Introduction
The HI35xx targeted version of buildroot I've been using in this series of blog posts has support for an overlay filesystem. An init script has been added to the skeleton filesystem that uses arguments provided through the kernels command line parameters to mount the overlay. When it works, the result is that the overlay gets merged into the root filesystem at boot. However, I had some trouble getting it working.
Getting the Overlay Filesystem Working
The init script has been written to look for "overlay" and "overlayfstype" arguments in the kernel command line parameters. The typical use case is that you create a jffs2 partition on the flash and then set the "overlay" argument to point to that partition and set the "overlayfstype" argument to jffs2. I gave this a go but it did not work. After spending a bit of time investigating I realised the init script was not getting run. The idea is that the init script should get run as the first process and it should in turn run /sbin/init. However, it looked as if the kernel was running /sbin/init directly and skipping the init script. The solution was to explicitly tell the kernel to run the init script using the "init" kernel command line argument.
An example of the resulting boot arguments I needed to pass to the kernel was:
setenv bootargs mem=42M console=ttyAMA0,115200 root=/dev/mtdblock2 rootfstype=cramfs mtdparts=hi_sfc:256K(boot),1472K(uImage),5760K(rootfs),512K(overlay) overlay=/dev/mtdblock3 overlayfstype=jffs2 init=/init
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