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Subject: Re: Regression in 5.1.0-rc2: PowerBook G4 Aluminum fails to boot -
 bisected to commit 0df977eafc792
To:     Larry Finger <Larry.Finger@lwfinger.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        ppc-dev <linuxppc-dev@lists.ozlabs.org>
Cc:     LKML <linux-kernel@vger.kernel.org>
References: <acc2d5a5-2fd4-4367-c8ef-497da951c208@lwfinger.net>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <1c1f7eae-0c49-dcb2-f27b-96a83d811df7@c-s.fr>
Date:   Mon, 25 Mar 2019 09:46:39 +0100
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Le 25/03/2019 à 01:49, Larry Finger a écrit :
> A build of kernel 5.1.0-rc2 resulted in a failure to boot on my 
> PowerBook G4 Aluminum. The bootstrap loads the initial kernel and issues 
> the appropriate start, but the machine hangs at that point.
> 
> The problem does not depend on the choice of PPC32 processor type. This 
> machine has a 7447A according to /proc/cpuinfo.
> 
> The problem was bisected to the following:
> 
> commit 0df977eafc792a5365a7f81d8d5920132e03afad
> Author: Christophe Leroy <christophe.leroy@c-s.fr>
> Date:   Thu Feb 21 10:37:54 2019 +0000
> 
>      powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while 
> in RTAS
> 
>      When calling RTAS, the stack pointer is stored in SPRN_SPRG2
>      in order to be able to restore it in case of machine check in RTAS.
> 
>      As machine check is not a perfomance critical path, this patch
>      frees SPRN_SPRG2 by using a field in thread struct instead.
> 
>      Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>      Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> 
> I reverted this patch and found that the system began execution, and 
> then failed, likely due to the reassignment of SPRN_SPRG2.
> 
> I had found this problem with 5.1.0-rc1, but -rc2 was out by the time I 
> finished the bisection. Unfortunately, none of the changes in -rc2 fixed 
> the problem.

I think I identified the issue, see 
https://patchwork.ozlabs.org/patch/1063962/

It is now booting properly under QEMU with your .config

Could you please test it on your target ?

Thanks
Christophe

> 
> Attached is the .config that I used.
> 
> Thanks,
> 
> Larry