Re: [PATCHv2] Documentation: dt-bindings: Add binding documentation for TI clkctrl clocks

[Tony Lindgren <tony@atomide.com>]

* Tero Kristo <t-kristo@ti.com> [170123 06:45]:
> On 18/01/17 00:53, Tony Lindgren wrote:
> > Texas Instruments omap variant SoCs starting with omap4 have a clkctrl
> > clock controller instance for each interconnect target module. The clkctrl
> > controls functional and interface clocks for the module.
> > 
> > The clkctrl clocks are currently handled by arch/arm/mach-omap2 hwmod code.
> > With this binding and a related clock device driver we can start moving the
> > clkctrl clock handling to live in drivers/clk/ti.
> > 
> > Note that this binding allows keeping the clockdomain related parts out of
> > drivers/clock. The CLKCTCTRL and DYNAMICDEP registers can be handled by
> > a separate driver in drivers/soc/ti and genpd. If the clockdomain driver
> > needs to know it's clocks, we can just set the the clkctrl device
> > instances to be children of the related clockdomain device.
> > 
> > Each clkctrl clock can have multiple optional gate clocks, and multiple
> > optional mux clocks. To represent this in device tree, it seems that
> > it is best done using four clock cells #clock-cells = <4> property.
> > 
> > The reasons for using #clock-cells = <4> are:
> > 
> > 1. We need to specify the clkctrl offset from the instance base. Otherwise
> >    we end up with a large number of device tree nodes that need to be
> >    patched when new clocks are discovered in a clkctrl clock with minor
> >    hardware revision changes for example
> > 
> > 2. On omap5 CM_L3INIT_USB_HOST_HS_CLKCTRL has ten OPTFCLKEN bits. So we
> >    need to use a separate cell for optional gate clocks to avoid address
> >    space conflicts
> > 
> > 3. Some clkctrl instances can also also optional mux clocks. To address
> >    them properly we need also a separate cell for the optional mux
> >    clock index
> > 
> > 4. The modulemode clock needs a flag passed to it for hardware or
> >    software controlled mode
> 
> Hi Tony,
> 
> I think #clock-cells = <4> is too much. I believe we only need 2:
> 
> - one entry for clkctrl offset
> - one entry for clock offset within the clkctrl entry (0 = module clock, 8+
> = opt-clocks / mux clocks / dividers)

OK the less #clock-cells the better as long as it's enough :)

> Fields 2 / 3 in your proposal are mutually exclusive, if either field is
> non-zero, the other one must be zero. And, the opt clocks / mux / divs
> always have different values for these.

OK. Just to confirm the assumptions then:

1. The optional mux clock the consumer needs to select the right
   source clock with with clk_set_parent()

2. The optional divider clock rate must be set by the consumer
   using clk_set_rate()

And in that case we again don't need to define any artificial
clock indexes, which is good if new clocks are discovered between
various SoC revisions.

> Field 4 is kind of redundant also, as the module clock must be registered at
> the clkctrl probe time, it is too late for the clock consumer to provide the
> proper setting for the clock during its own probe. It seems I need to add
> static data to driver which basically has this information in place already.

OK yeah good point, the "clocks" is a consumer property.

So in that case we must also assume that if any clock consumer needs
to change between HWSUP or SWSUP, it needs to be done with some yet
to be determined API. We have not needed that so far AFAIK though.

If there are no issues with the above, I'm naturally fine using the
#clock-cells = <2> :)

Regards,

Tony