I want to compile a Linux kernel for the Raspberry Pi 4.
When compiling a Linux kernel, menuconfig is used to configure the Linux kernel before compiling it.
In my case, Linux 5.10 from raspberrypi git would by default compile module brd as a built-in module, meaning it is embedded in the kernel image vmlinuz or kernel8.img by default, unless otherwise configured with menuconfig.
The problem is that the brd module cannot be used at all if it is a built-in module. It must be a loadable kernel module in order to be usable for creating a RAMdisk block device (/dev/ram0). See also: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1593293
My problem is that I cannot make brd a loadable kernel module in my testing inside initrd.img:
BusyBox v1.30.1 (Ubuntu 1:1.30.1-6ubuntu2) built-in shell (ash)$ uname -aLinux ubuntu 5.10.20-v8+ #1 SMP PREMPT Mon Mar 8 11:33:05 UTC 2021 aarch64 GNU/Linux$ rmmod brdrmmod: ERROR: Module brd is builtin.$ echo $?1# normal usage: create RAM block device of 128MiB (rd_size is in kilobytes)$ modprobe brd rd_nr=1 rd_size=$(( 128 * 1024 ))$ echo $?1# note: no error message, only exit code 1# usage of the verbose flag to modprobe does not change anything:$ modprobe -v brd rd_nr=1 rd_size=$(( 128 * 1024 ))$ echo $?1# again: only exit code 1 and no output at all, even though -v (verbose) was specifically requested# lsmod is unavailable inside initrd$ lsmod/bin/sh: lsmod: not found$ cat /proc/modules# -> lists modules such as radi10 md_mod and spi_bcm2835 but not brd$ cat /sys/module/brdcat: can' t open '/sys/module/brd': No such file or directory
My question is: how can I force brd to become a loadable kernel module, and not loaded already. And what is going on with brd being a builtin module, but not listed in /proc/modules? I don't understand what is going on.