Re: [PATCH v5 03/10] clk: renesas: clk-vbattb: Add VBATTB clock driver

[Krzysztof Kozlowski <krzk@kernel.org>]

On 31/10/2024 10:26, Claudiu Beznea wrote:
> Hi, Geert, Krzysztof,
> 
> On 31.10.2024 10:43, Geert Uytterhoeven wrote:
>> Hi Krzysztof,
>>
>> On Thu, Oct 31, 2024 at 8:48 AM Krzysztof Kozlowski <krzk@kernel.org> wrote:
>>> On Wed, Oct 30, 2024 at 01:01:13PM +0200, Claudiu 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.
>>>>
>>>> Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
>>>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
>>
>>>> --- a/drivers/clk/renesas/Kconfig
>>>> +++ b/drivers/clk/renesas/Kconfig
>>>> @@ -237,6 +237,10 @@ config CLK_RZV2H
>>>>       bool "RZ/V2H(P) family clock support" if COMPILE_TEST
>>>>       select RESET_CONTROLLER
>>>>
>>>> +config CLK_RENESAS_VBATTB
>>>> +     bool "Renesas VBATTB clock controller"
>>>
>>> tristate
>>
>> Good point.
>> However, does it work as a module, or would that break the RTC?
> 
> On RZ/G3S the RTC counter needs the clock provided by VBATTB.
> 
> I'll try with this as a module.

So it will defer, why would this be a problem? This does not look like
critical core component, which would halt the system probe (and even
then systems like Android put everything as modules).

Best regards,
Krzysztof