PPC32: Boot regression on Nintendo Wii, after create_branch rework in 5.8

[Jonathan Neuschäfer]

Hi,

On Wed, Apr 28, 2021 at 11:33:24AM +1000, Jordan Niethe wrote:
> On Mon, Apr 26, 2021 at 1:40 AM Jonathan Neuschäfer
> <j.neuschaefer at gmx.net> wrote:
> >
> > Hi,
> >
> > I recently booted my Wii again, and I noticed a regression at boot time.
> > Output stops after the "Finalizing device tree... flat tree at 0xXXXXXX"
> > message. I bisected it to this commit in the 5.8 development cycle:
> >
> > commit 7c95d8893fb55869882c9f68f4c94840dc43f18f
> > Author: Jordan Niethe <jniethe5 at gmail.com>
> > Date:   Wed May 6 13:40:25 2020 +1000
> >
> >     powerpc: Change calling convention for create_branch() et. al.
[...]
> > Do you have any hints on how to debug and/or fix this issue?
> Thanks for bisecting and reporting.
> The "Finalizing device tree... flat tree at 0xXXXXXX" message comes
> from the bootwrapper so if that is the last output it must be crashing
> pretty early.
> Commit 7c95d8893fb5 ("powerpc: Change calling convention for
> create_branch() et. al.") made a change to machine_init() in
> setup_32.c which seems like it might be a likely culprit for causing
> early crashing.
> The branch that is created and patched is just for optimization, so to
> see if that is in fact the problem it might be worth trying to boot
> with a patch like below
> 
> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
> --- a/arch/powerpc/kernel/setup_32.c
> +++ b/arch/powerpc/kernel/setup_32.c
> @@ -87,9 +87,6 @@ notrace void __init machine_init(u64 dt_ptr)
> 
>         patch_instruction_site(&patch__memcpy_nocache, ppc_inst(PPC_INST_NOP));
> 
> -       create_cond_branch(&insn, addr, branch_target(addr), 0x820000);
> -       patch_instruction(addr, insn);  /* replace b by bne cr0 */
> -
>         /* Do some early initialization based on the flat device tree */
>         early_init_devtree(__va(dt_ptr));

This brings no improvement, unfortunately. The output is still:

top of MEM2 @ 13F00000

zImage starting: loaded at 0x00b00000 (sp: 0x01145f90)
Allocating 0xae3970 bytes for kernel...
Decompressing (0x00000000 <- 0x00b11000:0x01143576)...
Done! Decompressed 0xa65fdc bytes

Linux/PowerPC load: root=/dev/mmcblk0p2 rootwait console=usbgecko1
Finalizing device tree... flat tree at 0x11467a0


I'll probably dig deeper next weekend.



Jonathan
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