A true mainline Linux Kernel for the Pinebook Pro

[DrYak]

I've spent some time investigating the failing to load time from RTC at boot. (Some people are circumventing it by fumbling around with NTP daemons).

Looking at the logs:

Code:

$ journalctl --boot | grep -P 'rtc0|timesyncd'|head -n 3
Dec 17 13:44:08 nereid kernel: hctosys: unable to open rtc device (rtc0)
Dec 17 13:44:13 nereid kernel: rk808-rtc rk808-rtc: registered as rtc0
Dec 17 13:44:17 nereid systemd-timesyncd[791]: System clock time unset or jumped backwards, restoring from recorded timestamp: Tue 2019-12-17 23:29:02 CET

Seems that there's a race condition: kernel's `hctosys` tries to fetch time before the RTC driver has actually registered a device to fetch from, and thus when timesyncd starts the clock is still wrong and gets restored from last shutdown timestamp.

It looks like a time race that looks exactly like this bug report.

In pinebook pro's situation:

Code:

$ zgrep -P 'RTC_(HCTOSYS|SYSTOHC|DRV_RK808)' /proc/config.gz
CONFIG_RTC_HCTOSYS=y
CONFIG_RTC_HCTOSYS_DEVICE="rtc0"
# CONFIG_RTC_SYSTOHC is not set
CONFIG_RTC_DRV_RK808=m

it seems that we hit the same situation:
- hctosys is a built-in kernel function and is ran immediately during kernel start-up, at which time there is no RTC available yet.
- rk808 is a module and is loaded much later (5 sec later), by which time it's too late.

Compare with a PC Workstation:

Code:

> zgrep -E 'RTC_(HCTOSYS|SYSTOHC|DRV_CMOS)' /proc/config.gz
CONFIG_RTC_HCTOSYS=y
CONFIG_RTC_HCTOSYS_DEVICE="rtc0"
CONFIG_RTC_SYSTOHC=y
CONFIG_RTC_SYSTOHC_DEVICE="rtc0"
CONFIG_RTC_DRV_CMOS=y

and thus

Code:

> sudo dmesg | grep rtc0
[    1.384356] rtc_cmos 00:04: registered as rtc0

RTC is loaded almost immediately and there are no error from hctosys.

Would it be possible to:
- either compile the RK808 driver as builtin? (CONFIG_RTC_DRV_RK808=y like the bug report mentionned above)
- or add a udev rule to `hwclock --hctosys` as soon as rtc0 is registered ?