We recently had a need for a quick RAM-disk image and went to the tool we've been using for years: Buildroot and took some notes along the way.

With less than 10 minutes of effort, I had a newly built RAM disk image that will work on either i.MX5x or i.MX6x boards, complete with login, and the filesystem utilities I was after. I was about to build

Try doing that, Yocto!

Getting right to the point, the steps I took were these:

Grab the latest snapshot

~/$ wget https://buildroot.org/downloads/buildroot-2012.08.tar.gz
~/$ tar zxvf buildroot-2012.08.tar.gz

Configure for i.MX

~/$ cd buildroot-2012.08
~/buildroot-2012.08$ make menuconfig

Using the kconfig interface, I chose:

• Target ARM (Little-Endian)
• Target Cortex A-8
• Under toolchain: enable large file support and WCHAR
• Under System Configuration, chose ttymxc1 as the "Port to run a getty (login prompt) on",
• Under Package Selection:Hardware, enabled dosfstools and e2fsprogs
• Under Filesystem Images, selected Output cpio - gzipped

Please read for a nice (and short) guide to usage. In particular, pay attention to this one:

~/buildroot-2012.08$ make busybox-menuconfig

After making those selections, a simple make resulted in a gzipped cpio archive.

Build the image

~/buildroot-2012.08$ make
~/buildroot-2012.08$ ls -l ./output/images/rootfs.cpio.gz
-rw-r--r-- 1 user group 1380949 2012-09-11 13:09 ./output/images/rootfs.cpio.gz

Less than 1.4MB for a working filesystem image with the tools I needed. Very nice!

Wrap it for U-Boot

In order to boot this using the 2-parameter U-Boot bootm command, all that was left is to wrap it up:

~/buildroot-2012.08$ mkimage -A arm -O linux -T ramdisk -n "Initial Ram Disk"
          -d output/images/rootfs.cpio.gz uramdisk.img
Image Name:   Initial Ram Disk
Created:      Tue Sep 11 15:26:51 2012
Image Type:   ARM Linux RAMDisk Image (gzip compressed)
Data Size:    1380949 Bytes = 1348.58 kB = 1.32 MB
Load Address: 0x00000000
Entry Point:  0x00000000

And boot it

I booted this on an i.MX6 by hand by placing it and a kernel on an SD card like so:

MX6Q SABRELITE U-Boot > mmc dev 0
MX6Q SABRELITE U-Boot > fatload mmc 0 12000000 uImage
reading uImage
3837484 bytes read
MX6Q SABRELITE U-Boot > fatload mmc 0 12500000 uramdisk.img
reading uramdisk.img
1381013 bytes read
MX6Q SABRELITE U-Boot > bootm 12000000 12500000
## Booting kernel from Legacy Image at 12000000 ...
   Image Name:   Linux-3.0.35-1968-gd3f7f36-02004
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    3837420 Bytes =  3.7 MB
   Load Address: 10008000
   Entry Point:  10008000
   Verifying Checksum ... OK
## Loading init Ramdisk from Legacy Image at 12500000 ...
   Image Name:   Initial Ram Disk
   Image Type:   ARM Linux RAMDisk Image (gzip compressed)
   Data Size:    1380949 Bytes =  1.3 MB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
   Loading Kernel Image ... OK
OK
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
...
Welcome to Buildroot
buildroot login: root
#

For more information about hacking RAM disks, check out this post.