[yocto] busybox and mdev, how does it really work?

[Anders Darander]

* Hans Beckérus <hans.beckerus at gmail.com> [140121 11:05]:
> Things looked ok for a while, /dev was populated properly after boot
> and all necessary file systems / mount points were created.
> We were using an ext2 fs in RAM for mounting /.
> Now, we made a change to instead use a CPIO image compressed using xz
> and enabled the support in kernel to handle this.
> This is when our problems started :( Suddenly our system booted with
> just a very minimalistic /dev folder, containing basically only a few
> of the devices probed at boot time.

> So, the questions now are:

> - how was /dev populated before when there was no /etc/init.d/mdev?

I haven't checked poky-tiny, or more specifically the kernel
configurations for it. But based on the description that I cited above,
I'd guess that poky-tiny has 
	CONFIG_DEVTMPFS=y
	CONFIG_DEVTMPFS_MOUNT=y
enabled.

That would mean the the kernel itself were managing /dev for you when
you were using ext2. Unfortunately, CONFIG_DEVTMPFS_MOUNT do only
automount devmptfs on /dev if you're not in an initramfs (or initrd). 

Thus, you should add a simple initscript which mounts devtmpfs on /dev.
Thereafter, you should be fine.

Regarding your other questions on mdev, it's been a little while since I
used mdev. Let us know if the ideas above is enough, or if you want to
dig more into mdev.

Cheers,
Anders

-- 
Anders Darander
ChargeStorm AB / eStorm AB