Openelec

I want an OpenELEC media PC for my TV. And I want it to boot over NFS...

Here goes!

Get your hands on a release!

I picked the v1.95.5 build from http://openelec.tv/get-openelec.

Investigate the release

$ tar -xvf OpenELEC-Generic.i386-1.95.5.tar.bz2 
OpenELEC-Generic.i386-1.95.5/
OpenELEC-Generic.i386-1.95.5/licenses/
OpenELEC-Generic.i386-1.95.5/licenses/LGPL2.txt
OpenELEC-Generic.i386-1.95.5/licenses/GPL3.txt
OpenELEC-Generic.i386-1.95.5/licenses/MIT_Modified.txt
OpenELEC-Generic.i386-1.95.5/licenses/GPL.txt
OpenELEC-Generic.i386-1.95.5/licenses/BSD_3_Clause.txt
OpenELEC-Generic.i386-1.95.5/licenses/ATI.txt
OpenELEC-Generic.i386-1.95.5/licenses/FDL1_2.txt
OpenELEC-Generic.i386-1.95.5/licenses/MPL1_1.txt
OpenELEC-Generic.i386-1.95.5/licenses/Artistic.txt
OpenELEC-Generic.i386-1.95.5/licenses/LGPL3.txt
OpenELEC-Generic.i386-1.95.5/licenses/Clarified_Artistic.txt
OpenELEC-Generic.i386-1.95.5/licenses/Radeon_rlc.txt
OpenELEC-Generic.i386-1.95.5/licenses/FDL.txt
OpenELEC-Generic.i386-1.95.5/licenses/MIT.txt
OpenELEC-Generic.i386-1.95.5/licenses/Public_Domain.txt
OpenELEC-Generic.i386-1.95.5/licenses/GPL2.txt
OpenELEC-Generic.i386-1.95.5/licenses/FDL1_3.txt
OpenELEC-Generic.i386-1.95.5/licenses/Info-ZIP.txt
OpenELEC-Generic.i386-1.95.5/licenses/NVIDIA.txt
OpenELEC-Generic.i386-1.95.5/licenses/LGPL2_1.txt
OpenELEC-Generic.i386-1.95.5/licenses/APSL.txt
OpenELEC-Generic.i386-1.95.5/licenses/BSD_4_Clause.txt
OpenELEC-Generic.i386-1.95.5/licenses/BSD_2_Clause.txt
OpenELEC-Generic.i386-1.95.5/licenses/BSD.txt
OpenELEC-Generic.i386-1.95.5/create_installstick
OpenELEC-Generic.i386-1.95.5/create_virtualimage
OpenELEC-Generic.i386-1.95.5/CHANGELOG
OpenELEC-Generic.i386-1.95.5/openelec.ico
OpenELEC-Generic.i386-1.95.5/README.md
OpenELEC-Generic.i386-1.95.5/3rdparty/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/extlinux.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/isolinux.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/CodingStyle.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/mboot.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/logo/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/logo/syslinux-100.png
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/comboot.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/menu.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/rfc5071.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/pxelinux.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/gpt.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/syslinux.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/keytab-lilo.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/SubmittingPatches.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/memdisk.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/usbkey.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/distrib.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/doc/sdi.txt
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/win64/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/win64/syslinux64.exe
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/NEWS
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/dos/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/dos/syslinux.com
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/win32/
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/win32/syslinux.exe
OpenELEC-Generic.i386-1.95.5/3rdparty/syslinux/README
OpenELEC-Generic.i386-1.95.5/3rdparty/md5sum/
OpenELEC-Generic.i386-1.95.5/3rdparty/md5sum/md5sum.exe
OpenELEC-Generic.i386-1.95.5/3rdparty/md5sum/README
OpenELEC-Generic.i386-1.95.5/RELEASE
OpenELEC-Generic.i386-1.95.5/sample.conf/
OpenELEC-Generic.i386-1.95.5/sample.conf/extlinux.conf
OpenELEC-Generic.i386-1.95.5/sample.conf/grub.conf
OpenELEC-Generic.i386-1.95.5/sample.conf/syslinux.cfg
OpenELEC-Generic.i386-1.95.5/sample.conf/syslinux_installer.cfg
OpenELEC-Generic.i386-1.95.5/create_installstick.exe
OpenELEC-Generic.i386-1.95.5/target/
OpenELEC-Generic.i386-1.95.5/target/SYSTEM
OpenELEC-Generic.i386-1.95.5/target/KERNEL.md5
OpenELEC-Generic.i386-1.95.5/target/SYSTEM.md5
OpenELEC-Generic.i386-1.95.5/target/KERNEL
OpenELEC-Generic.i386-1.95.5/create_installstick.bat
OpenELEC-Generic.i386-1.95.5/INSTALL

