Re: [PATCH v2] ARM: Drop fixed 200 Hz timer requirement from Samsung platforms

From: Arnd Bergmann
Date: Fri Nov 18 2016 - 03:47:47 EST

On Friday, November 18, 2016 9:16:58 AM CET Krzysztof Kozlowski wrote:
> 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.
> Exynos uses its own MCT or arch timer and can work with all HZ values.
> Older platforms use newer Samsung PWM timer driver which should handle
> down to 100 Hz.
> 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 200_HZ from S3C/S5P suggested by Arnd]
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Krzysztof Kozlowski <krzk@xxxxxxxxxx>
> Cc: Kukjin Kim <kgene@xxxxxxxxxx>
> Tested-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>

Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

Maybe add a paragraph about the specific problem:

"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".