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

[Jiaxun Yang <jiaxun.yang@flygoat.com>]

> 2023年5月30日 12:07，Maciej W. Rozycki <macro@orcam.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