Message-ID: <20150416192014.19585.9663@quantum>
Date:	Thu, 16 Apr 2015 12:20:14 -0700
From:	Michael Turquette <mturquette@...aro.org>
To:	Ray Jui <rjui@...adcom.com>, "Stephen Boyd" <sboyd@...eaurora.org>,
	"Matt Porter" <mporter@...aro.org>,
	"Alex Elder" <elder@...aro.org>,
	"Rob Herring" <robh+dt@...nel.org>,
	"Pawel Moll" <pawel.moll@....com>,
	"Mark Rutland" <mark.rutland@....com>,
	"Ian Campbell" <ijc+devicetree@...lion.org.uk>,
	"Russell King" <linux@....linux.org.uk>,
	"Arnd Bergmann" <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	"Scott Branden" <sbranden@...adcom.com>,
	"Dmitry Torokhov" <dtor@...gle.com>,
	"Anatol Pomazau" <anatol@...gle.com>, linux-kernel@...r.kernel.org,
	bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v6 1/6] clk: iproc: define Broadcom iProc clock binding

Quoting Ray Jui (2015-04-14 12:10:35)
> Hi Mike,
> 
> On 4/13/2015 12:40 PM, Ray Jui wrote:
> > Hi Mike,
> > 
> > On 4/12/2015 11:02 PM, Michael Turquette wrote:
> >> Quoting Ray Jui (2015-04-12 21:08:32)
> >>>
> >>>
> >>> 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@...adcom.com>
> >>>>> Reviewed-by: Scott Branden <sbranden@...adcom.com>
> >>>>> ---
> >>>>>  .../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.
> >>
> >> OK, that makes sense to me, if those registers live in addresses ranges
> >> which correspond to different IP blocks.
> >>
> >>>
> >>> 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?
> >>
> >> This doesn't make sense to me. If I understand correctly, the register
> >> range that controls the post-divider clock (e.g. axi21) is the same
> >> register range that control's genpll. This is a reasonable indicator to
> >> me that the leaf clocks are part of the same clock generator IP block as
> >> the PLL controls. As such they should be on node.
> >>
> >>>
> >>> 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.
> >>
> >> Sure, my argument above doesn't hinge on the fact that the pll and child
> >> clocks use the exact same register. It still looks to me like *pll and
> >> it's child dividers belong in the same IP block. Is there an open data
> >> sheet or technical reference manual I can look at for this part? That is
> >> the best way to put my concerns at ease ;-)
> >>
> >> Looking over your binding again, it looks like your nodes are divided
> >> conveniently for the different clock types (e.g. pll versus
> >> post-divider), but our goal with DT is to accurately describe the
> >> hardware, not the C structures.
> >>
> >> Regards,
> >> Mike
> >>
> > 
> > Yes, the genpll and genpll_clks are controlled by the same IP block. I
> > can make the change to combine them to use one DT node. But before doing
> > that, I just need to get one thing clarified with you. In SW, the pll
> > and its leaf clocks will still be registered as separate two clock
> > providers, since we need to be able to configure the PLL vco frequency
> > and its leaf clock frequencies separately. The pll will be the parent of
> > its leaf clocks, and the leaf clocks will be used by various peripherals.
> > 
> > I plan to have the combined DT looks like this:
> > 
> > genpll_clks: genpll_clks {
> >     #clock-cells = <1>;
> >     compatible = "brcm,cygnus-genpll-clk";
> >     reg = <0x0301d000 0x2c>;
> > 
> >     assigned-clocks = <&genpll_clks>;
> >     /* this sets the PLL rate at the time when the PLL clock is
> > registered */
> >     assigned-clock-rates = <4000000000>;
> > 
> >     clocks = <&osc>;
> > 
> >     clock-output-names = "axi21", "250mhz", "ihost_sys", "enet_sw",
> > "audio_125", "can";
> > };
> > 
> > peripheralA: peripheralA {
> >     /* use the first leaf clock of genpll */
> >     clocks = <&genpll_clks 0>;
> >     clock-frequency  = <100000000>;
> > };
> > 
> > If we register both the genpll and its leaf clocks to the clock
> > framework as two separate clock providers, would the above DT entries
> > still work? For example, peripheralA refers to the phandle of
> > genpll_clks, but how does the clock framework know to link it to the pll
> > clock or the leaf clock?

Hi Ray,

There are two options here: using clock-output-names or not using
clock-output-names.

