[PATCH v2 0/3] rtc: Set M41T82 & M41T83 xtal load capacitance from DT

[Dennis Lambe Jr <dennis@sparkcharge.io>]

I haven't heard any feedback on this series aside from the dt-bindings,
but with the holidays I'm not assuming that means they're perfect. I
don't want to sit on the fixes I've already incorporated any longer
though, so here's v2.

Changes in v2:
* dt-bindings: remove accidental wakeup-sources line
    suggested by Krzysztof Kozlowski
* spelling fixes in changelogs

The m41t82 and m41t83 have an adjustable internal capacitance that
defaults to 25 pF per xtal pin. This patch series adds the ability to
configure it via the devicetree.

Patch 1 just changes `#ifdef CONFIG_OF` to `if (IS_ENABLED(CONFIG_OF))`
in m41t80_probe() so that I don't need to use __maybe_unused on my new
functions and variables.

Patch 2 is the dt-bindings.

Patch 3 is the actual feature implementation.

The desired capacitance comes from the quartz-load-femtofarads property,
following the example of two other RTC ICs that have adjustable internal
load capacitance, the NXP pcf85063 and pcf8523. The m41t82 and m41t83
support much finer-grained control over the capacitance than those
chips, and ST calls the feature "analog calibration", but it looks to me
like it's essentially the same kind of thing.

My use case for this is:

ST specifies not to add any additional external load capacitance[1], but
the MikroElektronika RTC 9 Click board[2] has a 22 pF cap on each xtal
pin[3]. The resulting combined capacitance appears to be outside of the
operating range of the xtal, because when power is removed from the
boards I'm testing with, the RTC reports an Oscillator-Fail flag on the
next power on.

I found I could work around the problem by reducing the internal load
capacitance as low as it will go.

References:
[1] https://www.st.com/resource/en/application_note/an3060-applications-guide-for-serial-realtime-clocks-rtcs-stmicroelectronics.pdf
[2] https://www.mikroe.com/rtc-9-click
[3] https://download.mikroe.com/documents/add-on-boards/click/rtc-9/rtc-9-click-schematic-v100.pdf

Previous versions:
v1: https://lore.kernel.org/linux-rtc/20221219190915.3912384-1-dennis@sparkcharge.io/T/

Dennis Lambe Jr (3):
  rtc: m41t80: probe: use IS_ENABLED for CONFIG_OF
  dt-bindings: m41t80: add xtal load capacitance
  rtc: m41t80: set xtal load capacitance from DT

 .../devicetree/bindings/rtc/st,m41t80.yaml    | 16 ++++
 drivers/rtc/rtc-m41t80.c                      | 84 +++++++++++++++++--
 2 files changed, 92 insertions(+), 8 deletions(-)

-- 
2.25.1