[Bug 1960950] [NEW] Build focal pi images using the classic branch (aka to enable the pi zero 2 support)

[Łukasz Zemczak]

Public bug reported:

[Impact]

Right now focal pi images build using the 18 branch - the one we use for
UC18. This made sense at the time, as we wanted to keep using one common
branch for all use-cases as long as possible. Sadly, with time, classic
needs diverged from what we used for core. Also, the current state
anyway makes no sense as we build focal images with 18 (where UC18 was
bionic based?!) branch code - confusing!

Recently we needed to do changes to the pi-gadget tree to enable support
for a new Pi device, and doing it via the 18 branch would be risky -
since we'd also have to then add those changes to the UC18 code-base.
Using the 20 branch is unwise as UC20 has a different model than classic
(due to all the UC20 constrains). So it was decided that we'll simply
switch to using the classic branch as we do for all groovy+ images. Dave
prepared all the changes to make sure the classic branch conditionally
generates images per focal existing standards.

[Test Case]

* With the -proposed livecd-rootfs, build classic raspi arm64 and armhf images
* Boot them on a few Raspberry Pi images, making sure that they still build and work correctly
* Boot the same image on a Pi Zero 2, making sure it boots correctly
* Check with certification that the daily proposed images work correctly on all the tested Pi's

[Regression Potential]

As we're basically switching branches to a completely new series-dependent codebase using a lot of conditionals, there is a high risk of regressing. The newly created images should be thoroughly tested manually and by the certification team on all Pi images, making sure no boot issues have been introduced.
It's risky, but sadly we have no other choice if we want to sanely support the new device.

** Affects: livecd-rootfs (Ubuntu)
     Importance: Critical
         Status: Invalid

** Affects: livecd-rootfs (Ubuntu Focal)
     Importance: Critical
         Status: In Progress

** Also affects: livecd-rootfs (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Changed in: livecd-rootfs (Ubuntu Focal)
    Milestone: None => ubuntu-20.04.4

** Changed in: livecd-rootfs (Ubuntu Focal)
       Status: New => In Progress

** Changed in: livecd-rootfs (Ubuntu)
       Status: In Progress => Invalid

** Changed in: livecd-rootfs (Ubuntu Focal)
   Importance: Undecided => Critical

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https://bugs.launchpad.net/bugs/1960950

Title:
  Build focal pi images using the classic branch (aka to enable the pi
  zero 2 support)

Status in livecd-rootfs package in Ubuntu:
  Invalid
Status in livecd-rootfs source package in Focal:
  In Progress

Bug description:
  [Impact]

  Right now focal pi images build using the 18 branch - the one we use
  for UC18. This made sense at the time, as we wanted to keep using one
  common branch for all use-cases as long as possible. Sadly, with time,
  classic needs diverged from what we used for core. Also, the current
  state anyway makes no sense as we build focal images with 18 (where
  UC18 was bionic based?!) branch code - confusing!

  Recently we needed to do changes to the pi-gadget tree to enable
  support for a new Pi device, and doing it via the 18 branch would be
  risky - since we'd also have to then add those changes to the UC18
  code-base. Using the 20 branch is unwise as UC20 has a different model
  than classic (due to all the UC20 constrains). So it was decided that
  we'll simply switch to using the classic branch as we do for all
  groovy+ images. Dave prepared all the changes to make sure the classic
  branch conditionally generates images per focal existing standards.

  [Test Case]

  * With the -proposed livecd-rootfs, build classic raspi arm64 and armhf images
  * Boot them on a few Raspberry Pi images, making sure that they still build and work correctly
  * Boot the same image on a Pi Zero 2, making sure it boots correctly
  * Check with certification that the daily proposed images work correctly on all the tested Pi's

  [Regression Potential]

  As we're basically switching branches to a completely new series-dependent codebase using a lot of conditionals, there is a high risk of regressing. The newly created images should be thoroughly tested manually and by the certification team on all Pi images, making sure no boot issues have been introduced.
  It's risky, but sadly we have no other choice if we want to sanely support the new device.

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