linux-kernel - Re: [PATCH 06/12] rtc: renesas-rtca3: Add driver for RTCA-3 available on Renesas RZ/G3S SoC

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Message-ID: <41981721-b90e-4eed-9258-9aa96ea9cc07@tuxon.dev>
Date: Tue, 16 Jul 2024 11:01:38 +0300
From: claudiu beznea <claudiu.beznea@...on.dev>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc: geert+renesas@...der.be, mturquette@...libre.com, sboyd@...nel.org,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, lee@...nel.org,
 magnus.damm@...il.com, linux-renesas-soc@...r.kernel.org,
 linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org,
 Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH 06/12] rtc: renesas-rtca3: Add driver for RTCA-3 available
 on Renesas RZ/G3S SoC

Hi, Alexandre,

On 17.06.2024 10:31, claudiu beznea wrote:
> On 17.06.2024 10:25, Alexandre Belloni wrote:
>> On 14/06/2024 14:06:38+0300, claudiu beznea wrote:
>>>>> +	/*
>>>>> +	 * Stop the RTC and set to 12 hours mode and calendar count mode.
>>>>> +	 * RCR2.START initial value is undefined so we need to stop here
>>>>> +	 * all the time.
>>>>> +	 */
>>>> Certainly not, if you stop the RTC on probe, you lose the time
>>>> information, this must only be done when the RTC has never been
>>>> initialised. The whole goal of the RTC is the keep time across reboots,
>>>> its lifecycle is longer than the system.
>>> This was also my first thought when I read the HW manual.
>>>
>>> It has been done like this to follow the HW manual. According to HW manual
>>> [1], chapter 22.3.19 RTC Control Register 2 (RCR2), initial value of START
>>> bit is undefined.
>>>
>>> If it's 1 while probing but it has never been initialized, we can falsely
>>> detect that RTC is started and skip the rest of the initialization steps.
>>> W/o initialization configuration, the RTC will not be able to work.
>>>
>>> Even with this implementation we don't loose the time b/w reboots. Here is
>>> the output on my board [2]. The steps I did were the following:
>>> 1/ remove the power to the board (I don't have a battery for RTC installed
>>>    at the moment)
>>> 2/ boot the board and issue hwclock -w
>>> 3/ reboot
>>> 4/ check the systime and rtc time
>>> 5/ poweroff
>>> 6/ poweron
>>> 7/ boot and check systime and RTC time
>>>
>>> As you can see the time is not lost but continue to increment. I presume
>>> the hardware takes into account that time needs to increment when initial
>>> configuration is executed.
>> I don't think so, I guess it stops ticking but the registers are not
>> reset so when ts starts ticking again, you are not too far from the time
>> that it should report.
> I'll double check with hardware team on this and return with an answer.

It has been confirmed me that the HW manual has a mistake and we can avoid
stopping the RTC here every time. I'll adjust it in the next version.

Thank you,
Claudiu Beznea