Raspbian Kernel panic after RTC module .ko - How to unload modules at boot time?

Question

I am playing with the RasClock RTC module. After baking my own rtc-pcf2127a.ko, and following the tutorial from the website, I got a kernel panic when booting (Null pointer etc.) Could not find any decent emergency plan to recover my Raspbian, config.txt or cmdline.txt wouldn't help here, would they? How to disable a stalling .ko or enable emergency shell to sort things out? Thanks. BR

Answer 1

Here's two methods you can try; both require you edit files on the SD card.

Blacklist via cmdline.txt:

You can add an instruction to /boot/cmdline.txt. /boot is actually the first, small vfat partition on the SD card. Mount that somewhere and add to the beginning of the text in cmdline.txt:

modprobe.blacklist=rtc-pcf2127a

It doesn't have to be at the beginning, but don't add a line break anywhere. That file should contain one line only, it doesn't matter how long it is.

Blacklist via a modprobe .conf file

The preferred method for blacklisting is to add a line to a .conf file in /etc/modprobe.d (on the second, ext4 partition).

blacklist rtc-pcf2127a

And/or

install rtc-pcf2127a /bin/false

The last is needed if some other module depends on it, since blacklisting is not enough to prevent loading of dependencies. You can create a new file in that directory if you want, make sure to use the .conf suffix.

A possible issue with this method is you may have to run depmod -ae afterward to make it effective, meaning you'll have to use the cmdline.txt method first, to get the system running. I'd try this one first (without being able to call depmod, obviously) just to see.