The OpenELEC-Generic.i386-1.95.5/target/SYSTEM and OpenELEC-Generic.i386-1.95.5/target/KERNEL files are the most interesting here. The rest is really support fluff...

Look inside the SYSTEM image

mkdir system && sudo mount -o loop ./SYSTEM ./system

/usr/bin/Xorg is present, and /usr/lib/xbmc/xbmc.bin

Look inside the KERNEL image

$ file KERNEL 
KERNEL: Linux kernel x86 boot executable bzImage, version 3.2.21 (jetstream@1801-lin007) #1 SMP Fri Jul 6 01:46:03 CEST 2, RO-rootFS, swap_dev 0x4, Normal VGA
$ grep -P -a -b --only-matching $'\xfd\x37\x7a\x58\x5a\x00' KERNEL 
14946:?7zXZ
5133180:?7zXZ

Despite the fact that file states that this is a bzImage, I don't believe it... There are two xz magic headers, lets dig...

$ dd if=KERNEL bs=1 skip=14946 of=KERNEL.a.xz
5120734+0 records in
5120734+0 records out
5120734 bytes (5.1 MB) copied, 9.08123 s, 564 kB/s
$ dd if=KERNEL bs=1 skip=5133180 of=KERNEL.b.xz
2500+0 records in
2500+0 records out
2500 bytes (2.5 kB) copied, 0.0062067 s, 403 kB/s
$ file KERNEL.a.xz KERNEL.b.xz 
KERNEL.a.xz: XZ compressed data
KERNEL.b.xz: XZ compressed data

These files will start with XZ compressed data, but when that ends, there is some other binary data. To decompress we need to use the --single-stream argument to xz so that it won't label the data corrupt because of this.

$ cat KERNEL.a.xz | xz -d --single-stream > KERNEL.a
$ cat KERNEL.b.xz | xz -d --single-stream > KERNEL.b
xz: (stdin): Compressed data is corrupt

Looks like the second Magic header was present for some other reason. Perhaps it's part of the decompression engine?

Now we have the uncompressed kernel, we also have the initramfs and the contents of /proc/config.gz avaliable (inside KERNEL.a).

$ grep -P -a -b --only-matching $'\x1f\x8b\x08\x00' KERNEL.a
5229342:
6605512:
10308181:
$ for i in $(grep -P -a -b --only-matching $'\x1f\x8b\x08\x00' KERNEL.a | cut -d : -f 1); do dd if=KERNEL.a of=KERNEL.a.${i}.gz bs=1 skip=${i}; done
7551598+0 records in
7551598+0 records out
7551598 bytes (7.6 MB) copied, 14.7439 s, 512 kB/s
6175428+0 records in
6175428+0 records out
6175428 bytes (6.2 MB) copied, 11.0656 s, 558 kB/s
2472759+0 records in
2472759+0 records out
2472759 bytes (2.5 MB) copied, 4.82829 s, 512 kB/s
$ file KERNEL.a.*.gz
KERNEL.a.10308181.gz: gzip compressed data, ASCII, has comment, comment
KERNEL.a.5229342.gz:  gzip compressed data, ASCII, has CRC, was "", last modified: Tue May  7 14:40:32 2013, max speed
KERNEL.a.6605512.gz:  gzip compressed data, from Unix, max compression

Looking good... I wonder which is the kernel config?

cat KERNEL.a.6605512.gz | gzip -d | less

And does it support "Root file system on NFS" / CONFIG_ROOT_NFS (yes) and "IP: kernel level autoconfiguration" / CONFIG_IP_PNP (yes)

CONFIG_ROOT_NFS=y
CONFIG_IP_PNP=y
CONFIG_IP_PNP_DHCP=y

Good news, at least we don't have to rebuild the kernel.