linux-kernel - Re: [PATCH v9 2/6] dt: bindings: add mt7621-clk device tree binding documentation

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Date:   Sat, 6 Mar 2021 09:52:48 +0800
From:   Chuanhong Guo <gch981213@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Sergio Paracuellos <sergio.paracuellos@...il.com>,
        "open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        John Crispin <john@...ozen.org>, NeilBrown <neil@...wn.name>,
        "open list:COMMON CLK FRAMEWORK" <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v9 2/6] dt: bindings: add mt7621-clk device tree binding documentation

Hi Rob!

On Sat, Mar 6, 2021 at 6:48 AM Rob Herring <robh@...nel.org> wrote:
>
> On Thu, Feb 18, 2021 at 08:07:05AM +0100, Sergio Paracuellos wrote:
> > Adds device tree binding documentation for clocks in the
> > MT7621 SOC.
> >
> > Signed-off-by: Sergio Paracuellos <sergio.paracuellos@...il.com>
> > ---
> >  .../bindings/clock/mediatek,mt7621-clk.yaml   | 66 +++++++++++++++++++
> >  1 file changed, 66 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/clock/mediatek,mt7621-clk.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/clock/mediatek,mt7621-clk.yaml b/Documentation/devicetree/bindings/clock/mediatek,mt7621-clk.yaml
> > new file mode 100644
> > index 000000000000..842a0f2c9d40
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/clock/mediatek,mt7621-clk.yaml
> > @@ -0,0 +1,66 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/clock/mediatek,mt7621-clk.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: MT7621 Clock Device Tree Bindings
> > +
> > +maintainers:
> > +  - Sergio Paracuellos <sergio.paracuellos@...il.com>
> > +
> > +description: |
> > +  The MT7621 has a PLL controller from where the cpu clock is provided
> > +  as well as derived clocks for the bus and the peripherals. It also
> > +  can gate SoC device clocks.
> > +
> > +  Each clock is assigned an identifier and client nodes use this identifier
> > +  to specify the clock which they consume.
> > +
> > +  All these identifiers could be found in:
> > +  [1]: <include/dt-bindings/clock/mt7621-clk.h>.
> > +
> > +properties:
> > +  compatible:
> > +    const: mediatek,mt7621-clk
> > +
> > +  "#clock-cells":
> > +    description:
> > +      The first cell indicates the clock number, see [1] for available
> > +      clocks.
> > +    const: 1
> > +
> > +  ralink,sysctl:
> > +    $ref: /schemas/types.yaml#/definitions/phandle
> > +    description:
> > +      phandle of syscon used to control system registers
> > +
> > +  ralink,memctl:
> > +    $ref: /schemas/types.yaml#/definitions/phandle
> > +    description:
> > +      phandle of syscon used to control memory registers
>
> I assume one of these phandles are the main registers for the clocks?
> Make this a child node and drop that phandle.

On MT7621, CPU clock can be chosen from 3 sources: crystal clock,
a fixed 500MHz clock or a clock created by the memory controller.
sysctl contains a bootstrap register to determine crystal clock, a
clock mux for choosing between the 3 sources for CPU clock, and
a clock gate register for various peripherals. The ralink,memctl
phandle here is to read the cpu clock frequency from the memory
controller.
The original implementation hides this hardware detail to avoid
splitting the driver into three just for the CPU clock.
Is this approach okay and we can put it under sysctl node,
or this driver needs to be further splitted?

-- 
Regards,
Chuanhong Guo