[PATCH v3] ARM: Drop fixed 200 Hz timer requirement from Samsung platforms

From: Krzysztof Kozlowski
Date: Fri Nov 18 2016 - 06:15:45 EST

All Samsung platforms, including the Exynos, are selecting HZ_FIXED with
200 Hz. Unfortunately in case of multiplatform image this affects also
other platforms when Exynos is enabled.

This looks like an very old legacy code, dating back to initial
upstreaming of S3C24xx. Probably it was required for s3c24xx timer
driver, which was removed in commit ad38bdd15d5b ("ARM: SAMSUNG: Remove
unused plat-samsung/time.c").

Since then, this fixed 200 Hz spread everywhere, including out-of-tree
Samsung kernels (SoC vendor's and Tizen's). I believe this choice
was rather an effect of coincidence instead of conscious choice.

On S3C24xx, the PWM counter is only 16 bit wide, and with the
typical 12MHz input clock that overflows every 5.5ms. This works
with HZ=200 or higher but not with HZ=100 which needs a 10ms
interval between ticks. On Later chips (S3C64xx, S5P and EXYNOS),
the counter is 32 bits and does not have this problem.

The new samsung_pwm_timer driver solves the problem by scaling the input
clock by a factor of 50 on S3C24xx, which makes it less accurate but
allows HZ=100 as well as CONFIG_NO_HZ with fewer wakeups.

Few perf mem and sched tests on Odroid XU3 board (Exynos5422, 4x Cortex
A7, 4x Cortex A15) show no regressions when switching from 200 Hz to
other values.

Reported-by: Lee Jones <lee.jones@xxxxxxxxxx>
[Dropping of 200_HZ from S3C/S5P was suggested by Arnd]
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Krzysztof Kozlowski <krzk@xxxxxxxxxx>
Cc: Kukjin Kim <kgene@xxxxxxxxxx>
[Tested on Exynos5800]
Tested-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>
Acked-by: Kukjin Kim <kgene@xxxxxxxxxx>
[Tested on S3C2440]
Tested-by: Sylwester Nawrocki <s.nawrocki@xxxxxxxxxxx>


Changes since v2:
1. Extend message.
2. Add Kukjin's ack.
3. Add Sylwester's tested-by.

Changes since v1:
1. Add Javier's tested-by.
2. Drop HZ_FIXED also from ARCH_S5PV210 and ARCH_S3C24XX after Arnd
suggestions and analysis.
arch/arm/Kconfig | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index b5d529fdffab..ced2e08a9d08 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1496,8 +1496,7 @@ source kernel/Kconfig.preempt

config HZ_FIXED
- default 200 if ARCH_EBSA110 || ARCH_S3C24XX || \
+ default 200 if ARCH_EBSA110
default 128 if SOC_AT91RM9200
default 0