As of now ARM_LINUX_KERNEL_AS_BL33 is only supported when RESET_TO_BL31=1, along with it you need to pass PRELOADED_BL33_BASE as well as ARM_PRELOADED_DTB_BASE.
AFAIK This feature is not tested for platforms which uses all the BL(1/2/31) stages from TF-A . The most likely reason for this is loading and authentication of Linux Image.
BL2 which is responsible for loading of various images, does not have support to load linux image.
With platforms having RESET_TO_BL31, TF-A relies on prior loader which loads kernel and device tree blobs at respective address.

In short, if your platform has RESET_TO_BL31=1, it will be quite easy else you need to understand the BL2 loading mechanism and see if you can extend it for loading Linux and DTB.

Kernel Output format zImage/Image should work.

Hope this helps!
From: TF-A <tf-a-bounces@lists.trustedfirmware.org> on behalf of François Ozog via TF-A <tf-a@lists.trustedfirmware.org>
Sent: 15 April 2020 13:31
To: tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.org>
Subject: [TF-A] Linux as BL33
 
I want to use Linux as BL33 on a Marvell Macchiatobin.

Currently I have the successful boot flow:
TFA (mainline v2.2) -> U-Boot (Mainline 2020.04rc5) -> Kernel (5.6.3)
with U-root initrd (6.0.0, https://github.com/u-root/u-root ) ->
Ubuntu 19.10

The 5.6.3 "intermediary" kernel is 5.5MB uncompressed , u-root initrd
is 3.5MB compressed (some form of golang based busybox).

I was pointed to the ARM_LINUX_KERNEL_AS_BL33 option which is not
supported on the Macchiatobin.

It does not look too difficult to add, but I'd like to have some
feedback/guidance on how to do it:
- how to add the option to the TFA platform
- how to generate a usable kernel (compile options? non relocatable
kernel? output format, i.e. Image, zImage, uImage...)

Thanks for your help

-FF
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