linux-kernel - Re: [PATCH v6 2/7] dt-binding: cpu-topology: Move cpu-map to a common binding.

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Date:   Mon, 3 Jun 2019 10:05:31 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Atish Patra <atish.patra@....com>
Cc:     Jeremy Linton <jeremy.linton@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Will Deacon <will.deacon@....com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Rob Herring <robh@...nel.org>,
        Anup Patel <anup@...infault.org>,
        Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Rob Herring <robh+dt@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Otto Sabart <ottosabart@...erm.com>,
        Sudeep Holla <sudeep.holla@....com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v6 2/7] dt-binding: cpu-topology: Move cpu-map to a
 common binding.

On Mon, Jun 03, 2019 at 01:49:13AM -0700, Atish Patra wrote:
> On 5/30/19 1:55 PM, Jeremy Linton wrote:
> > Hi,
> >
> > On 5/29/19 4:13 PM, Atish Patra wrote:
> > > cpu-map binding can be used to described cpu topology for both
> > > RISC-V & ARM. It makes more sense to move the binding to document
> > > to a common place.
> > >
> > > The relevant discussion can be found here.
> > > https://lkml.org/lkml/2018/11/6/19
> > >
> > > Signed-off-by: Atish Patra <atish.patra@....com>
> > > Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> > > Reviewed-by: Rob Herring <robh@...nel.org>
> > > ---
> > >    .../topology.txt => cpu/cpu-topology.txt}     | 82 +++++++++++++++----
> > >    1 file changed, 66 insertions(+), 16 deletions(-)
> > >    rename Documentation/devicetree/bindings/{arm/topology.txt => cpu/cpu-topology.txt} (86%)
> > >

[...]

> > <nit picking>
> >
> > While socket is optional, its probably a good idea to include the node
> > in the example even if the result is the same.
>
> Sure. I will update that.
>
> That is because at least
> > on arm64 the DT clusters=sockets decision had performance implications
> > for larger systems.
> >
> > Assuring the socket information is correct is helpful by itself to avoid
> > having to explain why a single socket machine is displaying some other
> > value in lscpu.
> >
> Just for my understanding, can you give a example?
>

That's simple. Today any ARM{32,64} DT based platform sets their cluster
id to physical package id, which is exposed to userspace. The userspace
can/must interpret that as multi-socket system. E.g. TC2/Juno which
2 clusters show up as 2 socket systems which is wrong and needs fixing.
We have fixed it for ARM64 ACPI based systems but for DT(mostly used in
mobile/embedded) we need to make sure we don't break anything else before
we fix it.

--
Regards,
Sudeep