I would recommend against using clock-output-names. If both your
clock driver and the peripheral devices that consume these clocks are
all represented in DT then you can skip clock-output-names and instead
create phandle linkage between the clocks inside of your clock provider
and the consumer devices. String names are still important here, but now
you only need to specify the string name as an *input* to the consumer
device, and not as an *output* from the clock provider node. Let's look
at how the qcom bindings do it:

gcc is a clock generator ip block.

arch/arm/boot/dts/qcom-apq8084.dtsi:
                gcc: clock-controller@...00000 {
                        compatible = "qcom,gcc-apq8084";
                        #clock-cells = <1>;
                        #reset-cells = <1>;
                        reg = <0xfc400000 0x4000>;
                };

This same file includes this header:
#include <dt-bindings/clock/qcom,gcc-apq8084.h>

That header creates a map of clocks by numbers. An example:
#define GCC_BLSP2_UART2_APPS_CLK                        142

We can see how it used by the serial controller in
arch/arm/boot/dts/qcom-apq8084.dtsi:
                serial@...5e000 {
                        compatible = "qcom,msm-uartdm-v1.4", "qcom,msm-uartdm";
                        reg = <0xf995e000 0x1000>;
                        interrupts = <0 114 0x0>;
                        clocks = <&gcc GCC_BLSP2_UART2_APPS_CLK>, <&gcc GCC_BLSP2_AHB_CLK>;
                        clock-names = "core", "iface";
                        status = "disabled";
                };

Note the 'clock-names' property. This lists a string name which is a property
of the *consuming* device (uart controller), not the *providing* device (gcc
clock generator).

Finally, it all comes together in the driver simply using clk_get:
        msm_port->clk = devm_clk_get(&pdev->dev, "core");
        if (IS_ERR(msm_port->clk))
                return PTR_ERR(msm_port->clk);

        if (msm_port->is_uartdm) {
                msm_port->pclk = devm_clk_get(&pdev->dev, "iface");
                if (IS_ERR(msm_port->pclk))
                        return PTR_ERR(msm_port->pclk);

                clk_set_rate(msm_port->clk, 1843200);
        }

This works because we have created linkage between the clock phandle and
the consuming device node. The shared header is used by the Linux kernel
to look up the clock as well as the consumer node to map the string name
onto a clock input.

Back to clock-output-names:

If you decide to keep using clock-output-names you can still represent
the pll clock in your "clock-output-names". In a perfect world DT nodes
that use clock-output-names would only expose the leaf clocks that your
peripheral devices consume, but there is no technical reason why your
pll can't be a part of the clock-output-names array. It is simply an
array of string names that maps to an index. There is no sense of
parent-child hierarchy in the DT binding, so there is no problem adding
a parent pll clock to the array of clock-output-names. I think that this
will tactically solve your problem in the short term, but I encourage
you to inspect the qcom binding example I gave above to see if it a
better fit for you long term.

Regards,
Mike

> > 
> > Thanks,
> > 
> > Ray
> > 
> 
> So I've changed the code to merge pll and pll_clks nodes and have played
> around with it a bit. Let me try to explain the issue I'm having here,
> and see if you can help to shed some light:
> 
> As far as I know, the correct way to configure a clock's rate at the
> time of registering to the clock framework, as you mentioned previously,
> is to use the "assigned-clock" and "assigned-clock-rates" properties.
> 
> Assuming we have a pll clock called "pll", with 6 leaf clocks derived
> from the pll called "clk-0", "clk-1", etc., and a peripheral using the
> first leaf clock: "clk-0".
> 
> pll: pll {
>    #clock-cell = <1>;
>    compatible = "brcm,pll";
>    clocks = <&osc>;
>    clock-output-names = "clk-0", "clk-1", ...;
> };
> 
> peripheral: peripheral {
>     clocks = <&pll 0>;
> };
> 
> With your proposed way of constructing the above clock device tree,
> i.e., to combine "pll" and "clk-x" to use the same DT node, now how can
> one configure the frequency of the "pll" through device tree at the time
> of clock registration? There's now no access to "pll" through device
> tree but only access to its leaf clocks, e.g., clocks = <&pll 0>.
> 
> The requirement comes from the fact that we need the peripheral to run
> at a certain clock rate off the leaf clock, and to achieve that we need
> the pll to run at a specific rate, since we only have an integer based
> post-divider between the pll and its leaf clocks. I was able to achieve
> this previously by separating the pll and its leaf clocks into two DT
> nodes, but I am really not sure how one can do this when merging pll and
> its leaf clocks into the same node.
> 
> Any idea how this can be done cleanly with a single DT node representing
> the pll and its leaf clocks?
> 
> Thanks,
> 
> Ray
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