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

[Paul Cercueil]

Le sam. 22 août 2020 à 3:29, Maciej W. Rozycki <macro@xxxxxxxxxxxxxx> 
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