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
