Re: [PATCH v4 03/13] mips: Add MIPS Release 5 support

Serge Semin
2020-05-22T13:15:39Z

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 ]