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

From: Sylwester Nawrocki
Date: Fri Nov 18 2016 - 05:43:29 EST

On 11/18/2016 09:46 AM, Arnd Bergmann wrote:
> 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".

I've tested on S3C2440 SoC based board and I didn't notice any
issues with HZ=100.

Clock frequencies look a bit different because AFAIU MPLL
clock is mostly used as a root clock. The 12 MHz oscillator clock
is used a root clock for the MPLL.

refclk: 12000 kHz
mpll: 405000 kHz
upll: 48000 kHz
fclk: 405000 kHz
hclk: 101250 kHz
pclk: 50625 kHz

So frequency of the timer block's source clock (PCLK) is 50.625 MHz.
This is further divided by 50 in the prescaler as you pointed out.

So the 16-bit is clocked with 1012500 Hz clock. I added some printks
to verify this.

Here is boot log for HZ=200:
and HZ=100

samsung_clocksource_init:351 pclk: 50625000, timer clock_rate: 1012500
sched_clock: 16 bits at 1012kHz, resolution 987ns, wraps every 32362962ns

I just don't understand why the log says timer overflow is every 32.362 ms
and not twice this value (65536 * 1/1012500).