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SubjectRe: [PATCH] MIPS: Limit check_bugs32() under CONFIG_32BIT
On Thu, 9 Apr 2020, Jiaxun Yang wrote:

> > > There is no need to build and call check_bugs32() under
> > > CONFIG_64BIT, just limit it under CONFIG_32BIT.
> >
> > Since 32bit is subset of 64bit, and due to the code, I think that the
> > initial purpose
> > of check_bugs32() is also willing to run even with CONFIG_64BIT.
> >
> > For example, if we have a CPU which is 64bit, and work well on 64bit
> > mode, while has a bug only on 32bit mode, check_bugs32 should be used
> > here.
> >
> > Loongson's 3A 1000 is the example, I cannot support FP32 mode well.
>
> In this case bugs32 only contains a workaround for MIPS34K, which is a
> MIPS32 processor. It's safe to do so.

This is because commit c65a5480ff29 ("[MIPS] Fix potential latency
problem due to non-atomic cpu_wait.") moved the other generic workaround
elsewhere.

The intent has been since historical commit 450ad16ba0ab ("Get rid of
arch/mips64/kernel. 9116 lines of code gone.") that `check_bugs32' is for
generic errata affecting both 32-bit and 64-bit operation (e.g. 32-bit
instructions, which naturally may occur in both cases) and `check_bugs64'
is for errata affecting 64-bit operation only (e.g. 64-bit instructions).

But currently it appears we have no generic errata handled, as surely a
34K erratum cannot affect 64-bit operation. So I think such a change
makes sense in principle (if a generic erratum appears in the future we
can add a third category, which includes workarounds that are always
applied), but I think it has to be made in a cleaner way.

Specifically `check_errata' has to be renamed to `check_errata32', some
commentary added as to the intent, and last but not least a proper change
description added that not only repeats what the change does (and what
everyone sees regardless), but actually justifies why the change is made.
Saying: "There is no need[...]" does not tell us *why* there is no need.

> But my suggestion is if you're going to clean-up bugs and workarounds
> you'd better establish a file for silicon bugs and provide Kconfig
> options to enable & disable them. Manage bug dependencies by Kconfig
> will be easier.

Why is using Kconfig supposed to be better? Several configurations
support multiple processor types (e.g. swappable CPU daugthercards or FPGA
soft-cores) and having to list CPU types across platforms as CPUs are
added is going to be a maintenance nightmare. Whereas having workarounds
or panics associated with run-time determination of the actual CPU type
guarantees they will trigger where necessary. The use of `init' sections
assures the reclaim of memory for use after bootstrap.

OTOH I agree splitting off errata handling to a separate file may make
sense for structural reasons; we have it already for `check_bugs64'.

Maciej

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