[LEDE-DEV] Proper way to deal with "dual firmware" ar71xx devices

[Mathias Kresin]

2017-04-23 20:40 GMT+02:00 Bjørn Mork <bjorn at mork.no>:
> Hello,
>
> Many devices make use of "dual firmware" configurations, splitting the
> available flash and allowing two complete and independent installations.
> This works fine for devices like the Linksys WRT1900AC etc, where the
> boot loader make sure the kernel command line "root=" parameter matches
> the booted kernel.
>
> It does not work so well with ar71xx devices like the Ubiquiti UniFi AC
> Pro. The original firmware use this layout:
>
> dev:    size   erasesize  name
> mtd0: 00060000 00010000 "u-boot"
> mtd1: 00010000 00010000 "u-boot-env"
> mtd2: 00790000 00010000 "kernel0"
> mtd3: 00790000 00010000 "kernel1"
> mtd4: 00020000 00010000 "bs"
> mtd5: 00040000 00010000 "cfg"
> mtd6: 00010000 00010000 "EEPROM"
>
>
> The current LEDE images configure this as:
>   MTDPARTS = spi0.0:384k(u-boot)ro,64k(u-boot-env)ro,7744k(firmware),7744k(ubnt-airos)ro,128k(bs)ro,256k(cfg)ro,64k(EEPROM)ro
>
>
> Note that "kernel0" is statically mapped to "firmware", and that
> "kernel1" (or "ubnt-airos") is made read-only.  This sort of works as
> long as LEDE is installed on "kernel0". But LEDE/OpenWrt does its magic
> partition splitting based of the "firmware" partifion name.  And it will
> do this even if the currently booting LEDE kernel is located on
> "ubnt-airos"/"kernel1".
>
> Due to limited understanding of how the Ubiquiti U-Boot selects between
> "kernel0" and "kernel1", there are instructions out there telling users
> to try to install LEDE on both "kernel0" and "kernel1".  But what
> happens if the boot loader is actually loading the "kernel1" image? We
> will then have a system with the kernel loaded from "kernel1" but the
> rootfs loaded from "kernel0".  This is bad.  When sysupgrading, the
> image on "kernel0" (aka "firmare") is replaced, But the boot loader will
> still continue to load the old LEDE kernel from "kernel1".  If you are
> lucky, it will boot successfully using the new rootfs.  You can then use
> the mtd-rw package to make "ubnt-airos" writeable and copy the new
> kernel there.  Extremely confusing and unfriendly to users...
>
> This should be fixed somehow.  But I don't know how.  The best would be
> to make the kernel dynamically figure out which of the partitions it
> booted from and then force the rootfs there.  But I don't know if this
> can be done without the help of the boot loader?

Hey Bjørn,

I had a similar problem with the dual image feature of brnboot in 
lantiq. It resulted in the following two patches:

https://git.lede-project.org/d7438ce3152e2e0dc1678dcb24ca050dbc2733c8
https://git.lede-project.org/60ac4852745c69ae13a770dc343b648925b73ca2

Since lantiq uses the device tree it is a device tree focused solution. 
It is for sure not the holy grial in terms of reusability and there 
might be more elegant ways to do it. But it might give you an idea.

As there is no sysupgrade support for the board at all, I never had to 
think about ways to change the active partition bit from userspace. But 
as far as I understand your mail, replicating the dual boot feature in 
LEDE isn't your primary objective.

Mathias

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