clocksource/drivers/exynos_mct: Prioritise Arm arch timer on arm64

All arm64 CPUs feature an architected timer, which offers a relatively
low-latency interface to a per-cpu clocksource and timer. For the most
part, using this interface is a no-brainer, with the exception of SoCs
where it cannot be used to wake up from deep idle state (i.e.
CLOCK_EVT_FEAT_C3STOP is set).

On the contrary, the Exynos MCT is extremely slow to access yet can be
used as a wakeup source. In preparation for using the Exynos MCT as a
potential wakeup timer for the Arm architected timer, reduce its ratings
so that the architected timer is preferred.

This effectively reverts the decision made in 6282edb72b
("clocksource/drivers/exynos_mct: Increase priority over ARM arch timer")
for arm64, as the reasoning for the original change was to work around
a 32-bit SoC design.

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Chanwoo Choi <cw00.choi@samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com> # exynos-5422
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210608154341.10794-2-will@kernel.org

drivers/clocksource/exynos_mct.c

@@ -51,6 +51,15 @@
 
 #define TICK_BASE_CNT	1
 
+#ifdef CONFIG_ARM
+/* Use values higher than ARM arch timer. See 6282edb72bed. */
+#define MCT_CLKSOURCE_RATING		450
+#define MCT_CLKEVENTS_RATING		500
+#else
+#define MCT_CLKSOURCE_RATING		350
+#define MCT_CLKEVENTS_RATING		350
+#endif
+
 enum {
 	MCT_INT_SPI,
 	MCT_INT_PPI
@@ -206,7 +215,7 @@ static void exynos4_frc_resume(struct clocksource *cs)
 
 static struct clocksource mct_frc = {
 	.name		= "mct-frc",
-	.rating		= 450,	/* use value higher than ARM arch timer */
+	.rating		= MCT_CLKSOURCE_RATING,
 	.read		= exynos4_frc_read,
 	.mask		= CLOCKSOURCE_MASK(32),
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
@@ -457,7 +466,7 @@ static int exynos4_mct_starting_cpu(unsigned int cpu)
 	evt->set_state_oneshot_stopped = set_state_shutdown;
 	evt->tick_resume = set_state_shutdown;
 	evt->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
-	evt->rating = 500;	/* use value higher than ARM arch timer */
+	evt->rating = MCT_CLKEVENTS_RATING,
 
 	exynos4_mct_write(TICK_BASE_CNT, mevt->base + MCT_L_TCNTB_OFFSET);