Re: [PATCH] arm64: dts: mt8173: add clock_null

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Jul 07, 2015 at 10:15:29PM +0800, Daniel Kurtz wrote:
> On Tue, Jul 7, 2015 at 9:07 PM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > On Thu, Jun 18, 2015 at 01:29:11PM +0800, Eddie Huang wrote:
> >> Add clk_null, which represents clocks that can not / need not
> >> controlled by software.
> >> There are many clocks' parent set to clk_null.
> >>
> >> Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
> >> Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
> >> ---
> >> Base on 4.1-rc1
> >>
> >> Change-Id: I4db9b40d07e28f54f7bae9b676316cbd6a962124
> >> ---
> >>  arch/arm64/boot/dts/mediatek/mt8173.dtsi | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/arch/arm64/boot/dts/mediatek/mt8173.dtsi b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >> index 924fdb6..4798f44 100644
> >> --- a/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >> +++ b/arch/arm64/boot/dts/mediatek/mt8173.dtsi
> >> @@ -81,6 +81,12 @@
> >>               cpu_on        = <0x84000003>;
> >>       };
> >>
> >> +     clk_null: clk_null {
> >> +             compatible = "fixed-clock";
> >> +             clock-frequency = <0>;
> >> +             #clock-cells = <0>;
> >> +     };
> >
> > The discussion around this patch shows that we don't want to have this
> > clock in the device tree as it is not a hardware description.
> >
> > Ok, fine. Eddie, you told us that the rate of the current clk_null children
> > is not interesting. What's the motivation to send this patch anyway
> > then? Why can't you keep its children on the orphan list where they are
> > already now?
> >
> > Another possibility would be to instantiate the clk_null clock from C
> > code rather than from the device tree. This way we wouldn't put any
> > wrong descriptions into the device tree and still can implement the
> > support for the real parent clocks when we actually need them.
> 
> Some device nodes, like mmc, use a clk_null phandle as one of their clocks:
> 
> mmc1: mmc@11240000 {
>         compatible = "mediatek,mt8173-mmc",
>                      "mediatek,mt8135-mmc";
>         reg = <0 0x11240000 0 0x1000>;
>         interrupts = <GIC_SPI 72 IRQ_TYPE_LEVEL_LOW>;
>         clocks = <&pericfg CLK_PERI_MSDC30_1>,
>                  <&clk_null>;
>         clock-names = "source", "hclk";
>         status = "disabled";
> };

This is another case than the one we discussed about. In the case above
I motivated using a dummy clock since the clock exists in the system,
but is not software controllable. To abstract this from the driver
(which needs this clock since it exists) we here have the dummy clock.
However, of course I can't prove the clock is indeed not software
controllable; that's only the information I have.

Sascha

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