Re: deployment fallback and u-boot

[Colin Walters <walters verbum org>]

Hi,

On Tue, Aug 22, 2017, at 05:52 AM, Guy Shapiro wrote:
> Hi all,
>
> I am working with OSTree in the context of embedded device updates,
> utilizing the meta-updater Yocto layer.

Great!

> I want to add support for fallback in case of unbootable deployment.

A lot boils down to the precise definition of "unbootable"; there's
a related issue here:
https://github.com/ostreedev/ostree/issues/380

This is also potentially interesting/related:
https://github.com/projectatomic/rpm-ostree/pull/892

What specific scenarios are you thinking of here?  I can think of some:

 - Kernel breaks on just one of a few supported device types
 - Filesystem/drive corruption affecting just new tree
 - Hardware-independent logical error (e.g. shared library update breaks
   critical userspace daemon startup)

The last one for example I'd say should usually be caught by testing
in VMs/containers on the OS creator side, but perhaps it's a userspace daemon that interacts
with hardware (think video capture or the like)?
 
> Looks like most of the needed logic is already in place:
> OSTree stores the last bootable deployment. On bootloaders other than
> u-boot, both the new deployment and the last booted one available on the
> boot loader menu.  U-boot itself has mechanism for counting failed boots
> and running an alternative boot commands in case the count exceeds a
> limit. [1]

How does it measure "failed"?   Does userspace have to set something
in a config file in /boot or so? 
 
> The options I see are:
> 1) Write a second file, "uEnv2.txt", to "/boot/loader/".
> 2) Add kernel_image2/ramdisk_image2/ostree2 lines to "uEnv.txt"
>
> Do you have reason to prefer one of these options? Other options I
> haven't thought about?
> Does this approach sounds reasonable at all?

Honestly, I don't have a lot of expertise with u-boot myself; not enough
to be able to have an informed opinion on 1) vs 2) there.   Maybe someone
else does?

I did take a glance at
http://git.denx.de/?p=u-boot.git;a=blob;f=README;h=392b5fdbbbba334b3844b543c1e38eba1b4b0adf;hb=HEAD
and
http://git.denx.de/?p=u-boot.git;a=tree;f=doc;h=d43977d6bf2ced7ff3eb0f1b81080c967fd908fa;hb=HEAD
for the first time....impressive.