On 30/05/2024 15:33:11+0200, Alexander Dahl wrote:
> Hei hei,
>
> my current task is to bring up a new board featuring an at91 sam9x60
> SoC and a nxp pcf8523 RTC. The intention is to get a longer time the
> RTC can run on a capacitor because the external RTC draws less current
> than the internal RTC of the SoC.
>
> To save boards space and parts the 32768 Hz crystal of the SoC should
> be removed and the clkout of the external RTC should be used as a slow
> clock input instead.
>
> This is a somewhat known setup, at least I found other drivers
> implementing it for certain boards, see commits
> v5.15-rc1-16-g4c8a7b80d5f3 ("rtc: pcf85063: add support for fixed
> clock") or v5.13-rc1-2-gf765e349c3e1 ("rtc: m41t80: add support for
> fixed clock") for reference.
>
> Problem with the pcf8523: it has a shared !INT1 / CLKOUT pin, and you
> probably can use only either one or the other function. Default after
> reset is to have CLKOUT providing 32768 Hz, proved that with an
> oscilloscope.
>
> The naive way to add support is to just add a node to the dts and
> cross fingers like this:
>
> &flx5 {
> atmel,flexcom-mode = <ATMEL_FLEXCOM_MODE_TWI>;
> status = "okay";
>
> i2c5: i2c@600 {
> pinctrl-0 = <&pinctrl_flx5_default>;
> status = "okay";
>
> pcf8523: rtc@68 {
> compatible = "nxp,pcf8523";
> reg = <0x68>;
> };
> };
> };
>
> This actually works, but I suspect this is fragile, because it does
> not describe the clock I want to use, nor how to handle the irq.
>
> I noticed commit v5.12-rc2-22-g13e37b7fb75d ("rtc: pcf8523: add alarm
> support") and the commit message somehow suggests clkout is always
> disabled now? Well not always, only if some irq was defined in … but
> where exactly? In my setup the clkout is not disabled, I confirmed
> that with a scope.
>
> This is where all kinds of questions rise. The documentation on rtc
> and i2c device tree bindings is somewhat sparse. :-/
>
> From my understanding if I wanted to use the !INT1 pin as an actual
> interrupt line, I would probably hook it up to either a GPIO of the
> SoC or some wakeup input of the PMIC. What else is possible and how
> would the different ways be expressed in .dts? I found some .dts and
> .dtsi files having an 'interrupts' property in the rtc node, but I did
> not find the place in the code where this gets evaluated.
>
> For clkout it is probably like in
> arch/arm64/boot/dts/freescale/imx8mq-tqma8mq.dtsi and the driver
> changes would be maybe somewhat similar to the changes done in
> drivers/rtc/rtc-pcf85063.c right?
>
> Any hints on how to express in dts to _explicitly_ disable use as an
> interrupt pin and explicitly as fixed clkout? And what changes to the
> pcf8523 driver would have to be done, if any?
I see this email just now. Ideally, we would get a pinmuxing part in the
RTC driver so the device tree can explicitly describe which function is
used. Also, as you point out, this also needs a clock driver part so we
benefit from the CCF for clock enabling and recounting.
--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
