID: <20221204005117.53452-3-doug@schmorgal.com> (raw)
In-Reply-To: <20221204005117.53452-1-doug@schmorgal.com>

timer_read() was using an empty 100-iteration loop to wait for the
TMR_CVWR register to capture the latest timer counter value. The delay
wasn't long enough. This resulted in CPU idle time being extremely
underreported on PXA168 with CONFIG_NO_HZ_IDLE=y.

Switch to the approach used in the vendor kernel, which implements the
capture delay by reading TMR_CVWR a few times instead.

Fixes: 49cbe78637eb ("[ARM] pxa: add base support for Marvell's PXA168 processor line")
Signed-off-by: Doug Brown <doug@schmorgal.com>
---
 arch/arm/mach-mmp/time.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm/mach-mmp/time.c b/arch/arm/mach-mmp/time.c
index 41b2e8abc9e6..708816caf859 100644
--- a/arch/arm/mach-mmp/time.c
+++ b/arch/arm/mach-mmp/time.c
@@ -43,18 +43,21 @@
 static void __iomem *mmp_timer_base = TIMERS_VIRT_BASE;
 
 /*
- * FIXME: the timer needs some delay to stablize the counter capture
+ * Read the timer through the CVWR register. Delay is required after requesting
+ * a read. The CR register cannot be directly read due to metastability issues
+ * documented in the PXA168 software manual.
  */
 static inline uint32_t timer_read(void)
 {
-	int delay = 100;
+	uint32_t val;
+	int delay = 3;
 
 	__raw_writel(1, mmp_timer_base + TMR_CVWR(1));
 
 	while (delay--)
-		cpu_relax();
+		val = __raw_readl(mmp_timer_base + TMR_CVWR(1));
 
-	return __raw_readl(mmp_timer_base + TMR_CVWR(1));
+	return val;
 }
 
 static u64 notrace mmp_read_sched_clock(void)
-- 
2.34.1

