[PATCH 4/4] Documentation: Add device tree bindings for Freescale FTM PWM

[Xiubo Li-B47053]

Hi Tomasz,



> > > If the meaning of flags cell is the same as in generic, default PWM
> > > specifier format, then it should be noted here and generic PWM
> > > binding documentation mentioned.
> >
> > OK, How about the following ?
> > - #pwm-cells: Should be 3. See pwm.txt in this directory for a
> >   description of the cells format.
> 
> I meant just the last cell, which stores flags, but actually this might
> be a good idea, but with slightly extended description. Something among
> those
> lines:
> 
>  - #pwm-cells: Should be 3. The default three cell format specified by
> generic PWM bindings are used. Refer to the documentation of generic PWM
> bindings for more information about the meaning of cells.
> 

That's perfect.


> > > > +- fsl,pwm-clk-ps: the ftm0 pwm clock's prescaler,
> > > > divide-by 2^n(n = 0 ~ 7).
> > >
> > > Is it a hardware-specific property?
> >
> > Yes, I will revise it in v2.
> 
> I'd like to hear a bit more elaborate description of this property. What
> are the factors that decide what value should be used here?
> 

Sorry, "the ftm0 pwm clock's prescaler" maybe also confuse you, it should be
"the ftm pwm counter clock input prescaler".

The ftm's counter clock can be selected as :
System clock,
Fixed frequency clock,
External clock.

And the ftm module clock has only system clock source.

The fixed frequency clock is an alternative clock source for the FTM counter that allows
the selection of a clock other than the system clock or an external clock. This clock input
is defined by chip integration. Due to FTM hardware implementation limitations, the 
frequency of the fixed frequency clock must not exceed 1/2 of the system clock frequency.

The external clock passes through a synchronizer clocked by the system clock to assure
that counter transitions are properly aligned to system clock transitions.Therefore, to
meet Nyquist criteria considering also jitter, the frequency of the external clock source
must not exceed 1/4 of the system clock frequency.

So, if the fixed frequency clock or external clock equal or exceed the system clock...


> > > > +- fsl,pwm-number: the number of PWM devices, and is must equal to
> > > > the
> > > > number +  of "fsl,pwm-channels".
> > >
> > > This is redundant, because you can simply count how many entries
> > > have been specified in fsl,pwm-channels.
> >
> > Yes, I will revise it in v2.
> > And this would be renamed to " fsl,pwm-channel-number", which can be
> > more Readable and understood.
> 
> I meant that you don't need to specify how many entries other property
> has in another property, because device tree provides information about
> sizes of all properties. So, in parsing code, you would be able to simply
> get the size of "fsl,pwm-channels" property in bytes, divide that by
> sizeof(u32) and get the numer of cells specified.
> 

OK, I will revise it in v2.

> > > > +- fsl,pwm-channels: the channels' order which is be used for pwm
> > > > +in
> > > > ftm0 +  module, and they must be one or some of 0 ~ 7, because the
> > > > ftm0 only has +  8 channels can be used.
> > >
> > > Could you explain meaning of this property more precisely? I'm
> > > interested especially how is this related to the PWM IP block and
> > > boards.
> > Yes.
> > There are 8 channels most. While the pinctrls of 4th and 5th channels
> > could be used by uart's Rx and Tx, then these 2 channels won't be used
> > for pwm output, so there will be 6 channels available by the pwm.
> > Thus, the pwm chip will register only 6 pwms(6 channels)
> > most("fsl,pwm-channel-orders = {0 1 2 3 6 7}").And also the
> > "fsl,pwm-channel-number" will be 6.
> >
> > If hasn't any other problems, I will revise It in v2.
> > And this will be renamed to "fsl,pwm-channel-orders", which can be
> > more readable and understood.
> 
> As Sascha Hauer already suggested, you could get rid of both this and
> "fsl,pwm-channel-number" properties and simply register all the channels.
> This way you will have a deterministic 1:1 mapping of real hardware
> channels to channels specified in device tree, which is definitely the
> way to go.
> 
> Now as a safety measure, you could simply move the specification of
> pinctrl states from SoC family or SoC level dtsi file to board level dts,
> where only pinctrl states of channels used by a particular board would be
> specified, so nothing could break operation of other devices that share
> pin muxes with remaining channels.

Yes, I was also considering it, but not very be clear yet.
Thanks very much for your and Sascha's help.
I will try to implement it in v2.


Thanks very much.

--
Best regards,
Xiubo