Re: [PATCH v6 03/17] dt-bindings: rtc: sun6i: Add H616 compatible string

[Samuel Holland]

On 6/7/21 7:59 AM, Andre Przywara wrote:
> On Thu, 20 May 2021 21:37:34 -0500
> Samuel Holland <samuel@xxxxxxxxxxxx> wrote:
> 
> Hi,
> 
>> On 5/19/21 5:41 AM, Andre Przywara wrote:
>>> Add the obvious compatible name to the existing RTC binding.
>>> The actual RTC part of the device uses a different day/month/year
>>> storage scheme, so it's not compatible with the previous devices.
>>>
>>> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
>>> ---
>>>  .../devicetree/bindings/rtc/allwinner,sun6i-a31-rtc.yaml     | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/devicetree/bindings/rtc/allwinner,sun6i-a31-rtc.yaml b/Documentation/devicetree/bindings/rtc/allwinner,sun6i-a31-rtc.yaml
>>> index b1b0ee769b71..178c955f88bf 100644
>>> --- a/Documentation/devicetree/bindings/rtc/allwinner,sun6i-a31-rtc.yaml
>>> +++ b/Documentation/devicetree/bindings/rtc/allwinner,sun6i-a31-rtc.yaml
>>> @@ -26,6 +26,7 @@ properties:
>>>            - const: allwinner,sun50i-a64-rtc
>>>            - const: allwinner,sun8i-h3-rtc
>>>        - const: allwinner,sun50i-h6-rtc
>>> +      - const: allwinner,sun50i-h616-rtc
>>>  
>>>    reg:
>>>      maxItems: 1
>>> @@ -97,7 +98,9 @@ allOf:
>>>        properties:
>>>          compatible:
>>>            contains:
>>> -            const: allwinner,sun50i-h6-rtc
>>> +            enum:
>>> +              - allwinner,sun50i-h6-rtc
>>> +              - allwinner,sun50i-h616-rtc
>>>  
>>>      then:
>>>        properties:
>>>   
>>
>> This binding is missing a clock reference for the pll-periph0-2x input
>> to the 32kHz clock fanout.
> 
> Right. So do I get this correctly that we don't model the OSC24M input
> explicitly so far in the DT? I only see one possible input clock, which
> is for an optional 32K crystal oscillator.
> And this means we need to change some code also? Because at the moment
> a clock specified is assumed to be the 32K OSC, and having this clock
> means we switch to the external 32K OSC.

Right. The code would need updates to follow the binding.

> And who would decide which clock source to use? What would be the
> reason to use PLL_PERIPH(2x) over the RC16M based clock or the
> divided down 24MHz?

Because it would be more accurate. 24MHz/750 == 32000 Hz, while the RTC
expects 32768 Hz.

> So shall we ignore the PLL based input clock for now, put "0 input
> clocks" in the current binding, then later on extend this to allow
> choosing the PLL? And have it that way that having the PLL reference
> means we use it?

No, the device tree represents the hardware, not whatever happens to be
used by Linux drivers at the time. It should be in the binding
regardless of what the driver does with it.

Though the circular dependency between the clock providers does cause
problems. We cannot get a clk_hw for the PLL-based clock, so we would
have to hardcode a global name for it, which means we aren't really
using the device tree.

We already see this "not really using the binding" with the other CCUs:
the H616 CCU hardcodes the name "osc24M", while the A100 CCU hardcodes
"dcxo24M" for the same clock. So moving that clock into the RTC clock
provider would require using both names in one clk_hw simultaneously (or
rather fixing the CCU drivers to get a clk_hw from the DT instead of
referencing by name).

And trying to deal with optional clocks by index is only going to get
more painful over time. For example, with the R329 and D1, the RTC has
the following inputs:
 * DCXO24M (unless you model it inside the RTC)
 * External OSC32k (optional!)
 * The RTC bus gate/reset from the PRCM
 * R-AHB from the PRCM for the RTC SPI clock domain

So it seems time to start using clock-names in the RTC binding.

>> It is also missing a clock reference to the RTC register gate (and that
>> clock is in turn missing from the r_ccu driver).
> 
> Do you mean a gate bit somewhere in the PRCM? Do you have any
> pointer/documentation for that?

Yes, it's bit 0 of PRCM+0x20c, documented in the BSP[1], used in
mainline[2], and verified by experiment.

[1]:
https://github.com/orangepi-xunlong/linux-orangepi/blob/orange-pi-4.9-sun50iw9/drivers/clk/sunxi/clk-sun50iw9.h#L169
[2]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/clk/sunxi-ng/ccu-sun50i-a100-r.c#n129

> Cheers,
> Andre

Regards,
Samuel