Re: [linux-sunxi] [PATCH 0/3] Add basic support for RTC on Allwinner H6 SoC

From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Chen-Yu Tsai <wens@csie.org>, Mark Rutland <mark.rutland@arm.com>,
	Alessandro Zummo <a.zummo@towertech.it>,
	devicetree <devicetree@vger.kernel.org>,
	Maxime Ripard <maxime.ripard@bootlin.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-sunxi <linux-sunxi@googlegroups.com>,
	Rob Herring <robh+dt@kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	linux-rtc@vger.kernel.org
Subject: Re: [linux-sunxi] [PATCH 0/3] Add basic support for RTC on Allwinner H6 SoC
Date: Fri, 9 Aug 2019 01:39:30 +0200	[thread overview]
Message-ID: <20190808233930.GM3600@piout.net> (raw)
In-Reply-To: <20190808121237.g6twq2nh3sayu3vx@core.my.home>

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@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com