Sunday, March 17, 2013

Chromebook Tips, Tricks & Errata

1. If, like me, you ever choose to clone partitions from your SSD/HDD to external media and then boot from that media - you'll swear someone slipped a hallucinogen into your coffee!  Be sure to complete the process by assigning the clone a new UUID like this:
$ sudo cgpt add -i 3 -u $(uuidgen) /dev/sdb
and avoid taking the trip to Wonderland all together.


2. To identify the current rootfs & kernel partitions: 
$ rootdev -s
will return the partition mounted as / like this:
where # is 3, 5, or 7. So, just subtract 1 from # to identify the current kernel partition.


3. Interestingly, the Acer C7 seems unable to boot from an SD card in the on-board card reader.  I have successfully created bootable SD cards using the slot, but must move them to an external USB reader to boot from them.  The only conclusion I can draw is that the firmware can only boot from a device connected via USB and that the card reader slot is, in fact,  not USB.


4. To fix the erratic trackpad behavior in Ubuntu:

sudo gedit /usr/share/X11/xorg.conf.d/50-synaptics.conf

after MatchIsTouchPad "on" in the file, insert these lines:

Option "FingerLow" "4"
Option "FingerHigh" "10"

Reboot and the problem should be resolved.  However, it's possible that a system upgrade (i.e. 12.04 to 12.10) may wipe out this change, so keep that in mind.


5. Another factor to consider regarding Linux Distros on Chromebooks is the version of kernel that the distro ships with.  Since Google's current CrOS kernel is version 3.4.0, I've generally stuck to distros that included kernel 3.4.0 or earlier.  While the CrOS kernel apparently incorporates some features of later kernels, for optimal compatibility, 3.4.0 seems like a good logical target.

Postscript - 05/30/13 - Just ran across this excellent explanation of the issue:
I was merely speculating based on reasoning and experience.

Saturday, March 9, 2013

Installation Method

[Edit]  This method is now deprecated in favor of the USB Bootable Media Toolbox.  This post remains  for historical reference only.  [08/31/13]

Here's a rather verbose & needlessly complex explanation of how I test Linux Distros on the Acer C7.  However, I think it should work with any Chromebook capable of booting from USB, but might require modification in some cases.  It is and will always be a "work-in-progress", so please let me know if something isn't clear.


     1. A Chromebook in "Developer Mode."
     2. Same Chromebook running Developer Firmware.
     3. Same Chromebook with USB Boot enabled.
     4. USB bootable media (thumb drive or SD card).  From 4 - 16 GB will suffice.  16 GB will match the internal storage of every Chromebook prior to the Pixel, except for the Acer C7.  So, you could duplicate your SSD for development purposes.  See Tips #1 & #3 for relevant info.

The Method

     A. From ChromeOS, use cgpt to create a GPT partition table on your media (I'm using a 16GB SD card in the reader slot), add a kernel partition #6 which is 16 MB in size, and add a rootfs partition #7 using the remaining space.  Something like this:

cgpt create /dev/mmcblk0
cgpt show /dev/mmcblk0
       start        size    part  contents
           0           1          PMBR
           1           1          Pri GPT header
           2          32          Pri GPT table
    31537119          32          Sec GPT table
    31537151           1          Sec GPT header
cgpt add /dev/mmcblk0 -i 6 -b 34 -s 32768 -t kernel -l KERN-C -P 1 -T 1

The size of partition #7 here is the beginning sector of "Sec GPT table" (31537119) MINUS the beginning sector of partition #7 (32802).  Yes, the font is very tiny, but it preserves the appearance of the output.

cgpt add /dev/mmcblk0 -i 7 -b 32802 -s 31504317 -t rootfs -l ROOT-C
cgpt show /dev/mmcblk0
       start        size    part  contents
           0           1          PMBR
           1           1          Pri GPT header
           2          32          Pri GPT table
          34       32768       6  Label: "KERN-C"
                                  Type: ChromeOS kernel
                                  UUID: B6BF18B8-5B01-384F-AA69-7AD61AB56646
                                  Attr: priority=1 tries=1 successful=0
       32802    31504317       7  Label: "ROOT-C"
                                  Type: ChromeOS rootfs
                                  UUID: F33BC249-4577-D94A-8B7B-5B13D68DE423
    31537119          32          Sec GPT table
    31537151           1          Sec GPT header

Please note that (at least on the C7) although you can use the built-in card reader to create such media, you may be unable to boot from it without an external USB card reader.  At least, I haven't been able to.  So, it may be more practical to simply use a thumb drive. See Tip #3 for a theory.

     B. Create a debugging-enabled kernel from the current kernel, copy it to partition #6 and prioritize it to boot once.  See Tip #2 for an explanation here.

rootdev -s
dd if=/dev/sda2 of=kernel_2.blob
dump_kernel_config kernel_2.blob > kernel_2.cfg
cp kernel_2.cfg kernel_debug.cfg

Now, replace the contents of kernel_debug.cfg with the following:


These are commandline arguments passed to the kernel, so they should only be separated by spaces.  Here, I've alphabetized them on separate lines for clarity.  Now, wrap the new kernel with the verified block and  the new config:

vbutil_kernel --repack kernel_debug.blob \
  --keyblock /usr/share/vboot/devkeys/kernel.keyblock \
  --version 1 \
  --signprivate /usr/share/vboot/devkeys/kernel_data_key.vbprivk \
  --config=kernel_debug.cfg \
  --oldblob kernel_2.blob

Make sure the new kernel verifies OK and finally copy it to partition #6 and prioritize it.

vbutil_kernel --verify kernel_debug.blob
dd if=kernel_debug.blob of=/dev/mmcblk0p6
cgpt add /dev/mmcblk0 -i 6 -P 1 -T 1

     C. Boot your chosen distro on another machine (real or virtual) and install it onto partition #7 of your bootable media.  In some cases, you may need to create a filesystem on partition #7 prior to the installation, so I do just in case.  I've used both Ext2 and Ext4 successfully.  Either skip installing a bootloader or attempt to place the bootloader on partition #7.  I assume the distro must support GPT disks in order for this to work.

     D. Double check the GPT partition table for any changes that might prevent booting.

     E. Copy the necessary stuff from the current Chrome OS rootfs to partition #7.  Include the following complete folders:


And these files as well:


Overkill perhaps, but handy for working on kernels from Linux too.

     F. Cross your fingers, reboot & attempt to boot from USB.  If it boots, reprioritize partition #6 and mark it as having booted successfully.

cgpt add /dev/sdb -i 6 -P 1 -T 0 -S 1

I adapted this method from the technique published by Olof Johannson, a member of the Chrome OS development team.