On Thu, 29 Apr 2021 00:29:49 +0200, Sebastian Reichel wrote: > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The > modules SQW clock output defaults to 32768 Hz. This behaviour is > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed, > the clock is disabled and all i.MX6 functionality depending on > the 32 KHz clock has undefined behaviour. For example when using > the hardware watchdog the system will likely do arbitrary reboots. > > Referencing the m41t62 directly results in a deadlock. The kernel > will see, that i.MX6 system clock needs the RTC clock and do probe > deferral. But the i.MX6 I2C module never becomes usable without the > i.MX6 CKIL clock and thus the RTC's clock will not be probed. So > from the kernel's perspective this is a chicken-and-egg problem. > > Technically everything is fine by not touching anything, since > the RTC clock correctly enables the clock on reset (i.e. on > battery backup power loss) and also the bootloader enables it > in case an something (e.g. an unpatched kernel) disabled this > incorrectly. > > A workaround for this issue is describing the square wave pin > as fixed-clock, which is registered early and basically how > this pin is used on the i.MX6. > > Suggested-by: Saravana Kannan <saravanak@xxxxxxxxxx> > Signed-off-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx> > --- > Documentation/devicetree/bindings/rtc/rtc-m41t80.txt | 9 +++++++++ > drivers/rtc/rtc-m41t80.c | 12 ++++++++++++ > 2 files changed, 21 insertions(+) > Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
