[PATCH 0/2] mips: Introduce some IO-accessors optimizations

[Serge Semin]

It has been discovered that on our MIPS P5600-based CPU the IO accessors
aren't that rapid as they could be even taking into account a relatively
slow AXI2APB bridge embedded into the system interconnect. Turned out we
can introduce two types of optimizations. First we can remove the
execution barriers from the relaxed IO-accessors as our CPU conforms to
the MIPS Coherency Protocol Specification [1, 2]. Of course it also
concerns the IO interconnect implementation. So in accordance with [3] we
suggest to remove the barriers at least for the platforms which conform
the specification the same way as ours. Second there is a dedicated
Coherency Manager control register, which can be also used to tune the IO
methods up. For some reason it hasn't been added to the MIPS arch code so
far, while it provides flags for instance to speed the SYNC barrier for
the platforms with non-re-ordering IO interconnect, to set the cache ops
serialization limits, enable the speculative reads, etc. For now we
suggest to add just the macro with the CM2 GCR_CONTROL register accessors
and fields description. So any platform could use it to activate the
corresponding optimization. Our platform-wise we'll do this in the
framework of our Baikal-T1 platform code in the prom_init() method.

[1] MIPS Coherence Protocol Specification, Document Number: MD00605,
    Revision 01.01. September 14, 2015, 4.2 Execution Order Behavior,
    p. 33

[2] MIPS Coherence Protocol Specification, Document Number: MD00605,
    Revision 01.01. September 14, 2015, 4.8.1 IO Device Access, p. 58

[3] "LINUX KERNEL MEMORY BARRIERS", Documentation/memory-barriers.txt,
Section "KERNEL I/O BARRIER EFFECTS"

Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
Cc: Pavel Parkhomenko <Pavel.Parkhomenko@xxxxxxxxxxxxxxxxxxxx>
Cc: Vadim Vlasov <V.Vlasov@xxxxxxxxxxxxxxxxxxxx>
Cc: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

Serge Semin (2):
  mips: Add strong UC ordering config
  mips: Introduce MIPS CM2 GCR Control register accessors

 arch/mips/Kconfig               |  8 ++++++++
 arch/mips/include/asm/io.h      | 20 ++++++++++----------
 arch/mips/include/asm/mips-cm.h | 15 +++++++++++++++
 3 files changed, 33 insertions(+), 10 deletions(-)

-- 
2.27.0