linux-kernel - Re: [PATCH 11/13] dt-bindings: mips: Add loongson cpus & boards

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Date:   Tue, 27 Aug 2019 20:41:11 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
CC:     Rob Herring <robh+dt@...nel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        Huacai Chen <chenhc@...ote.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....co>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 11/13] dt-bindings: mips: Add loongson cpus & boards

Hi guys,

On Tue, Aug 27, 2019 at 10:18:46PM +0800, Jiaxun Yang wrote:
> On 2019/8/27 下午8:45, Rob Herring wrote:
> > On Tue, Aug 27, 2019 at 3:55 AM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
> > > diff --git a/Documentation/devicetree/bindings/mips/loongson/cpus.yaml b/Documentation/devicetree/bindings/mips/loongson/cpus.yaml
> > > new file mode 100644
> > > index 000000000000..410d896a0078
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/mips/loongson/cpus.yaml
> > > @@ -0,0 +1,38 @@
> > > +# SPDX-License-Identifier: GPL-2.0
> > Dual license for new bindings please:
> > 
> > (GPL-2.0-only OR BSD-2-Clause)
> > 
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/mips/loongson/cpus.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: Loongson CPUs bindings
> > > +
> > > +maintainers:
> > > +  - Jiaxun Yang <jiaxun.yang@...goat.com>
> > > +
> > > +description: |+
> > > +  The device tree allows to describe the layout of CPUs in a system through
> > > +  the "cpus" node, which in turn contains a number of subnodes (ie "cpu")
> > > +  defining properties for every cpu.
> > > +
> > > +  Bindings for CPU nodes follow the Devicetree Specification, available from:
> > > +
> > > +  https://www.devicetree.org/specifications/
> > > +
> > > +properties:
> > > +  reg:
> > > +    maxItems: 1
> > > +    description: |
> > > +      Physical ID of a CPU, Can be read from CP0 EBase.CPUNum.
> > Is this definition specific to Loongson CPUs or all MIPS?
> 
> Currently it's specific to Loongson CPU only, as other processors may using
> different method to express CPU map.
> 
> Different from Arm, MIPS family of processors seems less uniform and have
> their own designs.
> 
> For this point, we'd better ask Paul's opinion.

In general on MIPS we detect CPU properties at runtime from coprocessor
0 registers & similar sources of information, so there's not really a
need to specify anything about the CPU in devicetree. For example here
you say yourself that the value for this property can be read from
EBase.CPUNum - so why specify it in DT?

Thanks,
    Paul