Documentation/devicetree/bindings/clock/baikal,bt1-ccu-pll.yaml

# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
# Copyright (C) 2020 BAIKAL ELECTRONICS, JSC
%YAML 1.2
---
$id: http://devicetree.org/schemas/clock/baikal,bt1-ccu-pll.yaml#
$schema: http://devicetree.org/meta-schemas/core.yaml#

title: Baikal-T1 Clock Control Unit PLL

maintainers:
  - Serge Semin <fancer.lancer@gmail.com>

description: |
  Clocks Control Unit is the core of Baikal-T1 SoC System Controller
  responsible for the chip subsystems clocking and resetting. The CCU is
  connected with an external fixed rate oscillator, which signal is transformed
  into clocks of various frequencies and then propagated to either individual
  IP-blocks or to groups of blocks (clock domains). The transformation is done
  by means of PLLs and gateable/non-gateable dividers embedded into the CCU.
  It's logically divided into the next components:
  1) External oscillator (normally XTAL's 25 MHz crystal oscillator, but
     in general can provide any frequency supported by the CCU PLLs).
  2) PLLs clocks generators (PLLs) - described in this binding file.
  3) AXI-bus clock dividers (AXI).
  4) System devices reference clock dividers (SYS).
  which are connected with each other as shown on the next figure:

          +---------------+
          | Baikal-T1 CCU |
          |   +----+------|- MIPS P5600 cores
          | +-|PLLs|------|- DDR controller
          | | +----+      |
  +----+  | |  |  |       |
  |XTAL|--|-+  |  | +---+-|
  +----+  | |  |  +-|AXI|-|- AXI-bus
          | |  |    +---+-|
          | |  |          |
          | |  +----+---+-|- APB-bus
          | +-------|SYS|-|- Low-speed Devices
          |         +---+-|- High-speed Devices
          +---------------+

  Each CCU sub-block is represented as a separate dts-node and has an
  individual driver to be bound with.

  In order to create signals of wide range frequencies the external oscillator
  output is primarily connected to a set of CCU PLLs. There are five PLLs
  to create a clock for the MIPS P5600 cores, the embedded DDR controller,
  SATA, Ethernet and PCIe domains. The last three domains though named by the
  biggest system interfaces in fact include nearly all of the rest SoC
  peripherals. Each of the PLLs is based on True Circuits TSMC CLN28HPM core
  with an interface wrapper (so called safe PLL' clocks switcher) to simplify
  the PLL configuration procedure. The PLLs work as depicted on the next
  diagram:

      +--------------------------+
      |                          |
      +-->+---+    +---+   +---+ |  +---+   0|\
  CLKF--->|/NF|--->|PFD|...|VCO|-+->|/OD|--->| |
          +---+ +->+---+   +---+ /->+---+    | |--->CLKOUT
  CLKOD---------C----------------+          1| |
       +--------C--------------------------->|/
       |        |                             ^
  Rclk-+->+---+ |                             |
  CLKR--->|/NR|-+                             |
          +---+                               |
  BYPASS--------------------------------------+
  BWADJ--->

  where Rclk is the reference clock coming  from XTAL, NR - reference clock
  divider, NF - PLL clock multiplier, OD - VCO output clock divider, CLKOUT -
  output clock, BWADJ is the PLL bandwidth adjustment parameter. At this moment
  the binding supports the PLL dividers configuration in accordance with a
  requested rate, while bypassing and bandwidth adjustment settings can be
  added in future if it gets to be necessary.

  The PLLs CLKOUT is then either directly connected with the corresponding
  clocks consumer (like P5600 cores or DDR controller) or passed over a CCU
  divider to create a signal required for the clock domain.

  The CCU PLL dts-node uses the common clock bindings with no custom
  parameters. The list of exported clocks can be found in
  'include/dt-bindings/clock/bt1-ccu.h'. Since CCU PLL is a part of the
  Baikal-T1 SoC System Controller its DT node is supposed to be a child of
  later one.

properties:
  compatible:
    const: baikal,bt1-ccu-pll

  reg:
    maxItems: 1

  "#clock-cells":
    const: 1

  clocks:
    description: External reference clock
    maxItems: 1

  clock-names:
    const: ref_clk

additionalProperties: false

required:
  - compatible
  - "#clock-cells"
  - clocks
  - clock-names

examples:
  # Clock Control Unit PLL node:
  - |
    clock-controller@1f04d000 {
      compatible = "baikal,bt1-ccu-pll";
      reg = <0x1f04d000 0x028>;
      #clock-cells = <1>;

      clocks = <&clk25m>;
      clock-names = "ref_clk";
    };
  # Required external oscillator:
  - |
    clk25m: clock-oscillator-25m {
      compatible = "fixed-clock";
      #clock-cells = <0>;
      clock-frequency  = <25000000>;
      clock-output-names = "clk25m";
    };
...