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

[Pali Rohár]

Hello,

On Saturday 23 July 2022 14:42:22 Christophe Leroy wrote:
> Hello,
> 
> Le 22/07/2022 à 11:09, Pali Rohár a écrit :
> > 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.
> 
> Can you provide more information ? Which defconfig or .config, which 
> version of gcc, etc ... ?

I used default defconfig for mpc85xx with gcc 8, compilation for e500
cores.

If you need exact .config content I can send it during week.

> > 
> > 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?
> 
> Setting the eh flag to 0 is not supposed to be a change introduced by 
> that commit. Indeed that commit is not supposed to change anything at 
> all in the generated code.

My understanding of that commit is that it changed eh flag parameter
from 1 to 0 for 32-bit powerpc, including also p2020.

> Christophe
> 
> > 
> > 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.