linux-kernel - Re: [PATCH RFC v3 2/2] clk: Add handling of clk parent and rate assigned from DT

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Date:	Thu, 27 Mar 2014 14:57:56 +0100
From:	Sylwester Nawrocki <s.nawrocki@...sung.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-arm-kernel@...ts.infradead.org
Cc:	mturquette@...aro.org, mark.rutland@....com,
	devicetree@...r.kernel.org, linux@....linux.org.uk,
	gregkh@...uxfoundation.org, t.figa@...sung.com,
	sw0312.kim@...sung.com, linux-kernel@...r.kernel.org,
	kyungmin.park@...sung.com, robh+dt@...nel.org,
	galak@...eaurora.org, grant.likely@...aro.org,
	m.szyprowski@...sung.com
Subject: Re: [PATCH RFC v3 2/2] clk: Add handling of clk parent and rate
 assigned from DT

Hi Laurent,

On 27/03/14 14:24, Laurent Pinchart wrote:
> On Thursday 27 March 2014 13:16:19 Sylwester Nawrocki wrote:
>> This function adds a helper function to configure clock parents and rates
>> as specified in clock-parents, clock-rates DT properties for a consumer
>> device and a call to it before driver is bound to a device.
>>
>> Signed-off-by: Sylwester Nawrocki <s.nawrocki@...sung.com>
>> ---
[...]
>> ---
>>  .../devicetree/bindings/clock/clock-bindings.txt   |   26 ++++++
>>  drivers/base/dd.c                                  |    7 ++
>>  drivers/clk/Makefile                               |    1 +
>>  drivers/clk/clk-conf.c                             |   87 +++++++++++++++++
>>  drivers/clk/clk.c                                  |   10 ++-
>>  include/linux/clk/clk-conf.h                       |   19 +++++
>>  6 files changed, 149 insertions(+), 1 deletion(-)
>>  create mode 100644 drivers/clk/clk-conf.c
>>  create mode 100644 include/linux/clk/clk-conf.h
>>
>> diff --git a/Documentation/devicetree/bindings/clock/clock-bindings.txt
>> b/Documentation/devicetree/bindings/clock/clock-bindings.txt index
>> 7c52c29..b452f80 100644
>> --- a/Documentation/devicetree/bindings/clock/clock-bindings.txt
>> +++ b/Documentation/devicetree/bindings/clock/clock-bindings.txt
>> @@ -115,3 +115,29 @@ clock signal, and a UART.
>>    ("pll" and "pll-switched").
>>  * The UART has its baud clock connected the external oscillator and its
>>    register clock connected to the PLL clock (the "pll-switched" signal)
>> +
>> +==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).
>> +
>> +    uart@...0 {
>> +        compatible = "fsl,imx-uart";
>> +        reg = <0xa000 0x1000>;
>> +        ...
>> +        clocks = <&clkcon 0>, <&clkcon 3>;
>> +        clock-names = "baud", "mux";
>> +
>> +        clock-parents = <0>, <&pll 1>;
>> +        clock-rates = <460800>;
>> +    };
>> +
>> +In this example the pll is set as parent of "mux" clock and frequency of
>> "baud"
>> +clock is specified as 460800 Hz.
> 
> I'm curious, what should happen when two devices have conflicting requirements 
> ? If a different device required the <&clkcon 3> parent to be set to <&pll 2> 
> for instance, who should win ? Shouldn't a warning be printed ?

In general, the assumption is that the <&clkcon 3> clock would be used only
by the uart@...0 device. If a clock is shared I'd say it shouldn't be put
in a multiple consumer device nodes. Instead it should be put in a clock
provider node, as I was trying to explain in the sentence below.

A warning could be useful, but it could complicate the code. We would need,
for example, to store information about already configured clocks in a list and
scan it before actually altering any clock parent or rate.

>> +For clocks which are not directly connected to any consumer device
>> similarly
>> +clocks, clock-parents and/or clock-rates properties should be specified in
>> +assigned-clocks subnode of a clock controller DT node.
> 
> It might be that I'm not familiar enough with the clock framework, but this 
> sounds unclear to me. I'm not sure what you mean exactly.

Sorry about not being precise here, would something like below be more
clear ?

"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.:

    clkcon {
        ...
        #clock-cells = <1>;

        assigned-clocks {
            clocks = <&clkcon 16>, <&clkcon 17>;
            clock-parents = <0>, <&clkcon 1>;
            clock-rates = <200000>;
        };
    };	
"

Naturally it's this just an RFC, any critics or suggestions are welcome.:)

--
Regards,
Sylwester
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