Re: [PATCH v2 3/9] arm: twr-k70f120m: clock driver for Kinetis SoC

[Paul Osmialowski <pawelo@king.net.pl>]

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Hi Arnd,

I'm attaching excerpt from Kinetis reference manual that may make 
situation clearer.

These MCG and SIM registers are used only to determine configuration 
(clock fixed rates and clock signal origins) at run time.

Namely, the real MCGOUTCLK source (in the middle) which is the parent for 
core clock (CCLK) and peripheral clock (PCLK) is determined at run time by 
reading MCG registers, let me quote commit message from Emcraft git repo:

      * Determine in run-time what oscillator module (OSC0 or OSC1) is used
     as clock source for the main PLL.
      * When OSC1 is selected, assume its frequency to be 12 MHz on all
     boards (there is a 12 MHz oscillator on XTAL1/EXTAL1 on K70-SOM and
     TWR-K70F120M boards).

In my .dts I'm trying to possibly follow real clock hierarchy, but to go 
anywhere behind MCGOUTCLK would require ability to rewrite .dtb e.g. by 
U-boot. But that's too demanding for any potential users of this BSP. So 
let's asume that MCGOUTCLK is the root clock and a parent for CCLK and 
PCLK.

In my most recent version I added OSC0ERCLK explicitly as one more root 
clock, since it is also used directly (through CG reg. 1 bit 0) by 
Freescale fec network device whose in-tree driver I'm trying to make 
usable for Kinetis.

On Sat, 4 Jul 2015, Arnd Bergmann wrote:

> On Friday 03 July 2015 00:08:27 Thomas Gleixner wrote:
>> On Thu, 2 Jul 2015, Paul Osmialowski wrote:
>>> On Thu, 2 Jul 2015, Arnd Bergmann wrote:
>>>
>>>> I wonder if you could move out the fixed rate clocks into their own
>>>> nodes. Are they actually controlled by the same block? If they are
>>>> just fixed, you can use the normal binding for fixed rate clocks
>>>> and only describe the clocks that are related to the driver.
>>>
>>> In my view having these clocks grouped together looks more convincing. After
>>> all, they all share the same I/O regs in order to read configuration.
>>
>> The fact that they share a register is not making them a group. That's
>> just a HW design decision and you need to deal with that by protecting
>> the register access, but not by trying to group them artificially at
>> the functional level.
>
> I'd disagree with that: The clock controller is the device that owns the
> registers and that should be one node in DT, as Paul's first version does.
>
> The part I'm still struggling with is understanding how the fixed-rate
> clocks are controlled through those registers. If they are indeed configured
> through the registers, the name is probably wrong and should be changed
> to whatever kind of non-fixed clock this is.
>
> 	Arnd
>

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