linux-kernel - Re: [PATCH 15/17] arm64: allwinner: dts: add DTSI file for R329 SoC

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Date:   Thu, 19 Aug 2021 22:06:43 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Icenowy Zheng <icenowy@...eed.com>,
        Maxime Ripard <maxime@...no.tech>
Cc:     Rob Herring <robh+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Andre Przywara <andre.przywara@....com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] arm64: allwinner: dts: add DTSI file for R329 SoC

On 8/18/21 9:32 PM, Samuel Holland wrote:
> On 8/18/21 4:15 AM, Icenowy Zheng wrote:
>> 于 2021年8月18日 GMT+08:00 下午5:01:39, Maxime Ripard <maxime@...no.tech> 写到:
>>> On Mon, Aug 02, 2021 at 02:22:10PM +0800, Icenowy Zheng wrote:
>>>> +		ccu: clock@...1000 {
>>>> +			compatible = "allwinner,sun50i-r329-ccu";
>>>> +			reg = <0x02001000 0x1000>;
>>>> +			clocks = <&osc24M>, <&rtc 0>, <&rtc 2>;
>>>> +			clock-names = "hosc", "losc", "iosc";
>>>
>>> Do we have a clock tree for the RTC? Is it the same than the H616?
>>
>> Nope, it's the same with H6 because of external LOSC crystal is
>> possible. (Although production M2A SoMs has it NC for cost control.)
> 
> It is not the same as the H6, either. The clock tree _is_ identical to the D1,
> which has three diagrams on pages 363-364 of its user manual here:
> 
> https://dl.linux-sunxi.org/D1/D1_User_Manual_V0.1_Draft_Version.pdf
> 
> Compared to the H6, the R329/D1:
>  - Loses the LOSC calibration circuit
>  - Gains a third mux input for LOSC (not external 32k) to fanout
>  - Gains a mux to choose between LOSC and HOSC/750 for the RTC clock
>  - Gains an SPI bus clock input divided from the PRCM AHB
> 
> Compared to the H616, the R329/D1:
>  - Has an external 32k crystal input
>    - Gains the IOSC vs. external 32k crystal mux for LOSC
>    - Switches fanout mux input #1 from pll_periph0/N to external 32k
>  - Gains a mux to choose between LOSC and HOSC/750 for the RTC clock
>  - Gains an SPI bus clock input divided from the PRCM AHB
> 
> So the R329/D1 RTC has three^Wfour inputs:
>  - SPI clock from PRCM
>  - 24 MHz DCXO crystal
>  - 32 kHz external crystal (optional)

Whoops, I missed one here:
 - Bus clock from PRCM

The SPI clock is new for R329, but the bus clock has been around since H6.

> and four outputs:
>  - 16 MHz "IOSC" RC oscillator
>  - 32 kHz "LOSC"
>  - ~1 kHz for RTC timekeeping

Even though this is internal to the RTC, it is still useful to model, as it can
be used to correct for known RTC drift. (For example, HOSC/750 is 32000 Hz
instead of 32768 Hz, so 2.34375% slow. But that is better than IOSC, which has
unknown error.)

>  - 32 kHz fanout
> 
> (Arguably, since the 24 MHz DCXO can be turned on/off from the RTC registers, it
> should be an "output" and not an "input".)
> 
> Regards,
> Samuel
>