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I've been running Mythbuntu on rPi2 for some time. I bought a couple of rPi3s in an effort to improve performance a bit. However, when I put the SD card from my rPi2 into the rPi3 the boot just shows a colorful image.

Am I wrong in thinking rPi3 should be binary compatible with rPi2?

PS. this is not power related. I specifically bought the official 2.5A rPi3 power supply to be sure.

  • I'm not sure if this is a Mythbuntu specific issue, but an sd card with Raspbian on it can be moved between Pi models and still work ok. See here for similar question: raspberrypi.org/forums/viewtopic.php?f=63&t=111083 – Kevin Hooke Jun 9 '16 at 5:52
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    "Am I wrong in thinking rPi3 should be binary compatible with rPi2?" -> No but as thlngan points out forward compatibility and backward compatibility are distinct concepts. Making things backward compatible, as is the case here, is commonplace. Making them forward compatible is much less so as it tends to complicate the possibility for "forward" to include "new" and "advance". – goldilocks Jun 9 '16 at 11:33
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The Raspberry Pi 3 needs different drivers than the Raspberry Pi 2. Generally speaking, kernels compiled for a Raspberry Pi will be made retro-compatible with previous iterations by including all relevant drivers, however, this does not make them forward-compatible. Is your specific version of Mythbuntu compatible with the Raspberry Pi 3, is the question you need to be asking.

2

Pi3 requires the lastest firmware and if you want to use the WiFi and Bluetooth new drivers.

There are hardware differences between the Pi3 & Pi2 which are handled by the latest kernel and Device Tree,

The rainbow screen indicates the firmware does not support Pi3.

Ubuntu MATE 16.04 will run on Pi3, and this will run on Pi2.

If you have a 16.04 Mythbuntu you should be able to upgrade to run on Pi3. There are instructions on the Wiki.

If you have an older version you will probably need a new installation. There is no supported upgrade path for Ubuntu MATE, I doubt there is for Mythbuntu.

2

To actually solve the problem...

  1. Get a distro image known to work on the Pi 3.

  2. Mount it on another system as described here where you can also mount your SD card.

  3. Make a backup copy of the stuff in your first partition (it is <25 MB) just in case, then delete all of it.

  4. Now copy everything from the first partition of the whatever image (you might as well use Raspbian jessie-lite, it is probably one of the smaller downloads) onto the first partition of your SD card.

  5. Go into the second/root fs partition of both things and look for the /lib/modules directory. From the Raspbian (or whatever) image, copy any directory over that ends -7+. If there is an identical one already there, you could back that up too; you don't have to bother deleting it as, e.g., cp -a will overwrite where appropriate.

The card should now work in both the 2 and 3.

You could also do this using the github "firmware" repo instead of a distro image (it is more than just "firmware", but that is somewhat flexible term; note both the contents of the first partition and the /lib/modules directly are there).

But...

If there is a newer version of Mythbuntu intended to work on the 3 a better choice is to upgrade to that. It will be backward compatible with the 2, so you can continue to use the same card/synced system on both.

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