[PATCH v1 2/6] clocksource/drivers/exynos_mct: Don't register as a sched_clock on arm64

[Will McVicker <willmcvicker@google.com>]

When using the Exynos MCT as a sched_clock, accessing the timer value
via the MCT register is extremely slow. To improve performance on Arm64
SoCs, use the Arm architected timer instead for timekeeping.

Note, ARM32 SoCs don't have an architectured timer and therefore
will continue to use the MCT timer. Detailed discussion on this topic
can be found at [1].

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

Signed-off-by: Donghoon Yu <hoony.yu@samsung.com>
Signed-off-by: Youngmin Nam <youngmin.nam@samsung.com>
[Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727
Signed-off-by: Will McVicker <willmcvicker@google.com>
---
 drivers/clocksource/exynos_mct.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
index da09f467a6bb..05c50f2f7a7e 100644
--- a/drivers/clocksource/exynos_mct.c
+++ b/drivers/clocksource/exynos_mct.c
@@ -219,12 +219,12 @@ static struct clocksource mct_frc = {
 	.resume		= exynos4_frc_resume,
 };
 
+#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 +250,13 @@ 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;
 }
-- 
2.49.0.472.ge94155a9ec-goog