On Sat, Oct 19, 2024 at 10:47 AM Claudiu [off-list ref] wrote:

From: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>

The VBATTB IP of the Renesas RZ/G3S SoC controls the clock that is used
by the RTC. The input to the VBATTB could be a 32KHz crystal
or an external clock device.

The HW block diagram for the clock generator is as follows:

           +----------+ XC   `\
RTXIN  --->|          |----->| \       +----+  VBATTCLK
           | 32K clock|      |  |----->|gate|----------->
           | osc      | XBYP |  |      +----+
RTXOUT --->|          |----->| /
           +----------+      ,

After discussions w/ Stephen Boyd the clock tree associated with this
hardware block was exported in Linux as:

vbattb-xtal
   xbyp
   xc
      mux
         vbattbclk

where:
- input-xtal is the input clock (connected to RTXIN, RTXOUT pins)
- xc, xbyp are mux inputs
- mux is the internal mux
- vbattclk is the gate clock that feeds in the end the RTC

to allow selecting the input of the MUX though assigned-clock DT
properties, using the already existing clock drivers and avoid adding
other DT properties. If the crystal is connected on RTXIN,
RTXOUT pins the XC will be selected as mux input. If an external clock
device is connected on RTXIN, RTXOUT pins the XBYP will be selected as
mux input.

The load capacitance of the internal crystal can be configured
with renesas,vbattb-load-nanofarads DT property.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
---

Changes in v4:
- dropped oscillator from patch description
- s/on-board/internal in patch description
- updated dt-binding included file name in the driver as it has been
  renamed to include/dt-bindings/clock/renesas,r9a08g045-vbattb.h
- dropped the "_BIT" from driver macros
- used "quartz-load-femtofarads" dt property instead of adding a new one
- register the "vbattclk" as critical clock as this feeds the RTC counter
  logic and it needs to stay on from the moment the RTC is configured;
  along with it, added a comment to express this.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds