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


Le sam. 22 août 2020 à 3:29, Maciej W. Rozycki <macro@linux-mips.org>
a écrit :
> 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.

Would run-time patching allow to drop dead code?

>> > 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.

Yes, Ingenic CPUs have a few bloopers here and there...

-Paul


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