Getting rid of falcon mode

Alex G.
2021-04-12T21:32:49Z

## Introduction

Today we use "falcon mode" to mean "boot linux straight from SPL". This
designation makes sense, since falcons "fly at high speed and change
direction rapidly" according to Wikipedia.

The way we implement falcon mode is to reserve two areas of storage:
* kernel area/partition
* dtb area/partition
By using some "special cases", and "spl export", SPL can more or less
figure out how to skip u-boot.

## The plot twist

People familiar with FIT, will have recognized that the above is
achievable with a very basic FIT image. With some advantages:

- No "special cases" in SPL code
- Signed kernel images
- Signed kernel devicetree
- Devicetree overlays
- Automatic selection of correct devicetree

## The problems

The advantages of FIT are not obvious by looking at SPL code. A
noticeable amount of SPL code is hidden under #ifdef CONFIG_SPL_OS_BOOT,
leading one to believe that SPL_OS_BOOT is very important. It must be
since it takes up so much code.

Enabling falcon mode is not well documented, and requires a lot of trial
and error. I've had to define 7 macros, and one new function to get it
working on my board -- and vividly remember the grief. This is an
antiquated way of doing things, and completely ignores the u-boot
devicetree -- we could just as well have defined those seven values in
the dtb.

SPL assumes that it must load u-boot, unless in instances under
CONFIG_SPL_OS_BOOT. This has cause me a huge amount of grief and
confusion over the past several months. I have no less than three patch
series trying to address shortfalls there. It's awful.

## The proposal

I propose we drop falcon mode support for legacy images.

- Drop CONFIG_SPL_OS_BOOT. Support for this is implied by SPL_FIT
- Drop the "dtb area/partition". The dtb is derived from the FIT
- Drop "spl export"

How do we deal with devicetrees in the FIT then? The options are to use
a modified devicetree which has the desired "/chosen" node, or use DTB
overlays.

What are the reasons why we shouldn't go this route?

Alex