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. > 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-kernel@xxxxxxxxxxxxxxxxxxx > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
