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

[Christophe Leroy]

Le 25/07/2022 à 14:52, Pali Rohár a écrit :
> On Monday 25 July 2022 18:20:01 Michael Ellerman wrote:
>> Pali Rohár <pali at kernel.org> writes:
>>> On Saturday 23 July 2022 14:42:22 Christophe Leroy wrote:
>>>> Le 22/07/2022 à 11:09, Pali Rohár a écrit :
>>>>> 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.
>>
>> Can you compare the disassembly before and after and find a place where
>> an instruction has changed?
>>
>> cheers
> 
> Yes, of course. Here is diff between output from objdump -d vmlinux.
> original version --- is from git master branch and modified version +++
> is the original version with reverted above problematic commit.
> So the +++ version is the one which is working.
> 
> --- vmlinux.master.dump	2022-07-25 14:43:45.922239496 +0200
> +++ vmlinux.revert.dump	2022-07-25 14:43:49.238259296 +0200
> @@ -1,5 +1,5 @@
>   
> -vmlinux.master:     file format elf32-powerpc
> +vmlinux.revert:     file format elf32-powerpc
>   
>   
>   Disassembly of section .head.text:
> @@ -11213,7 +11213,7 @@ c000b850:	3f a0 c1 0f 	lis     r29,-1611
>   c000b854:	81 02 00 04 	lwz     r8,4(r2)
>   c000b858:	3b fd 10 68 	addi    r31,r29,4200
>   c000b85c:	39 40 00 01 	li      r10,1
> -c000b860:	7d 20 f8 29 	lwarx   r9,0,r31,1
> +c000b860:	7d 20 f8 28 	lwarx   r9,0,r31
>   c000b864:	2c 09 00 00 	cmpwi   r9,0
>   c000b868:	40 82 00 10 	bne     c000b878 <die+0x68>
>   c000b86c:	7d 40 f9 2d 	stwcx.  r10,0,r31

That's really strange. I made a try with mpc85xx_defconfig with GCC 11 
and I don't get any such difference.

Does your version of GCC has anything special ?

Can you send you exact .config ?

Thanks
Christophe