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Date:   Thu, 09 Apr 2020 23:47:04 +0800
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Subject: Re: [PATCH] MIPS: Limit check_bugs32() under CONFIG_32BIT
To:     "Maciej W. Rozycki" <macro@linux-mips.org>
CC:     YunQiang Su <wzssyqa@gmail.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-mips <linux-mips@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-ID: <7A98E39B-EDCF-496D-9525-0160A368361B@flygoat.com>
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=E4=BA=8E 2020=E5=B9=B44=E6=9C=889=E6=97=A5 GMT+08:00 =E4=B8=8B=E5=8D=8811=
:07:46, "Maciej W=2E Rozycki" <macro@linux-mips=2Eorg> =E5=86=99=E5=88=B0:
>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=2E =20
>> >=20
>> > 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=2E
>> >=20
>> > 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=2E
>> >=20
>> > Loongson's 3A 1000 is the example, I cannot support FP32 mode well=2E
>>=20
>> In this case bugs32 only contains a workaround for MIPS34K, which is
>a
>> MIPS32 processor=2E It's safe to do so=2E
>
> This is because commit c65a5480ff29 ("[MIPS] Fix potential latency=20
>problem due to non-atomic cpu_wait=2E") moved the other generic
>workaround=20
>elsewhere=2E
>
> The intent has been since historical commit 450ad16ba0ab ("Get rid of=20
>arch/mips64/kernel=2E  9116 lines of code gone=2E") that `check_bugs32' i=
s
>for=20
>generic errata affecting both 32-bit and 64-bit operation (e=2Eg=2E 32-bi=
t=20
>instructions, which naturally may occur in both cases) and
>`check_bugs64'=20
>is for errata affecting 64-bit operation only (e=2Eg=2E 64-bit
>instructions)=2E
>
>But currently it appears we have no generic errata handled, as surely a
>
>34K erratum cannot affect 64-bit operation=2E  So I think such a change=
=20
>makes sense in principle (if a generic erratum appears in the future we
>
>can add a third category, which includes workarounds that are always=20
>applied), but I think it has to be made in a cleaner way=2E
>
>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=20
>description added that not only repeats what the change does (and what=20
>everyone sees regardless), but actually justifies why the change is
>made=2E =20
>Saying: "There is no need[=2E=2E=2E]" does not tell us *why* there is no
>need=2E
>
>> 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=2E Manage bug dependencies by Kconfig
>> will be easier=2E
>
> Why is using Kconfig supposed to be better?  Several configurations=20
>support multiple processor types (e=2Eg=2E swappable CPU daugthercards or
>FPGA=20
>soft-cores) and having to list CPU types across platforms as CPUs are=20
>added is going to be a maintenance nightmare=2E  Whereas having
>workarounds=20
>or panics associated with run-time determination of the actual CPU type
>
>guarantees they will trigger where necessary=2E  The use of `init'
>sections=20
>assures the reclaim of memory for use after bootstrap=2E

Actually I meant let bug checks depends on Kconfig's CPU selection=2E

It's guaranteed that you can only select one kind of CPU one time,
to prevent the overhead of checking bugs on irrelevant processors=2E

And we still have to check PRID/CPUTYPE during boot to enable
proper workarounds, because the Kconfig options are telling about the poss=
ibility,
which means a processor potentially has some kinds of bug=2E

In this case, M34K's errata should depends on or selected by=20
CPU_MIPS32_R2 in Kconfig=2E

So there won't be any nightmare, but only reduced code :-)

Probably we can build-up a general framework for checks & workarounds,
and display affected bugs in /proc/cpuinfo?
That's my personal thought=2E

Thanks=2E

>
>OTOH I agree splitting off errata handling to a separate file may make=20
>sense for structural reasons; we have it already for `check_bugs64'=2E
>
>  Maciej

--=20
Jiaxun Yang