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

[Daniel Kurtz <djkurtz@chromium.org>]

On Fri, Jul 10, 2015 at 3:27 PM, Eddie Huang <eddie.huang@mediatek.com> wrote:
> Hi all,
>
> On Wed, 2015-07-08 at 13:44 +0800, Sascha Hauer wrote:
>> On Wed, Jul 08, 2015 at 10:37:21AM +0800, Eddie Huang wrote:
>> > On Tue, 2015-07-07 at 23:10 +0800, Daniel Kurtz wrote:
>> > > On Tue, Jul 7, 2015 at 10:36 PM, Sascha Hauer <s.hauer@pengutronix.de> wrote:
>> > > > 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.
>> > >
>> > > I was trying to answer your question "What's the motivation to send
>> > > this patch anyway?".
>> > > The motivation is to send follow on patches that use the clk_null
>> > > phandle.  We need to provide some clock as the mmc1's hclk.  I do not
>> > > understand why this has to be "clk_null", though.  It seems like this
>> > > should be a real clock coming from one of the real clock_controller
>> > > nodes.  After all, the mmc driver is going to be enabling/disabling
>> > > this clock for power savings at runtime.  What does that even mean for
>> > > clk_null ?
>> >
>> > The original purpose of this patch is to provide a common dummy clock
>> > for both software don't care clock and clock that is not software
>> > controllable.I got comments that device tree should describe hardware
>> > and should put exact clock in device tree. I think this is true. So we
>> > will remove this clock_null patch, and:
>> >
>> > 1. For Mediatek SoC CCF driver, James will clarify clock usage further.
>> > Actually, we still think it's not necessary to describe whole tree that
>> > software don't care, James will deal this in clock driver.
>>
>> I think that aswell since the device tree is not affected in this case.
>> Should we realize later that we indeed need the missing clocks we can
>> still implement them without modifying the device tree.
>>
>> >
>> > 2. For other module that use SW not controllable clock (mmc case
>> > mentioned by Dan), because this is a real clock, we will put a dummy
>> > clock in device tree, like
>> >
>> > clk_mmchclk: dummyhclk {
>> >     compatible = "fixed-clock";
>> >     clock-frequency = <0>;
>> >     #clock-cells = <0>;
>> > };
>> >
>> > How about this modification ?
>>
>> I wouldn't name it 'dummy', this will again raise some eyebrows.
>>
>
> I got mmc hclk from our designer. HCLK is from AXI Bus directly (sorry,
> I gave wrong information to Dan and Sascha yesterday). Because there is
> no any mux or gate register to control this HCLK, so current
> clk-mt8173.c didn't model it. Since I know where this clock comes from,
> I will abandon this stupid dummy clock device tree patch. But there are
> two alternative ways:
>
> 1. In MMC device tree, use parent clock, like
>      <&topckgen CLK_TOP_AXI_SEL>
>
> 2. In clk-mt8173.c, add fix factor clock, like apll case
>    FACTOR(CLK_TOP_APLL1, "apll1_ck", "apll1", 1, 1),
>
> Either way works, but have different meaning. Any suggestion to handle
> thing like this.

I like (1), it seems more straightforward.
What is the advantage of (2)?

Thanks,
-Dan

> Thanks
> Eddie
>
>