Re: clkout vs. !int1 on nxp pcf8523

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

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