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Date:   Sat, 22 Aug 2020 01:19:16 +0200
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH 00/13] MIPS: Convert Ingenic to a generic board
To:     "Maciej W. Rozycki" <macro@linux-mips.org>
Cc:     Zhou Yanjie <zhouyanjie@wanyeetech.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Paul Burton <paulburton@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>, od@zcrc.me,
        linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org,
        =?UTF-8?b?5ryG6bmP5oyv?= <aric.pzqi@ingenic.com>,
        dongsheng.qiu@ingenic.com, rick.tyliu@ingenic.com,
        yanfei.li@ingenic.com, xuwanhao@wanyeetech.com
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Hi Maciej,

Le ven. 21 ao=FBt 2020 =E0 20:23, Maciej W. Rozycki=20
<macro@linux-mips.org> a =E9crit :
> On Fri, 7 Aug 2020, Paul Cercueil wrote:
>=20
>>  > I'm not too sure if remove "cpu-feature-overrides.h" will cause=20
>> some
>>  > problems for X2000, because according to my current test on=20
>> X2000, I found
>>  > that it is somewhat different from the SoCs using XBurst1 CPU=20
>> core, with the
>>  > kernel source code provided by Ingenic, for example, we must=20
>> configure
>>  > "#define cpu_has_tlbinv 1" in "cpu-feature-overrides.h" to make=20
>> the X2000
>>  > work normally, otherwise the kernel will get stuck. And X2000's=20
>> interrupt
>>  > controller has also been redesigned. If these differences make it=20
>> impossible
>>  > to share code, should we set a subdirectory of "xburst" and=20
>> "xburst2" in
>>  > "arch/mips/ingenic"? (I am just worried about this situation, so=20
>> far I have
>>  > not been able to successfully run the mainline kernel on X2000).
>>=20
>>  The <cpu-feature-overrides.h> is kind of a hack, to hardcode=20
>> settings in case
>>  the CPU is not properly detected. The cpu-probe.c should be able to
>>  auto-detect these settings, including the inverted TLB that the=20
>> X2000 has,
>>  reading from the CPU config registers ("TLB INV" info should be in=20
>> config4).
>>  Right now cpu_probe_ingenic() doesn't read config4 (not present on=20
>> older SoCs)
>>  but that's trivial to add.
>=20
>  FAOD <cpu-feature-overrides.h> is not a hack, but an optimisation=20
> measure
> so that features known to be hardwired for a given machine/CPU do not=20
> have
> to be dynamically queried every time referred.  In some cases that=20
> results
> in large portions of code being optimised away by the compiler as=20
> well.

Fair enough. Bloat-o-meter reports about ~100 KiB saved when that file=20
is present. But we can't use it in a generic kernel, unfortunately.

>  The hardcoded value for a feature defined in=20
> <cpu-feature-overrides.h>
> always has to be the same as one in the corresponding bit of the=20
> `options'
> member of `struct cpuinfo_mips', in this case MIPS_CPU_TLBINV.

In theory yes, in practice the CPU detection code is lagging behind...

Cheers,
-Paul