Regression: Linux v5.15+ does not boot on Freescale P2020

[Pali Rohár]

Hello!

Trying to boot mainline Linux kernel v5.15+, including current version
from master branch, on Freescale P2020 does not work. Kernel does not
print anything to serial console, seems that it does not work and after
timeout watchdog reset the board.

I run git bisect and it found following commit:

9401f4e46cf6965e23738f70e149172344a01eef is the first bad commit
commit 9401f4e46cf6965e23738f70e149172344a01eef
Author: Christophe Leroy <christophe.leroy at csgroup.eu>
Date:   Tue Mar 2 08:48:11 2021 +0000

    powerpc: Use lwarx/ldarx directly instead of PPC_LWARX/LDARX macros

    Force the eh flag at 0 on PPC32.

    Signed-off-by: Christophe Leroy <christophe.leroy at csgroup.eu>
    Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
    Link: https://lore.kernel.org/r/1fc81f07cabebb875b963e295408cc3dd38c8d85.1614674882.git.christophe.leroy@csgroup.eu

:040000 040000 fe6747e45736dfcba74914a9445e5f70f5120600 96358d08b65d3200928a973efb5b969b3d45f2b0 M      arch


If I revert this commit then kernel boots correctly. It also boots fine
if I revert this commit on top of master branch.

Freescale P2020 has two 32-bit e500 powerpc cores.

Any idea why above commit is causing crash of the kernel? And why it is
needed? Could eh flag set to 0 cause deadlock?

I have looked into e500 Reference Manual for lwarx instruction (page 562)
https://www.nxp.com/files-static/32bit/doc/ref_manual/EREF_RM.pdf and
both 0 and 1 values for EH flag should be supported.