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

[Serge Semin]

Thomas,
Any comment on the series? The changes aren't that comprehensive, so it would
be great to merge it in before the 5.10 merge window is opened.

-Sergey

On Sun, Sep 20, 2020 at 02:00:08PM +0300, Serge Semin wrote:
> 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
>