FreeBSD/Linux Kernel Cross Reference
sys/contrib/device-tree/Bindings/mfd/mfd.txt

     Multi-Function Devices (MFD)
     
     These devices comprise a nexus for heterogeneous hardware blocks containing
     more than one non-unique yet varying hardware functionality.
     
     A typical MFD can be:
     
     - A mixed signal ASIC on an external bus, sometimes a PMIC (Power Management
       Integrated Circuit) that is manufactured in a lower technology node (rough
      silicon) that handles analog drivers for things like audio amplifiers, LED
      drivers, level shifters, PHY (physical interfaces to things like USB or
      ethernet), regulators etc.
    
    - A range of memory registers containing "miscellaneous system registers" also
      known as a system controller "syscon" or any other memory range containing a
      mix of unrelated hardware devices.
    
    Optional properties:
    
    - compatible : "simple-mfd" - this signifies that the operating system should
      consider all subnodes of the MFD device as separate devices akin to how
      "simple-bus" indicates when to see subnodes as children for a simple
      memory-mapped bus. For more complex devices, when the nexus driver has to
      probe registers to figure out what child devices exist etc, this should not
      be used. In the latter case the child devices will be determined by the
      operating system.
    
    - ranges: Describes the address mapping relationship to the parent. Should set
      the child's base address to 0, the physical address within parent's address
      space, and the length of the address map.
    
    - #address-cells: Specifies the number of cells used to represent physical base
      addresses. Must be present if ranges is used.
    
    - #size-cells: Specifies the number of cells used to represent the size of an
      address. Must be present if ranges is used.
    
    Example:
    
    foo@1000 {
            compatible = "syscon", "simple-mfd";
            reg = <0x01000 0x1000>;
    
            led@8.0 {
                    compatible = "register-bit-led";
                    offset = <0x08>;
                    mask = <0x01>;
                    label = "myled";
                    default-state = "on";
            };
    };

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