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Date:   Fri, 21 Aug 2020 20:23:16 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@linux-mips.org>
To:     Paul Cercueil <paul@crapouillou.net>
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
Subject: Re: [PATCH 00/13] MIPS: Convert Ingenic to a generic board
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On Fri, 7 Aug 2020, Paul Cercueil wrote:

> > I'm not too sure if remove "cpu-feature-overrides.h" will cause some
> > problems for X2000, because according to my current test on X2000, I found
> > that it is somewhat different from the SoCs using XBurst1 CPU core, with the
> > kernel source code provided by Ingenic, for example, we must configure
> > "#define cpu_has_tlbinv 1" in "cpu-feature-overrides.h" to make the X2000
> > work normally, otherwise the kernel will get stuck. And X2000's interrupt
> > controller has also been redesigned. If these differences make it impossible
> > to share code, should we set a subdirectory of "xburst" and "xburst2" in
> > "arch/mips/ingenic"? (I am just worried about this situation, so far I have
> > not been able to successfully run the mainline kernel on X2000).
> 
> The <cpu-feature-overrides.h> is kind of a hack, to hardcode 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 X2000 has,
> reading from the CPU config registers ("TLB INV" info should be in config4).
> Right now cpu_probe_ingenic() doesn't read config4 (not present on older SoCs)
> but that's trivial to add.

 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. 

 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.

  Maciej