linux-kernel - Re: [RFC][PATCHv5 4/4] Documentation: Add device tree bindings for Freescale FTM PWM.

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Message-Id: <754E4595-A2C5-4BAD-BBA7-29091A83197B@codeaurora.org>
Date:	Tue, 29 Oct 2013 03:09:52 -0500
From:	Kumar Gala <galak@...eaurora.org>
To:	Xiubo Li-B47053 <B47053@...escale.com>
Cc:	Guo Shawn-R65073 <r65073@...escale.com>,
	"thierry.reding@...il.com" <thierry.reding@...il.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"pawel.moll@....com" <pawel.moll@....com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"matt.porter@...aro.org" <matt.porter@...aro.org>,
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"t.figa@...sung.com" <t.figa@...sung.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"rob@...dley.net" <rob@...dley.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [RFC][PATCHv5 4/4] Documentation: Add device tree bindings for Freescale FTM PWM.


On Oct 28, 2013, at 10:48 PM, Xiubo Li-B47053 wrote:

>>> This adds the Document for Freescale FTM PWM driver under
>>> Documentation/devicetree/bindings/pwm/.
>>> 
>>> Signed-off-by: Xiubo Li <Li.Xiubo@...escale.com>
>>> ---
>>> .../devicetree/bindings/pwm/pwm-fsl-ftm.txt        | 34
>> ++++++++++++++++++++++
>>> 1 file changed, 34 insertions(+)
>>> create mode 100644
>>> Documentation/devicetree/bindings/pwm/pwm-fsl-ftm.txt
>>> 
>>> diff --git a/Documentation/devicetree/bindings/pwm/pwm-fsl-ftm.txt
>>> b/Documentation/devicetree/bindings/pwm/pwm-fsl-ftm.txt
>>> new file mode 100644
>>> index 0000000..175b762
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/pwm/pwm-fsl-ftm.txt
>>> @@ -0,0 +1,34 @@
>>> +Freescale FTM PWM controller
>> 
>> What does FTM stand for, and can we spell out PWM at least once early on.
>> 
> 
> "FTM" is for short of "FlexTimer Module", I'll use the full name then.
> 
>>> +
>>> +Required properties:
>>> +- compatible: Should be "fsl,vf610-ftm-pwm"
>>> +- reg: Physical base address and length of the controller's registers
>>> +- #pwm-cells: Should be 3. See pwm.txt in this directory for a
>>> +description of
>>> +  the cells format.
>>> +- clock-names : Includes the following module clock source entries:
>>> +    "ftm0" (system clock),
>>> +    "ftm0_fix_sel" (fixed frequency clock),
>>> +    "ftm0_ext_sel" (external clock)
>>> +- clocks : Must contain a clock specifier for each entry in
>>> +clock-names,
>>> +  See ../clock/clock-bindings.txt for details of the property values.
>>> +- fsl,pwm-counter-clk: The FTM PWM counter clock source, should be
>>> +one of the
>>> +  entries in clock-names.
>> 
>> Why do we need this, why not just have only the clock-names/clocks
>> reference the clk that is actually used?
>> 
> 
> As I have replied before, the FTM has two clock sources: the module clock and the counter clock.
> 
> The counter clock source is selectable depends on different board and also the hardware design:
> +++++
> * FTM source clock is selectable
> * Source clock can be the system clock, the fixed frequency clock, or an external
> clock
> * Fixed frequency clock is an additional clock input to allow the selection of an on
> chip clock source other than the system clock
> * Selecting external clock connects FTM clock to a chip level input pin therefore
> allowing to synchronize the FTM counter with an off chip clock source
> -----
> From the above description we can see that the external clock source can allow to synchronize the
> FTM counter with an off chip clock source.
> 
> As the chip spec permits the counter clock source be selectable, so the different board maybe has different
> implementation, if the driver do not support, this will be a bug.

The binding specs the HW so what a driver does or doesn't support isn't relevant.  Since I assume only one of these clocks makes sense for a given system I don't see a reason the binding should not just state that a given dts will have the one the system specified rather than all 3.

>>> +- pinctrl-names: must contain a "default" entry.
>>> +- pinctrl-NNN: One property must exist for each entry in pinctrl-names.
>>> +  See ../pinctrl/pinctrl-bindings.txt for details of the property
>> values.
>> 
>> let's drop the .. when making directory references.
>> 
> 
> Is absolute path " Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt " or just "pinctrl/pinctrl-bindings.txt " ?

The later.

- k

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation

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