ath79: sysupgrade format policy for ar71xx migrations

[Sven Eckelmann]

Hi,

I am currently a little bit unsure about the preferred sysupgrade format when 
an ar71xx device is re-added as ath79 device. I know that there is a migration 
guide for users [1] but I didn't see one for developers.

First a little bit of background: The OpenMesh devices used the combined 
extended image format (CE01) for its factory image (used internally by their 
firmware) and the support for sysupgrade was added by Gabor Juhos and Marek 
Lindner. Support for this CE01 format was also added to ap51-flash - used 
since this point as primary method to flash these kind of devices over the 
network. These devices are using a bootloader which checks the vmlinux + 
rootfs partition before it boots it and switches to the "other" (recovery) 
area if the check failed. And they write to the currently not used area during 
a sysupgrade (+ change the boot order in u-boot).

But Matthias Schiffer (in gluon context) didn't like that there was only a 
factory image  but not sysupgrade image. So I've added a sysupgrade image as 
copy of the CE01 factory image. But this ended up as a problem when the first 
OpenMesh IPQ40xx devices were added. Mathias Kresin didn't like that there is 
(mostly) the same file as factory and sysupgrade file on the servers and that 
the sysupgrade file is not using something more common like sysupgrade-tar 
[2] to store kernel+rootfs in separate chunks. So he helped to implemented the 
sysupgrade-tar support for this kind of dual boot NOR devices and left the 
CE01 images as factory image for the IPQ40xx devices.

The OpenMesh company doesn't exist anymore since a while and they cannot be 
(officially) bought anymore. But various Freifunk installations are using 
these devices. So I have some interest in getting (at least parts) of these 
devices working for a while. At least when the dualboot bootloader partition 
selection problem is fixed [3] and the NOR flash size is big enough.


Now to the actual question: 

What should the OpenMesh devices use as images for ath79:

1. CE01 factory + sysupgrade-tar (like the OpenMesh IPQ40xx devices)
2. CE01 factory + CE01 sysupgrade (like ar71xx)
3. CE01 sysupgrade (like ar71xx but avoid having two files with the same 
   content)

Kind regards,
	Sven

[1] https://openwrt.org/docs/guide-user/installation/ar71xx.to.ath79
[2] Just as info: The sysupgrade-tar script is now creating unbootable images 
    for IPQ40xx OpenMesh devices in OpenWrt master
    https://github.com/openwrt/openwrt/pull/3616
[3] https://lists.openwrt.org/pipermail/openwrt-devel/2020-November/032261.html
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