Re: [PATCH RFC v5 2/2] clk: Add handling of clk parent and rate assigned from DT

[Mike Turquette]

Quoting Sylwester Nawrocki (2014-04-11 05:25:49)
> >> +==Assigned clock parents and rates==
> >> +
> >> +Some platforms require static initial configuration of parts of the clocks
> >> +controller. Such a configuration can be specified in a clock consumer node
> >> +through clock-parents and clock-rates DT properties. The former should
> >> +contain a list of parent clocks in form of phandle and clock specifier pairs,
> >> +the latter the list of assigned clock frequency values (one cell each).
> >> +To skip setting parent or rate of a clock its corresponding entry should be
> >> +set to 0, or can be omitted if it is not followed by any non-zero entry.
> >> +
> >> +    uart@a000 {
> >> +        compatible = "fsl,imx-uart";
> >> +        reg = <0xa000 0x1000>;
> >> +        ...
> >> +        clocks = <&clkcon 0>, <&clkcon 3>;
> >> +        clock-names = "baud", "mux";
> >> +
> >> +        clock-parents = <0>, <&pll 1>;
> >> +        clock-rates = <460800>;
> > 
> > Is this the input frequency or serial baud rate? Looks like a baud
> > rate, but the clock framework needs input (to the uart) frequency. I
> > would say this should be clock-frequency and specify the max baud rate
> > as is being done with i2c bindings. The uart driver should know how to
> > convert between input clock freq and baud rate.
> 
> This UART example is not quite representative for the issues I have been
> trying to address with this patch set. There is a need to set (an initial)
> input clock frequency. E.g. in case of multimedia devices there may be
> a need to set clock parent and frequency of an input clock to multiple IP
> blocks, so they are clocked synchronously and data is carried properly
> across a whole processing chain. Thus there may not be even clock output
> in an IP block, but still input clock needs to be set. IIUC there is
> similar issue with audio, where it is difficult to calculate the clock
> frequencies/determine parent clocks in individual drivers algorithmically.
> 
> >> +    };
> >> +
> >> +In this example the pll is set as parent of "mux" clock and frequency 
> >> of "baud"
> >> +clock is specified as 460800 Hz.
> > 
> > I don't really like clock-parents. The parent information is part of
> > the clock source, not the consumer.
> 
> I'm not sure we must always consider the parent information as property
> of a clock source. If for example we expose a structure like below as
> single clock object, supporting clock gating, parent and frequency
> setting the parent setting is still accessible from within a device
> driver.

The design of the ccf implementation certainly allows one to hide
individually addressable/configurable clock nodes within a single struct
clk. But should we? I have always maintained that a clock driver should
enumerate clocks in the same way that the data sheet or technical
reference manual states. I did make a recent exception[1], but that is
going to be rolled back after the coordinated clock rate changes land in
mainline.

> And clock parent selection may depend on a system configuration
> not immediately obvious from within a single device driver perspective.
> 
>                          MUX
>                        ,-------.     DIVIDER      GATE
> common clk source 1 -->|--.    |   ,--------.   ,--------.
>                        |   \   |   |        |   |        |
> common clk source 2 -->|-   '--|-->|        |-->|        |--> consumer
>        ...             |       |   |        |   |        |
> common clk source N -->|-      |   '--------'   '--------'
>                        '-------'
> 
> > We've somewhat decided against having every single clock defined in DT
> > and rather only describe a clock controller with leaf clocks to
> > devices. That is not a hard rule, but for complex clock trees that is
> > the norm. Doing something like this will require all levels of the
> > clock tree to be described. You may have multiple layers of parents
> > that have to be configured correctly. How are you configuring the rest
> > of the tree?
> 
> I believe even clock controllers where clocks are represented as flat
> array often describe the clock tree entirely by parenthood, the tree
> structure is just not obvious from the DT binding.
> In addition, there seems to be appearing more and more clock controller
> DT bindings describing their clocks individually.

I've been discouraging these per-clock node bindings in favor of the
per-controller node style.

> 
> >> +Configuring a clock's parent and rate through the device node that uses
> >> +the clock can be done only for clocks that have a single user. Specifying
> >> +conflicting parent or rate configuration in multiple consumer nodes for
> >> +a shared clock is forbidden.
> >> +
> >> +Configuration of common clocks, which affect multiple consumer devices
> >> +can be specified in a dedicated 'assigned-clocks' subnode of a clock
> >> +provider node, e.g.:
> > 
> > This seems like a work-around due to having clock-parents in the
> > consumer node. If (I'm not convinced we should) we have a binding for
> > parent config, it needs to be a single binding that works for both
> > cases.
> 
> When this issue was first raised during an ARM kernel summit it was
> proposed to add 'assigned' prefix to DT properties for such bindings.
> 

Yes, I like the "assigned-" prefix.

> How about separate properties for the default clock configuration,
> e.g. assigned-clocks/assigned-clock-parents/assigned-clock-rates ?
> So a clock provider would look like:
> 
>     clkcon {
>         ...
>         #clock-cells = <1>;
> 
>         assigned-clocks = <&clkcon 16>, <&clkcon 17>;
>         assigned-clock-parents = <0>, <&clkcon 1>;
>         assigned-clock-rates = <200000>;
>     };
> 
> And a consumer device node:
> 
>     uart@a000 {
>         compatible = "fsl,imx-uart";
>         reg = <0xa000 0x1000>;
>         ...
>         clocks = <&clkcon 0>;
>         clock-names = "baud";
> 
>         assigned-clocks = <&clkcon 3>, <&clkcon 0>;
>         assigned-clock-parents = <&pll 1>;
>         assigned-clock-rates = <0>, <460800>;
> };

It looks like this idea was dropped for v6. Can we revisit it? Take a
look at Tero's example implementation for OMAP using this binding:

http://www.spinics.net/lists/linux-omap/msg104705.html

There is a bogus "default-clocks" node made solely for storing this info
within the OMAP PRCM clock provider node. This is basically faking a
clock consumer. I think with the proposed solution above Tero could have
avoided that node entirely and done the following:

diff --git a/arch/arm/boot/dts/omap4.dtsi b/arch/arm/boot/dts/omap4.dtsi
index 649b5cd..e3ff1a7 100644
--- a/arch/arm/boot/dts/omap4.dtsi
+++ b/arch/arm/boot/dts/omap4.dtsi
@@ -145,6 +145,11 @@
 			cm2_clocks: clocks {
 				#address-cells = <1>;
 				#size-cells = <0>;
+
+				assigned-clocks = <&abe_dpll_refclk_mux_ck>,
+					<&dpll_usb_ck>, <&dpll_abe_ck>;
+				assigned-clock-parents = <&sys_32k_ck>;
+				assigned-clock-rates = <0>, <960000000>, <98304000>;
 			};
 
 			cm2_clockdomains: clockdomains {


Tero, what do you think?

Regards,
Mike

[1] http://www.spinics.net/lists/cpufreq/msg10071.html
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