STMicroelectronics STM32MP1 clock tree initialization
=====================================================

The STM32MP clock tree initialization is based on device tree information
for RCC IP and on fixed clocks.

-------------------------------
RCC CLOCK = st,stm32mp1-rcc-clk
-------------------------------

The RCC IP is both a reset and a clock controller but this documentation only
describes the fields added for clock tree initialization which are not present
in Linux binding.

Please refer to ../mfd/st,stm32-rcc.txt for all the other properties common
with Linux.

Required properties:

- compatible: Should be "st,stm32mp1-rcc-clk"

- st,clksrc : The clock source in this order

	for STM32MP15x: 9 clock sources are requested
		MPU AXI MCU PLL12 PLL3 PLL4 RTC MCO1 MCO2

	with value equals to RCC clock specifier as defined in
	dt-bindings/clock/stm32mp1-clksrc.h: CLK_<NAME>_<SOURCE>

- st,clkdiv : The div parameters in this order
	for STM32MP15x: 11 dividers value are requested
		MPU AXI MCU APB1 APB2 APB3 APB4 APB5 RTC MCO1 MCO2

	with DIV coding defined in RCC associated register RCC_xxxDIVR

	most the case, it is:
		0x0: not divided
		0x1: division by 2
		0x2: division by 4
		0x3: division by 8
		...

	but for RTC MCO1 MCO2, the coding is different:
		0x0: not divided
		0x1: division by 2
		0x2: division by 3
		0x3: division by 4
		...

Optional Properties:
- st,pll
    PLL children node for PLL1 to PLL4 : (see ref manual for details)
    with associated index 0 to 3 (st,pll@0 to st,pll@4)
    PLLx is off when the associated node is absent

    - Sub-nodes:

	- cfg:	The parameters for PLL configuration in this order:
		DIVM DIVN DIVP DIVQ DIVR Output

		with DIV value as defined in RCC spec:
			0x0: bypass (division by 1)
			0x1: division by 2
			0x2: division by 3
			0x3: division by 4
			...

		and Output = bitfield for each output value = 1:ON/0:OFF
			BIT(0) => output P : DIVPEN
			BIT(1) => output Q : DIVQEN
			BIT(2) => output R : DIVREN
		  NB : macro PQR(p,q,r) can be used to build this value
		       with p,p,r = 0 or 1

	- frac : Fractional part of the multiplication factor
		(optional, PLL is in integer mode when absent)

	- csg : Clock Spreading Generator (optional)
	        with parameters in this order:
		MOD_PER INC_STEP SSCG_MODE

		* MOD_PER: Modulation Period Adjustment
		* INC_STEP: Modulation Depth Adjustment
		* SSCG_MODE: Spread spectrum clock generator mode
		  you can use associated defines from stm32mp1-clksrc.h
		  * SSCG_MODE_CENTER_SPREAD = 0
		  * SSCG_MODE_DOWN_SPREAD = 1


- st,pkcs : used to configure the peripherals kernel clock selection
  containing a list of peripheral kernel clock source identifier as defined
  in the file dt-bindings/clock/stm32mp1-clksrc.h

  Example:

	rcc: rcc@50000000 {
		compatible = "syscon", "simple-mfd";

		reg = <0x50000000 0x1000>;

		rcc_clk: rcc-clk@50000000 {
			#clock-cells = <1>;
			compatible = "st,stm32mp1-rcc-clk";

			st,clksrc = <	CLK_MPU_PLL1P
					CLK_AXI_PLL2P
					CLK_MCU_HSI
					CLK_PLL12_HSE
					CLK_PLL3_HSE
					CLK_PLL4_HSE
					CLK_RTC_HSE
					CLK_MCO1_DISABLED
					CLK_MCO2_DISABLED
			>;

			st,clkdiv = <
				1 /*MPU*/
				0 /*AXI*/
				0 /*MCU*/
				1 /*APB1*/
				1 /*APB2*/
				1 /*APB3*/
				1 /*APB4*/
				5 /*APB5*/
				23 /*RTC*/
				0 /*MCO1*/
				0 /*MCO2*/
			>;

			st,pll@0 {
				cfg = < 1 53 0 0 0 1 >;
				frac = < 0x810 >;
			};
			st,pll@1 {
				cfg = < 1 43 1 0 0 PQR(0,1,1)>;
				csg = <10 20 1>;
			};
			st,pll@2 {
				cfg = < 2 85 3 13 3 0>;
				csg = <10 20 SSCG_MODE_CENTER_SPREAD>;
			};
			st,pll@3 {
				cfg = < 2 78 4 7 9 3>;
			};
			st,pkcs = <
					CLK_STGEN_HSE
					CLK_CKPER_HSI
					CLK_USBPHY_PLL2P
					CLK_DSI_PLL2Q
				  >;
		};
	};

--------------------------
other clocks = fixed-clock
--------------------------
The clock tree is also based on 5 fixed-clock in clocks node
used to define the state of associated ST32MP1 oscillators:
- clk-lsi
- clk-lse
- clk-hsi
- clk-hse
- clk-csi

At boot the clock tree initialization will
- enable the oscillator present in device tree
- disable HSI oscillator if the node is absent (always activated by bootrom)

Optional properties :

a) for external oscillator: "clk-lse", "clk-hse"

	4 optional fields are managed
	- "st,bypass" Configure the oscillator bypass mode (HSEBYP, LSEBYP)
	- "st,digbypass" Configure the bypass mode as full-swing digital signal
	  (DIGBYP)
	- "st,css" Activate the clock security system (HSECSSON, LSECSSON)
	- "st,drive" (only for LSE) value of the drive for the oscillator
	   (see LSEDRV_ define in the file dt-bindings/clock/stm32mp1-clksrc.h)

	Example board file:

	/ {
		clocks {
			clk_hse: clk-hse {
				#clock-cells = <0>;
				compatible = "fixed-clock";
				clock-frequency = <64000000>;
				st,bypass;
			};

			clk_lse: clk-lse {
				#clock-cells = <0>;
				compatible = "fixed-clock";
				clock-frequency = <32768>;
				st,css;
				st,drive = <LSEDRV_LOWEST>;
			};
	};

b) for internal oscillator: "clk-hsi"

	internally HSI clock is fixed to 64MHz for STM32MP157 soc
	in device tree clk-hsi is the clock after HSIDIV (ck_hsi in RCC doc)
	So this clock frequency is used to compute the expected HSI_DIV
	for the clock tree initialisation

	ex: for HSIDIV = /1

	/ {
		clocks {
			clk_hsi: clk-hsi {
				#clock-cells = <0>;
				compatible = "fixed-clock";
				clock-frequency = <64000000>;
			};
	};

	ex: for HSIDIV = /2

	/ {
		clocks {
			clk_hsi: clk-hsi {
				#clock-cells = <0>;
				compatible = "fixed-clock";
				clock-frequency = <32000000>;
			};
	};