[PATCH v4 2/6] clocksource/drivers/exynos_mct: Don't register as a sched_clock on arm64

[Will McVicker <willmcvicker@google.com>]

The MCT register is unfortunately very slow to access, but importantly
does not halt in the c2 idle state. So for ARM64, we can improve
performance by not registering the MCT for sched_clock, allowing the
system to use the faster ARM architected timer for sched_clock instead.

The MCT is still registered as a clocksource, and a clockevent in order
to be a wakeup source for the arch_timer to exit the "c2" idle state.

Since ARM32 SoCs don't have an architected timer, the MCT must continue
to be used for sched_clock. Detailed discussion on this topic can be
found at [1].

[1] https://lore.kernel.org/linux-samsung-soc/1400188079-21832-1-git-send-email-chirantan@chromium.org/

[Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727

Signed-off-by: Donghoon Yu <hoony.yu@samsung.com>
Signed-off-by: Youngmin Nam <youngmin.nam@samsung.com>
Reviewed-by: Youngmin Nam <youngmin.nam@samsung.com>
Acked-by: John Stultz <jstultz@google.com>
Tested-by: Youngmin Nam <youngmin.nam@samsung.com>
Signed-off-by: Will McVicker <willmcvicker@google.com>
---
 drivers/clocksource/exynos_mct.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
index da09f467a6bb..96361d5dc57d 100644
--- a/drivers/clocksource/exynos_mct.c
+++ b/drivers/clocksource/exynos_mct.c
@@ -219,12 +219,18 @@ static struct clocksource mct_frc = {
 	.resume		= exynos4_frc_resume,
 };
 
+/*
+ * Since ARM devices do not have an architected timer, they need to continue
+ * using the MCT as the main clocksource for timekeeping, sched_clock, and the
+ * delay timer. For AARCH64 SoCs, the architected timer is the preferred
+ * clocksource due to it's superior performance.
+ */
+#if defined(CONFIG_ARM)
 static u64 notrace exynos4_read_sched_clock(void)
 {
 	return exynos4_read_count_32();
 }
 
-#if defined(CONFIG_ARM)
 static struct delay_timer exynos4_delay_timer;
 
 static cycles_t exynos4_read_current_timer(void)
@@ -250,12 +256,13 @@ static int __init exynos4_clocksource_init(bool frc_shared)
 	exynos4_delay_timer.read_current_timer = &exynos4_read_current_timer;
 	exynos4_delay_timer.freq = clk_rate;
 	register_current_timer_delay(&exynos4_delay_timer);
+
+	sched_clock_register(exynos4_read_sched_clock, 32, clk_rate);
 #endif
 
 	if (clocksource_register_hz(&mct_frc, clk_rate))
 		panic("%s: can't register clocksource\n", mct_frc.name);
 
-	sched_clock_register(exynos4_read_sched_clock, 32, clk_rate);
 
 	return 0;
 }
-- 
2.50.0.rc2.761.g2dc52ea45b-goog