linux-kernel - Re: [PATCH v5 1/2] dt-bindings: brcm: clocks: add binding for brcmstb-cpu-clk-div

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Date:   Wed, 1 Feb 2017 11:50:51 -0800
From:   Markus Mayer <markus.mayer@...adcom.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     Markus Mayer <code@...yer.net>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>,
        Linux Clock List <linux-clk@...r.kernel.org>,
        Power Management List <linux-pm@...r.kernel.org>,
        Device Tree List <devicetree@...r.kernel.org>,
        ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] dt-bindings: brcm: clocks: add binding for brcmstb-cpu-clk-div

On 20 January 2017 at 16:52, Stephen Boyd <sboyd@...eaurora.org> wrote:
> On 01/18, Markus Mayer wrote:
>> diff --git a/Documentation/devicetree/bindings/clock/brcm,brcmstb-cpu-clk-div.txt b/Documentation/devicetree/bindings/clock/brcm,brcmstb-cpu-clk-div.txt
>> new file mode 100644
>> index 0000000..c4acb53
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/clock/brcm,brcmstb-cpu-clk-div.txt
>> @@ -0,0 +1,27 @@
>> +The CPU divider node serves as the sole clock for the CPU complex. It supports
>> +power-of-2 clock division, with a divider of "1" as the default highest-speed
>> +setting.
>> +
>> +Required properties:
>> +- compatible: shall be "brcm,brcmstb-cpu-clk-div"
>> +- reg: address and width of the divider configuration register
>> +- #clock-cells: shall be set to 0
>> +- clocks: phandle of clock provider which provides the source clock
>> +          (this would typically be a "fixed-clock" type PLL)
>> +- div-table: list of (raw_value,divider) ordered pairs that correspond to the
>> +             allowed clock divider settings
>> +- div-shift-width: least-significant bit position and width of divider value
>
> Are these properties used? Please don't put these types of
> details in DT.

Yeah, unfortunately they are. Luckily, I think the issue can be
resolved quite easily, because the user of those properties isn't
involved in this series.

They are currently being used by a clock driver
("drivers/clk/clk-brcmstb.c") that hasn't been upstreamed yet. I
performed some code archeology. While I wasn't 100% successful in
tracking down the origins of this interface, it looks like it was
designed this way a while back (4+ years or so), probably before
device tree best practices were developed or, at least, before they
were widely known.

So, what I can do is to remove the properties from the official
binding. (I'll send an update to that effect shortly.) Once the
binding is accepted upstream, we can work on fixing up the design of
clk-brcmstb.c, so it doesn't rely on these properties anymore (and
derives them from the compatible string instead), and then proceed to
upstream that, as well.

I don't think this approach would cause any conflicts or problems.

>> +
>> +Optional properties:
>> +- clock-names: the clock may be named
>> +
>> +Example:
>> +     cpuclkdiv: cpu-clk-div@...e257c {
>> +             compatible = "brcm,brcmstb-cpu-clk-div";
>> +             reg = <0xf03e257c 0x4>;
>
> This register really looks like some offset in something larger.
> Is there some clock controller? What's the hw block at
> 0xf03e2000? Maybe I already asked this.

It looks this way, but in this case, looks are deceiving. The address
and the length are really correct the way they are.

This memory area holds a range of only loosely related configuration
registers. It's called the Bus Interface Unit Register Set and deals
with configuring the CPU in general. At address 0xf03e257c, there
happens to be the clock divider register we need, and it's really just
one register, i.e. 4 bytes.

>> +             div-table = <0x00 1>;
>> +             div-shift-width = <0 5>;
>> +             #clock-cells = <0>;
>> +             clocks = <&cpupll>;
>> +             clock-names = "cpupll";
>> +     };
>>
>
> --
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