C720 News

According to Jay Lee (Author of the Chrubuntu Script), "Ron Minnich has confirmed all the post-Pixel x86 Chrome devices should do SeaBIOS CTRL+L legacy boot."  Reference

Makes one wonder why the pre-Pixel 3rd generation Intel devices haven't been given this ability.  I realize that upgrading the firmware isn't something most people would want to do, but the slot in the C710 firmware was left full of zeros anyway, so ... why not?

General Advice on Flashing Firmware

I guess relatively few people are as paranoid as I am, but given the ease and speed of flashing from CrOS or Linux on the C710 itself, I suggest the following process:

1. Before you flash an image, create a backup (BACKUP.BIN) of the current firmware and generate an md5sum for it.
2. Now, generate an md5sum for the image you're about to flash, let's say NEW_FW.BIN.
3. Flash the image.
4. Even if everything appears to go perfectly, create a backup of the firmware again, i.e. READBACK.BIN and either compare it to NEW_FW.BIN  or (I prefer to) generate an md5sum for READBACK.BIN and compare the md5sums of the two.

This will allow you to revert to the state of the firmware before you started (with your BACKUP.BIN) and guarantee that the EEPROM now contains what you intended.

How To ... Reconstruct your Firmware from an Invalid Backup

If you're stuck with an invalid backup of your factory firmware because you followed some other geek's flawed instructions ;), the best solution is to reconstruct your firmware by replacing the first 2 MB of code in your backup ROM image with the first 2 MB of code from the default C710 firmware (bios.bin in the shellball). This replaces the Intel Management Engine, which is the part that's trashed in the backup, but keeps the rest of your original ROM intact.

You want to keep the rest intact because the firmware contains the serial number of your C710 which is used by Google in some manner - I don't know exactly how.  So, anyway, here's the Intel Management Engine:

Download

Un-zipped, this file should be exactly 2048 kb or 2 MB. You need to replace the first 2 MB of your ROM backup with this 2 MB. So, the resulting COMBINED.ROM file will still be 8 MB in size. Then you can flash the COMBINED.ROM and you should be back to factory (or close enough).  

There are many ways to accomplish this, I used a Hex Editor to open my BACKUP.ROM and 01_SI_ALL.BIN, selected all of 01_SI_ALL.BIN, copied it to the clipboard, pasted it over the first 2 MB of BACKUP.ROM and then saved the new combined file as COMBINED.ROM.  Describing the process was more difficult than performing it, trust me.  And there are much simpler methods using the command line, but I hate typing, so you're on your own there! ;)

Special thanks to Chusheng Zheng for giving me a push to finally document this stuff!