linux-kernel - Re: [PATCH 2/3] Documentation: add sprd clock bindings

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Date:   Thu, 18 May 2017 19:12:02 -0700
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Chunyan Zhang <zhang.lyra@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Chunyan Zhang <chunyan.zhang@...eadtrum.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        xiaolong.zhang@...eadtrum.com,
        Orson Zhai (翟京) <orson.zhai@...eadtrum.com>,
        "geng.ren@...eadtrum.com" <geng.ren@...eadtrum.com>,
        linux-clk@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/3] Documentation: add sprd clock bindings

On 05/18, Chunyan Zhang wrote:
> On 18 May 2017 at 03:43, Arnd Bergmann <arnd@...db.de> wrote:
> > On Mon, May 15, 2017 at 10:35 AM, Chunyan Zhang
> > <chunyan.zhang@...eadtrum.com> wrote:
> >> diff --git a/Documentation/devicetree/bindings/clock/sprd/sprd,adjustable-pll-clock.txt b/Documentation/devicetree/bindings/clock/sprd/sprd,adjustable-pll-clock.txt
> >> new file mode 100644
> >> index 0000000..476e315
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/bindings/clock/sprd/sprd,adjustable-pll-clock.txt
> >> @@ -0,0 +1,17 @@
> >> +Spreadtrum adjustable pll clock driver
> >> +
> >> +Required properties:
> >> +
> >> +- compatible : must be one of:
> >> +       "sprd,sc9836-adjustable-pll-clock"
> >> +       "sprd,sc9860-adjustable-pll-clock"
> >> +
> >> +Example:
> >> +       clk_mpll0: clk@...00024 {
> >> +               compatible = "sprd,sc9860-adjustable-pll-clock";
> >> +               #clock-cells = <0>;
> >> +               reg = <0 0x40400024 0 0x4>,
> >> +                     <0 0x40400028 0 0x4>;
> >> +               clocks = <&clk_mpll_gates 2>;
> >> +               clock-output-names = "clk_mpll0";
> >> +       };
> >
> > The properties listed in the example must all be either
> > defined as "required" or "optional" properties and have a
> > description.
> 
> Since these common properties are documented in the common clock binding [1]
> 
> [1] Documentation/devicetree/bindings/clock/clock-bindings.txt
> 
> I will add explanation in this file like I do in other bindings
> introduced in this patch, and will address 'reg' which probably is not
> similar to the common usage.
> 
> >
> > The reg property here is a bit odd, as it lists two consecutive
> > 4-byte areas, and both are suspiciously close to a round
> > address (0x40400000), so I would guess that they are
> > in fact part of a clock controller with several registers.
> 
> They are PLL clock configuration registers.
> 
> Different PLL has different configurations which listed in pll_cfg.h,
> and probably has different number of registers for storing
> configurations, and on some Spreadtrum's SoCs PLL clock configuration
> registers aren't consecutive, but all PLLs on all Spreadtrum's SoCs
> (at least so far) are using the same driver.
> 
> >
> > If so, please define a node for the entire clock controller,
> > the DT description should reflect the design of the hardware
> > rather than the design of your device driver.
> 
> I also realized our implementation might not be easy to be understood,
> but I haven't thought out a better solution considering the hardware
> limitation I explained above.

This binding is going down the wrong path. Please look at how
drivers such as sunxi-ng have done the binding in comparison to
original sunxi design. We don't put this level of detail into DT,
instead we put the details into the driver code and have clock
controller nodes in DT. A quick glance shows that this binding is
making a node per-clk, which is not going to be accepted.

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