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Subject: Re: Regression in 5.1.0-rc2: PowerBook G4 Aluminum fails to boot -
 bisected to commit 0df977eafc792
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        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>
 <1c1f7eae-0c49-dcb2-f27b-96a83d811df7@c-s.fr>
From:   Larry Finger <Larry.Finger@lwfinger.net>
Message-ID: <4d7e3fc5-82d8-5bcf-0891-97445e284c36@lwfinger.net>
Date:   Mon, 25 Mar 2019 11:19:15 -0500
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On 3/25/19 3:46 AM, Christophe Leroy wrote:
> 
> 
> 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. That patch fixes the issue.

Larry