linux-kernel - Re: [PATCH 1/2] MIPS: Allow MIPS32R2 kernel to run on P5600 and M5150

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Date:   Tue, 30 May 2023 12:54:49 +0100
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Subject: Re: [PATCH 1/2] MIPS: Allow MIPS32R2 kernel to run on P5600 and M5150



> 2023年5月30日 12:07，Maciej W. Rozycki <macro@...am.me.uk> 写道：
> 
> On Tue, 30 May 2023, Jiaxun Yang wrote:
> 
>>>> M5150 and P5600 are two MIPS32R5 kernels, however as MIPS32R5 is
>>>> backward compatible with MIPS32R2 there is no reason to forbid
>>>> M5150 and P5600 on MIPS32R2 kernel.
>>> 
>>> What problem are you trying to solve?  The CONFIG_SYS_HAS_CPU_* settings 
>>> denote overall platform's support for the given CPU and have nothing to do 
>>> with what architecture level a given kernel has been configured for.  You 
>>> do need to get the settings right for your platform, just as you do in 
>>> 2/2, but this 1/2 part looks wrong to me.
>> 
>> Well the universal target is to allow R2 generic kernel to run on R5 CPUs.
>> As R5 is backward compatible we can just have one universal kernel binary.
> 
> Sure, but this change is not needed for it.  You just need to declare 
> which ISA revisions your platform supports and leave `__get_cpu_type' 
> alone.  It has worked like that for a decade now.

I’m afraid it won’t work as you expected.

Actually I ran into a problem that `case CPU_P5600` in c-r4k.c is optimised out
by compiler, because the codepath is marked as unreachable.

Thanks
- Jiaxun

> 
> Back in the day I used to run R1 kernels on R2 hardware myself.  And 
> maybe MIPS IV on R1 even, as we had MIPS Malta CPU modules with both MIPS 
> IV devices (QED RM5261/RM7061) and MIPS64r1 devices (MIPS 5Kc/20Kc/25Kf) 
> and switching the kernel when swapping modules was a nuisance.  The Malta 
> config still supports these devices although some may not exist anymore.
> 
>  Maciej