From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexandre Belloni Subject: Re: [PATCH 0/3] Add basic support for RTC on Allwinner H6 SoC Date: Fri, 9 Aug 2019 01:39:30 +0200 Message-ID: <20190808233930.GM3600@piout.net> References: <20190412120730.473-1-megous@megous.com> <20190806183045.edhm3qzpegscf2z7@core.my.home> <20190807105502.GK3600@piout.net> <20190808121237.g6twq2nh3sayu3vx@core.my.home> Reply-To: alexandre.belloni-LDxbnhwyfcJBDgjK7y7TUQ@public.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Return-path: Sender: linux-sunxi-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org Content-Disposition: inline In-Reply-To: <20190808121237.g6twq2nh3sayu3vx-9v8tmBix7cb9zxVx7UNMDg@public.gmane.org> List-Post: , List-Help: , List-Archive: , List-Unsubscribe: , To: Chen-Yu Tsai , Mark Rutland , Alessandro Zummo , devicetree , Maxime Ripard , linux-kernel , linux-sunxi , Rob Herring , linux-arm-kernel , linux-rtc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org On 08/08/2019 14:12:37+0200, Ond=C5=99ej Jirman wrote: > On Wed, Aug 07, 2019 at 12:55:02PM +0200, Alexandre Belloni wrote: > > Hi, > >=20 > > On 06/08/2019 20:30:45+0200, Ond=C5=99ej 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 bootload= er will > > > be using this info to gate/disable the osciallator. > > >=20 > >=20 > > 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). >=20 > 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. >=20 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 d= on't > drive it from 24MHz DCXO oscillator. The reason is that you'd have to dea= l with > the fact that the clock for RTC then becomes 24000000/750 (750 is fixed > divider), which is 32000. >=20 > So if you want to get 32768Hz for RTC by tuning the DCXO, it would have t= o have > 24 576 000 Hz. And even if you could achieve that (doubtful), it would th= row off > timings in the rest of the system (say UART, USB, CPU, display ctl) in a = major way. >=20 > 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 time= rs, > because of inaccuracies introduced during DVFS, for example. >=20 > regards, > o. >=20 > > I'm curious, what kind of options does this RTC have? > >=20 > > --=20 > > Alexandre Belloni, Bootlin > > Embedded Linux and Kernel engineering > > https://bootlin.com > >=20 > > _______________________________________________ > > linux-arm-kernel mailing list > > linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel --=20 Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com --=20 You received this message because you are subscribed to the Google Groups "= linux-sunxi" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to linux-sunxi+unsubscribe-/JYPxA39Uh5TLH3MbocFF+G/Ez6ZCGd0@public.gmane.org To view this discussion on the web, visit https://groups.google.com/d/msgid= /linux-sunxi/20190808233930.GM3600%40piout.net.