On Fri, Aug 09, 2019 at 01:39:30AM +0200, Alexandre Belloni wrote: > On 08/08/2019 14:12:37+0200, Ondřej Jirman wrote: > > On Wed, Aug 07, 2019 at 12:55:02PM +0200, Alexandre Belloni wrote: > > > Hi, > > > > > > On 06/08/2019 20:30:45+0200, Ondřej Jirman wrote: > > > > Maybe whether XO or DCXO is used also matters if you want to do some > > > > fine > > > > tunning of DCXO (control register has pletny of options), but that's > > > > probably > > > > better done in u-boot. And there's still no need to read HOSC source > > > > from DT. > > > > The driver can just check compatible, and if it is H6 and > > > > OSC_CLK_SRC_SEL is 1, > > > > it can do it's DCXO tunning, or whatever. But neither OS nor bootloader > > > > will > > > > be using this info to gate/disable the osciallator. > > > > > > > > > > It is actually useful to be able to tweak the crystal tuning at > > > runtime to be able to reduce clock drift and compare with a reliable > > > source (e.g. NTP). > > > > I don't think there's a Linux kernel API that you can use to achieve that, > > so > > that's a rather theoretical concern at the moment. > > > > There is /sys/class/rtc/rtcX/offset which is even properly documented. > > The reason I asked is that some RTCs have both analog (changing the > oscillator capacitance) and digital (changing the RTC counter) so I'm > wondering whether this interface should be extended.
As I wrote below, that can't be achieved by tuning DCXO. > > Also there are multiple clocks, that can drive the RTC, and you usually > > don't > > drive it from 24MHz DCXO oscillator. The reason is that you'd have to deal > > with > > the fact that the clock for RTC then becomes 24000000/750 (750 is fixed > > divider), which is 32000. > > > > So if you want to get 32768Hz for RTC by tuning the DCXO, it would have to > > have > > 24 576 000 Hz. And even if you could achieve that (doubtful), it would > > throw off > > timings in the rest of the system (say UART, USB, CPU, display ctl) in a > > major way. > > > > I guess you can try tuning 24MHz oscillator so that it's closer to the > > real-world 24MHz via NTP reference for other reasons. But it would be > > complicated, and require precise interaction with other components, like > > using > > HW timers sourced from 24MHz HOSC clock, because you can't use CPU's timers, > > because of inaccuracies introduced during DVFS, for example. > > > > regards, > > o. > > > > > I'm curious, what kind of options does this RTC have? > > > > > > -- > > > Alexandre Belloni, Bootlin > > > Embedded Linux and Kernel engineering > > > https://bootlin.com > > > > > > _______________________________________________ > > > linux-arm-kernel mailing list > > > linux-arm-ker...@lists.infradead.org > > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel > > -- > Alexandre Belloni, Bootlin > Embedded Linux and Kernel engineering > https://bootlin.com > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-ker...@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
