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Date:   Fri, 22 May 2020 16:15:26 +0300
From:   Serge Semin <Sergey.Semin@baikalelectronics.ru>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
CC:     Serge Semin <fancer.lancer@gmail.com>,
        Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
        Paul Burton <paulburton@kernel.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Rob Herring <robh+dt@kernel.org>, <devicetree@vger.kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Alexander Lobakin <alobakin@dlink.ru>,
        Huacai Chen <chenhc@lemote.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Cedric Hombourger <Cedric_Hombourger@mentor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
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        Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= <f4bug@amsat.org>,
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        Allison Randal <allison@lohutok.net>,
        Liangliang Huang <huanglllzu@gmail.com>,
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        <zhouyanjie@wanyeetech.com>, YunQiang Su <syq@debian.org>,
        Zou Wei <zou_wei@huawei.com>,
        Oleksij Rempel <linux@rempel-privat.de>,
        Kamal Dasu <kdasu.kdev@gmail.com>,
        <linux-mips@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <kvm@vger.kernel.org>
Subject: Re: [PATCH v4 03/13] mips: Add MIPS Release 5 support
Message-ID: <20200522131526.pmqtpmreq6ly3kou@mobilestation>
References: <20200521140725.29571-1-Sergey.Semin@baikalelectronics.ru>
 <20200521140725.29571-4-Sergey.Semin@baikalelectronics.ru>
 <20200522072743.GA7331@alpha.franken.de>
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On Fri, May 22, 2020 at 09:27:43AM +0200, Thomas Bogendoerfer wrote:
> On Thu, May 21, 2020 at 05:07:14PM +0300, Serge Semin wrote:
> > There are five MIPS32/64 architecture releases currently available:
> > from 1 to 6 except fourth one, which was intentionally skipped.
> > Three of them can be called as major: 1st, 2nd and 6th, that not only
> > have some system level alterations, but also introduced significant
> > core/ISA level updates. The rest of the MIPS architecture releases are
> > minor.
> > 
> > Even though they don't have as much ISA/system/core level changes
> > as the major ones with respect to the previous releases, they still
> > provide a set of updates (I'd say they were intended to be the
> > intermediate releases before a major one) that might be useful for the
> > kernel and user-level code, when activated by the kernel or compiler.
> > In particular the following features were introduced or ended up being
> > available at/after MIPS32/64 Release 5 architecture:
> > + the last release of the misaligned memory access instructions,
> > + virtualisation - VZ ASE - is optional component of the arch,
> > + SIMD - MSA ASE - is optional component of the arch,
> > + DSP ASE is optional component of the arch,
> > + CP0.Status.FR=1 for CP1.FIR.F64=1 (pure 64-bit FPU general registers)
> >   must be available if FPU is implemented,
> > + CP1.FIR.Has2008 support is required so CP1.FCSR.{ABS2008,NAN2008} bits
> >   are available.
> > + UFR/UNFR aliases to access CP0.Status.FR from user-space by means of
> >   ctc1/cfc1 instructions (enabled by CP0.Config5.UFR),
> > + CP0.COnfig5.LLB=1 and eretnc instruction are implemented to without
> >   accidentally clearing LL-bit when returning from an interrupt,
> >   exception, or error trap,
> > + XPA feature together with extended versions of CPx registers is
> >   introduced, which needs to have mfhc0/mthc0 instructions available.
> > 
> > So due to these changes GNU GCC provides an extended instructions set
> > support for MIPS32/64 Release 5 by default like eretnc/mfhc0/mthc0. Even
> > though the architecture alteration isn't that big, it still worth to be
> > taken into account by the kernel software. Finally we can't deny that
> > some optimization/limitations might be found in future and implemented
> > on some level in kernel or compiler. In this case having even
> > intermediate MIPS architecture releases support would be more than
> > useful.
> > 
> > So the most of the changes provided by this commit can be split into
> > either compile- or runtime configs related. The compile-time related
> > changes are caused by adding the new CONFIG_CPU_MIPS32_R5/CONFIG_CPU_MIPSR5
> > configs and concern the code activating MIPSR2 or MIPSR6 already
> > implemented features (like eretnc/LLbit, mthc0/mfhc0). In addition
> > CPU_HAS_MSA can be now freely enabled for MIPS32/64 release 5 based
> > platforms as this is done for CPU_MIPS32_R6 CPUs. The runtime changes
> > concerns the features which are handled with respect to the MIPS ISA
> > revision detected at run-time by means of CP0.Config.{AT,AR} bits. Alas
> > these fields can be used to detect either r1 or r2 or r6 releases.
> > But since we know which CPUs in fact support the R5 arch, we can manually
> > set MIPS_CPU_ISA_M32R5/MIPS_CPU_ISA_M64R5 bit of c->isa_level and then
> > use cpu_has_mips32r5/cpu_has_mips64r5 where it's appropriate.
> > 
> > Since XPA/EVA provide too complex alterationss and to have them used with
> > MIPS32 Release 2 charged kernels (for compatibility with current platform
> > configs) they are left to be setup as a separate kernel configs.
> > 
> > Co-developed-by: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
> > Signed-off-by: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
> > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> > Cc: Paul Burton <paulburton@kernel.org>
> > Cc: Ralf Baechle <ralf@linux-mips.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Cc: devicetree@vger.kernel.org
> > ---
> >  arch/mips/Kconfig                    | 56 +++++++++++++++++++++++++---
> >  arch/mips/Makefile                   |  2 +
> >  arch/mips/include/asm/asmmacro.h     | 18 +++++----
> >  arch/mips/include/asm/compiler.h     |  5 +++
> >  arch/mips/include/asm/cpu-features.h | 27 ++++++++++----
> >  arch/mips/include/asm/cpu-info.h     |  2 +-
> >  arch/mips/include/asm/cpu-type.h     |  7 +++-
> >  arch/mips/include/asm/cpu.h          | 10 +++--
> >  arch/mips/include/asm/fpu.h          |  4 +-
> >  arch/mips/include/asm/hazards.h      |  8 ++--
> >  arch/mips/include/asm/module.h       |  4 ++
> >  arch/mips/include/asm/stackframe.h   |  2 +-
> >  arch/mips/include/asm/switch_to.h    |  8 ++--
> >  arch/mips/kernel/cpu-probe.c         | 17 +++++++++
> >  arch/mips/kernel/entry.S             |  6 +--
> >  arch/mips/kernel/proc.c              |  4 ++
> >  arch/mips/kernel/r4k_fpu.S           | 14 +++----
> >  arch/mips/kvm/vz.c                   |  6 +--
> >  arch/mips/lib/csum_partial.S         |  6 ++-
> >  arch/mips/mm/c-r4k.c                 |  7 ++--
> >  arch/mips/mm/sc-mips.c               |  7 ++--
> >  21 files changed, 163 insertions(+), 57 deletions(-)
> 
> applied to mips-next. I've changed the two /* fall through */ by fallthrough;
> while appliny. Running checkpatch would have caught that ;-)

Good. Thanks. Actually I've seen that warning, but just didn't know what way to
choose.) So I've decided to leave the comment-based Fall-through fixup seeing
the rest of the file is using the older way. By doing so I've kept the locally
implemented coding style. Though I've heard the explicit attribute "fallthrough;"
utilization is a preferred way of marking combined case statements.

-Sergey

> 
> Thomas.
> 
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]