linux-kernel - Re: [PATCH] ARM: dts: imx6qdl-hummingboard: Add rtc0 and rtc1 aliases to fix hctosys

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Date: Fri, 19 Jan 2024 13:46:26 +0000
From: Josua Mayer <josua@...id-run.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: Rob Herring <robh+dt@...nel.org>, Krzysztof Kozlowski
	<krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>,
	Shawn Guo <shawnguo@...nel.org>, Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>, Fabio Estevam
	<festevam@...il.com>, NXP Linux Team <linux-imx@....com>, Yazan Shhady
	<yazan.shhady@...id-run.com>, Jon Nettleton <jon@...id-run.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: dts: imx6qdl-hummingboard: Add rtc0 and rtc1 aliases
 to fix hctosys

Am 19.01.24 um 13:07 schrieb Josua Mayer:
> Am 18.01.24 um 17:07 schrieb Russell King (Oracle):
>> On Thu, Jan 18, 2024 at 04:01:10PM +0100, Josua Mayer wrote:
>>> HummingBoard has two RTCs, first integrated within SoC that can be used to
>>> wake up from sleep - and a second on the carrier board including back-up
>>> battery which is intended for keeping time during power-off.
>>>
>>> Add aliases for both, ensuring that the battery-backed clock is primary
>>> rtc and used by default during boot for restoring system time.
>> Given that the snvs RTC isn't battery backed, should we even be enabling
>> that in DT?
> In imx6qdl.dtsi it is not disabled.
> According to Jon it is useful because it can wake up the soc from sleep,
> whereas the external rtc can't.
>> Also, have you seen any issues such as:
>>
>> [    0.933249] rtc-pcf8523 0-0068: failed to set xtal load capacitance: -11
>> [    0.933505] rtc-pcf8523: probe of 0-0068 failed with error -11
>>
>> which seems to be exhibiting itself on my SolidSense board?
> Not on my HummingBoard Gate Rev. 1.4., but indeed on my solidsense
> unit too, which is probably same age as yours.
> Only tested imx6dl-hummingboard2-emmc-som-v15.dtb,
> but solidsense one should make no difference.

I was reading control registers 1-3:
debian@...imx6:~$ sudo i2cget -y -a -f 0 0x68 0x00
0x00
debian@...imx6:~$ sudo i2cget -y -a -f 0 0x68 0x01
0x00
debian@...imx6:~$ sudo i2cget -y -a -f 0 0x68 0x02
0x04

^^ This means low voltage on back up battery

After a few power-cycles that error went away.
Why pcf8523_load_capacitance would ever return EAGAIN I don't see.

In any case now that probe succeeded, I read these values:
0x80
0x00
0x04

And consequently when trying to read or write time I receive
[   88.844038] rtc-pcf8523 0-0068: low voltage detected, time is unreliable

Apparently all of my HummingBoard-2 / SolidSense have completely
drained their backup batteries to 0.1V over the past 4 years
while not in use.
FYI replacement part is MS621T.

Long story short I don't think the EAGAIN during probe is related
to adding aliases.
HOWEVER imo pcf8523_probe should return an error when
pcf8523_load_capacitance fails.