linux-kernel - [PATCH 07/25] clocksource/drivers/meson6_timer: Implement the ARM delay timer

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Date:   Tue, 18 Dec 2018 22:28:25 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Carlo Caione <carlo@...one.org>,
        Kevin Hilman <khilman@...libre.com>,
        linux-arm-kernel@...ts.infradead.org (moderated list:ARM/Amlogic Meson
        SoC support),
        linux-amlogic@...ts.infradead.org (open list:ARM/Amlogic Meson SoC
        support)
Subject: [PATCH 07/25] clocksource/drivers/meson6_timer: Implement the ARM delay timer

From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>

Implement an ARM delay timer to be used for udelay(). This allows us to
skip the delay loop calibration at boot.

With this patch udelay() is now independent of CPU frequency changes.
This is a good thing on Meson8, Meson8b and Meson8m2 because changing
the CPU frequency requires running the CPU clock off the XTAL while
changing the PLL or it's dividers. After changing the CPU clocks we need
to wait a few usecs for the clock to become stable. So having an
udelay() implementation that doesn't depend on the CPU frequency is
beneficial.

Suggested-by: Jianxin Pan <jianxin.pan@...ogic.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
---
 drivers/clocksource/meson6_timer.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/clocksource/meson6_timer.c b/drivers/clocksource/meson6_timer.c
index 23c7638e2bb3..84bd9479c3f8 100644
--- a/drivers/clocksource/meson6_timer.c
+++ b/drivers/clocksource/meson6_timer.c
@@ -22,6 +22,10 @@
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
 
+#ifdef CONFIG_ARM
+#include <linux/delay.h>
+#endif
+
 #define MESON_ISA_TIMER_MUX					0x00
 #define MESON_ISA_TIMER_MUX_TIMERD_EN				BIT(19)
 #define MESON_ISA_TIMER_MUX_TIMERC_EN				BIT(18)
@@ -54,6 +58,18 @@
 
 static void __iomem *timer_base;
 
+#ifdef CONFIG_ARM
+static unsigned long meson6_read_current_timer(void)
+{
+	return readl_relaxed(timer_base + MESON_ISA_TIMERE);
+}
+
+static struct delay_timer meson6_delay_timer = {
+	.read_current_timer = meson6_read_current_timer,
+	.freq = 1000 * 1000,
+};
+#endif
+
 static u64 notrace meson6_timer_sched_read(void)
 {
 	return (u64)readl(timer_base + MESON_ISA_TIMERE);
@@ -192,6 +208,12 @@ static int __init meson6_timer_init(struct device_node *node)
 
 	clockevents_config_and_register(&meson6_clockevent, USEC_PER_SEC,
 					1, 0xfffe);
+
+#ifdef CONFIG_ARM
+	/* Also use MESON_ISA_TIMERE for delays */
+	register_current_timer_delay(&meson6_delay_timer);
+#endif
+
 	return 0;
 }
 TIMER_OF_DECLARE(meson6, "amlogic,meson6-timer",
-- 
2.17.1