Re: [PATCH v6 1/6] clk: iproc: define Broadcom iProc clock binding

[Ray Jui]

On 4/10/2015 5:12 PM, Michael Turquette wrote:
> Quoting Ray Jui (2015-03-17 22:45:17)
>> Document the device tree binding for Broadcom iProc architecture based
>> clock controller
>>
>> Signed-off-by: Ray Jui <rjui@xxxxxxxxxxxx>
>> Reviewed-by: Scott Branden <sbranden@xxxxxxxxxxxx>
>> ---
>>  .../bindings/clock/brcm,iproc-clocks.txt           |  171 ++++++++++++++++++++
>>  1 file changed, 171 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt
>>
>> diff --git a/Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt b/Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt
>> new file mode 100644
>> index 0000000..bf2316b
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt
>> @@ -0,0 +1,171 @@
>> +Broadcom iProc Family Clocks
>> +
>> +This binding uses the common clock binding:
>> +    Documentation/devicetree/bindings/clock/clock-bindings.txt
>> +
>> +The iProc clock controller manages clocks that are common to the iProc family.
>> +An SoC from the iProc family may have several PPLs, e.g., ARMPLL, GENPLL,
>> +LCPLL0, MIPIPLL, and etc., all derived from an onboard crystal. Each PLL
>> +comprises of several leaf clocks
>> +
>> +Required properties for PLLs:
>> +- compatible:
>> +    Should have a value of the form "brcm,<soc>-<pll>". For example, GENPLL on
>> +Cygnus has a compatible string of "brcm,cygnus-genpll"
>> +
>> +- #clock-cells:
>> +    Must be <0>
>> +
>> +- reg:
>> +    Define the base and range of the I/O address space that contain the iProc
>> +clock control registers required for the PLL
>> +
>> +- clocks:
>> +    The input parent clock phandle for the PLL. For all iProc PLLs, this is an
>> +onboard crystal with a fixed rate
>> +
>> +Example:
>> +
>> +       osc: oscillator {
>> +               #clock-cells = <0>;
>> +               compatible = "fixed-clock";
>> +               clock-frequency = <25000000>;
>> +       };
>> +
>> +       genpll: genpll {
>> +               #clock-cells = <0>;
>> +               compatible = "brcm,cygnus-genpll";
>> +               reg = <0x0301d000 0x2c>,
>> +                       <0x0301c020 0x4>;
>> +               clocks = <&osc>;
>> +       };
>> +
>> +Required properties for leaf clocks of a PLL:
>> +
>> +- compatible:
>> +    Should have a value of the form "brcm,<soc>-<pll>-clk". For example, leaf
>> +clocks derived from the GENPLL on Cygnus SoC have a compatible string of
>> +"brcm,cygnus-genpll-clk"
>> +
>> +- #clock-cells:
>> +    Have a value of <1> since there are more than 1 leaf clock of a
>> +given PLL
>> +
>> +- reg:
>> +    Define the base and range of the I/O address space that contain the iProc
>> +clock control registers required for the PLL leaf clocks
>> +
>> +- clocks:
>> +    The input parent PLL phandle for the leaf clock
>> +
>> +- clock-output-names:
>> +    An ordered list of strings defining the names of the leaf clocks
>> +
>> +Example:
>> +
>> +       genpll: genpll {
>> +               #clock-cells = <0>;
>> +               compatible = "brcm,cygnus-genpll";
>> +               reg = <0x0301d000 0x2c>,
>> +                       <0x0301c020 0x4>;
>> +               clocks = <&osc>;
>> +       };
>> +
>> +       genpll_clks: genpll_clks {
>> +               #clock-cells = <1>;
>> +               compatible = "brcm,cygnus-genpll-clk";
>> +               reg = <0x0301d000 0x2c>;
>> +               clocks = <&genpll>;
>> +               clock-output-names = "axi21", "250mhz", "ihost_sys",
>> +                       "enet_sw", "audio_125", "can";
>> +       };
> 
> Hi Ray,
> 
> Thanks for submitting the patch. It was nice meeting you at ELC as well.
> 
> This binding doesn't conform to the norms for clock bindings. It looks
> like for each type of controllable clock node (e.g. pll, leaf clock,
> etc) you have a dts node. Looking at the above example it seems that
> those two nodes (genpll and genpll_clks) share the same register.
> 
> /me checks patch #5
> 
> Yup, you re-use the same register address for the *pll and *pll_clks
> nodes. I'm not a DT expert but I think this is considered Wrong.
> 
> More generally your clock dt binding should be modeling the hardware in
> terms of IP blocks. If you have a clock generator IP block it may
> control many clock bits and output many clock signals. E.g. for your
> hardware (based only on the addresses in patch #5 and not having seen
> any data manual) I will hazard a guess that the genpll, lcpll and asiu
> clocks are all part of the same IP block.

Hi Mike,

In fact, lcpll, genpll, mipipll are similar but DIFFERENT IP blocks, and
asiu is completely different from any of them. All of these plls are
unique and have their own register banks, as you can see from the
bcm-cygnus-clock.dtsi file. Therefore, I think it's totally correct and
actually necessary to represent each of them with a separate device node.

Regarding the relationship between a PLL clock and its leaf clocks:
Taking the genpll as an example, physically they are connected as:

xtal -> genpll -> axi21, 250mhz, ihost_sys, ...

The 25 MHz crystal feeds to the genpll, and the genpll generates 6
different leaf clocks including axi21, 250mhz, ihost_sys, and etc. One
can choose to set the genpll's vco to one frequency, and based on that
frequency, different leaf clock frequencies can be generated with their
own post divider.

Therefore, I think it makes sense to represent xtal, genpll, genpll_clks
(including axi21, 250mhz, ihost_sys, and etc) each with a unique device
node, and genpll is the parent of these leaf clocks. Basically the
device nodes and the way how the "clocks" phandle is used represent the
hierarchy of the clock architecture within Cygnus (and in the future
other iProc SoCs). Does that make sense?

Regarding the register address overlapping, again, taking genpll as an
example, the genpll and the genpll_clks actually never access the same
set of registers, but the registers are sort of scattered within one
bank, i.e., on Cygnus, genpll uses registers at offset 0x0, 0x8 ~ 0x1c,
and genpll_clks uses registers at offset 0x4, 0x20 - 0x24.

Thanks,

Ray

> 
> If my guess is correct then these clocks should all be represented by a
> single node int DT. Lets say that the clock controller IP block that
> manages the genpll, lcpll and asiu clocks (including their child/leaf
> clocks) is called "foobar" in your data manual. You should have a dts
> node with a compatible string such as "brcm,cygnus-foobar" or
> "brcm,cygnus-foobar-clk".
> 
> Your clock driver should be responsible for registering all of the
> clocks controlled by this IP block, regardless of the "type" of clock
> node.
> 
> I think part of the reason for your approach is that the previous
> (deprecated) binding/dts had a bunch of fixed-rate clocks in it. This is
> a hack that we do every now and then to get a new platform up and
> running with a mainline kernel, but we don't do per-clock nodes in dts
> any more, we do them by clock controller ip block instead.
> 
> There are plenty of good examples of this for Exynos and QCOM SoCs if
> you want an example.
> 
> Regards,
> Mike
> 
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