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

From: Russell King - ARM Linux
Date: Mon Nov 21 2016 - 04:07:34 EST

On Mon, Nov 21, 2016 at 08:59:06AM +0000, Ben Dooks wrote:
> On 21/11/16 06:01, Tomasz Figa wrote:
> >2016-11-18 17:46 GMT+09:00 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".
> >
> >One thing to correct here is that the typical clock is PCLK, which is
> >derived from one of the PLLs and AFAIR is between 33-66 MHz on
> >s3c24xx. Technically you can drive the PWM block from an external
> >clock (12 MHz for some board-file based boards), but for simplicity
> >this functionality was omitted in the new PWM timer driver used for DT
> >boards (which worked fine with the PWM driven by PCLK).
> Given it was a clock mux option, that would not have been difficult to
> acheive. However these platforms are now so old people don't care, I
> think all my pre-armv7 stuff is now in a box.

However, there are still s3c2410 machines running out there... so we
should do our best not to break them.

> The use of the 12MHz input was to give something to run PWM timers from
> that wasn't interrupted by cpu frequency scaling as PCLK generally is
> half HCLK which is divided down from the core CPU clock.
> The original implementation was to go for the best accuracy from the
> timer at the expense of 200 irqs per second instead of the usual 100.

This sounds like a good enough reason not to switch away from using 200Hz
and the 12MHz input.

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