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. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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