Originally Posted by Eregoth
This is interesting. Is it possible to open or extract the .img files and investigate further what the differences might be?
Unlikely, I'm afraid... bootloader.img files are closed source and nobody knows what they contain except Asus (this is why there is no changelog, detailing the added features of v4.23 over v4.18, for example).
A bootloader is crypto-signed to the unique ID of a given Nexus 7, and when you flash a new one it 'inherits'
the previous bootloaders data. The bootloader acts as a kind of security gate keeper and becomes active every time you switch on the Nexus 7. It's a system critical component of the Nexus 7, and without it... nothing happens
. You can mess up pretty much any partition on the Nexus 7, and still recover from it... and is typically known as a softbrick
, and is characterized by such egregious behaviour as bootlooping or refusal to boot... but as long as can you see the White-on-Black Google logo (which is the bootloader) upon boot or the White-on-Black battery charging animation (also the bootloader) when charging from a fully shut down state... as long as you can see these, the device isn't hardbricked
and most of the time can be recovered.
If you corrupt, or erase the bootloader... the Nexus 7 is dead, from which there is known recovery. Although it's been speculated that an nvFlash tool could flash a new bootloader. I've used nvFlash before, on another tablet, and it's considerably more flexible/powerful than fastboot (you can change partition sizes, called 'layouts'
, for example)... but so far, nobody has been able to make nvFlash work on the Nexus 7
This PDF document
lodged by Nvidia Corporation with the United States Patent office in 2009 gives some indication of a what a bootloader is and what it's purpose is. Although somewhat technical, and a little out of date, it's worth reading.
All of which, though interesting, doesn't explain why the two v4.23 bootloaders are different. And reading through the posts here on this thread, it's clear there is a problem.
Fastboot flashing a bootloader is a risky procedure at the best of times, and always makes me nervous. And this kind of thing hardly inspires confidence in me.
Unless I've seriously misunderstood something, or I'm missing something, I think the only logical conclusion I can draw is Google have screwed up the bootloader
that ships with Build JWR66Y.
I've just done a Google search on this topic, to see if it's been flagged up anywhere else, outside of XDA... but so far, nothing. But it's early days, and build JWR66Y has only been available for around 24hrs... so who knows what may crop up over the next few days. Unless Google quietly fixes the problem, I suspect many people wishing to restore back to stock 4.3 are going to have similar problems as those who have reported on this thread.
Anyway... I guess we'll see what happens...
Still damned odd, though.