Re: [PATCH 00/13] MIPS: Convert Ingenic to a generic board

[Zhou Yanjie <zhouyanjie@wanyeetech.com>]

Hello Maciej & Paul,

在 2020/8/22 上午10:29, Maciej W. Rozycki 写道:
> Hi Paul,
>
>>> FAOD <cpu-feature-overrides.h> is not a hack, but an optimisation measure
>>> so that features known to be hardwired for a given machine/CPU do not have
>>> to be dynamically queried every time referred.  In some cases that results
>>> in large portions of code being optimised away by the compiler as well.
>> Fair enough. Bloat-o-meter reports about ~100 KiB saved when that file is
>> present. But we can't use it in a generic kernel, unfortunately.
>   Well, run-time patching might be an alternative to get the best of both
> worlds, but someone would have to reimplement our feature selection system
> to use it.
>
>>> The hardcoded value for a feature defined in <cpu-feature-overrides.h>
>>> always has to be the same as one in the corresponding bit of the `options'
>>> member of `struct cpuinfo_mips', in this case MIPS_CPU_TLBINV.
>> In theory yes, in practice the CPU detection code is lagging behind...
>   I wasn't aware of that.  In that case it has been a design abuse which
> has been missed by the maintainer when accepting patches.  It used to be
> the case that run-time detection was accurate and overrides were rather
> lazily added.
>
>   Also I note Ingenic must have had a CPU erratum if our `decode_configs'
> doesn't just work, as the interpretation of CP0.Config[5:0] registers has
> been architectural and mandatory, and that for a reason.  It's only legacy
> MIPS I-IV processors that should require special attention here.


I think I found the cause of the problem. Ingenic XBurst2's TLBINV is 
operate on entire MMU, according to the description of the MD00090 
document, the IE value in cp0 config4 should be 3 (Ingenic's XBurst2 
Core PM document also shows that the IE value is indeed 3). But the code 
in cpu-probe.c only detects the case where the IE value is equal to 2 
(TLBINV only operate on one TLB entry). Therefore, the kernel mistakenly 
believes that X2000 does not support TLBINV during detection.


Thanks and best regards!


>    Maciej