PHY SUBSYSTEM
  		  Kishon Vijay Abraham I <kishon@ti.com>
  
  This document explains the Generic PHY Framework along with the APIs provided,
  and how-to-use.
  
  1. Introduction
  
  *PHY* is the abbreviation for physical layer. It is used to connect a device
  to the physical medium e.g., the USB controller has a PHY to provide functions
  such as serialization, de-serialization, encoding, decoding and is responsible
  for obtaining the required data transmission rate. Note that some USB
  controllers have PHY functionality embedded into it and others use an external
  PHY. Other peripherals that use PHY include Wireless LAN, Ethernet,
  SATA etc.
  
  The intention of creating this framework is to bring the PHY drivers spread
  all over the Linux kernel to drivers/phy to increase code re-use and for
  better code maintainability.
  
  This framework will be of use only to devices that use external PHY (PHY
  functionality is not embedded within the controller).
  
  2. Registering/Unregistering the PHY provider
  
  PHY provider refers to an entity that implements one or more PHY instances.
  For the simple case where the PHY provider implements only a single instance of
  the PHY, the framework provides its own implementation of of_xlate in
  of_phy_simple_xlate. If the PHY provider implements multiple instances, it
  should provide its own implementation of of_xlate. of_xlate is used only for
  dt boot case.
  
  #define of_phy_provider_register(dev, xlate)    \
          __of_phy_provider_register((dev), THIS_MODULE, (xlate))
  
  #define devm_of_phy_provider_register(dev, xlate)       \
          __devm_of_phy_provider_register((dev), THIS_MODULE, (xlate))
  
  of_phy_provider_register and devm_of_phy_provider_register macros can be used to
  register the phy_provider and it takes device and of_xlate as
  arguments. For the dt boot case, all PHY providers should use one of the above
  2 macros to register the PHY provider.
  
  void devm_of_phy_provider_unregister(struct device *dev,
  	struct phy_provider *phy_provider);
  void of_phy_provider_unregister(struct phy_provider *phy_provider);
  
  devm_of_phy_provider_unregister and of_phy_provider_unregister can be used to
  unregister the PHY.
  
  3. Creating the PHY
  
  The PHY driver should create the PHY in order for other peripheral controllers
  to make use of it. The PHY framework provides 2 APIs to create the PHY.
  
  struct phy *phy_create(struct device *dev, const struct phy_ops *ops,
          struct phy_init_data *init_data);
  struct phy *devm_phy_create(struct device *dev, const struct phy_ops *ops,
  	struct phy_init_data *init_data);
  
  The PHY drivers can use one of the above 2 APIs to create the PHY by passing
  the device pointer, phy ops and init_data.
  phy_ops is a set of function pointers for performing PHY operations such as
  init, exit, power_on and power_off. *init_data* is mandatory to get a reference
  to the PHY in the case of non-dt boot. See section *Board File Initialization*
  on how init_data should be used.
  
  Inorder to dereference the private data (in phy_ops), the phy provider driver
  can use phy_set_drvdata() after creating the PHY and use phy_get_drvdata() in
  phy_ops to get back the private data.
  
  4. Getting a reference to the PHY
  
  Before the controller can make use of the PHY, it has to get a reference to
  it. This framework provides the following APIs to get a reference to the PHY.
  
  struct phy *phy_get(struct device *dev, const char *string);

  struct phy *phy_optional_get(struct device *dev, const char *string);

  struct phy *devm_phy_get(struct device *dev, const char *string);

  struct phy *devm_phy_optional_get(struct device *dev, const char *string);
  
  phy_get, phy_optional_get, devm_phy_get and devm_phy_optional_get can
  be used to get the PHY. In the case of dt boot, the string arguments
  should contain the phy name as given in the dt data and in the case of
  non-dt boot, it should contain the label of the PHY.  The two
  devm_phy_get associates the device with the PHY using devres on
  successful PHY get. On driver detach, release function is invoked on
  the the devres data and devres data is freed. phy_optional_get and
  devm_phy_optional_get should be used when the phy is optional. These
  two functions will never return -ENODEV, but instead returns NULL when
  the phy cannot be found.

  It should be noted that NULL is a valid phy reference. All phy
  consumer calls on the NULL phy become NOPs. That is the release calls,
  the phy_init() and phy_exit() calls, and phy_power_on() and
  phy_power_off() calls are all NOP when applied to a NULL phy. The NULL
  phy is useful in devices for handling optional phy devices.

  5. Releasing a reference to the PHY
  
  When the controller no longer needs the PHY, it has to release the reference
  to the PHY it has obtained using the APIs mentioned in the above section. The
  PHY framework provides 2 APIs to release a reference to the PHY.
  
  void phy_put(struct phy *phy);
  void devm_phy_put(struct device *dev, struct phy *phy);
  
  Both these APIs are used to release a reference to the PHY and devm_phy_put
  destroys the devres associated with this PHY.
  
  6. Destroying the PHY
  
  When the driver that created the PHY is unloaded, it should destroy the PHY it
  created using one of the following 2 APIs.
  
  void phy_destroy(struct phy *phy);
  void devm_phy_destroy(struct device *dev, struct phy *phy);
  
  Both these APIs destroy the PHY and devm_phy_destroy destroys the devres
  associated with this PHY.
  
  7. PM Runtime
  
  This subsystem is pm runtime enabled. So while creating the PHY,
  pm_runtime_enable of the phy device created by this subsystem is called and
  while destroying the PHY, pm_runtime_disable is called. Note that the phy
  device created by this subsystem will be a child of the device that calls
  phy_create (PHY provider device).
  
  So pm_runtime_get_sync of the phy_device created by this subsystem will invoke
  pm_runtime_get_sync of PHY provider device because of parent-child relationship.
  It should also be noted that phy_power_on and phy_power_off performs
  phy_pm_runtime_get_sync and phy_pm_runtime_put respectively.
  There are exported APIs like phy_pm_runtime_get, phy_pm_runtime_get_sync,
  phy_pm_runtime_put, phy_pm_runtime_put_sync, phy_pm_runtime_allow and
  phy_pm_runtime_forbid for performing PM operations.
  
  8. Board File Initialization
  
  Certain board file initialization is necessary in order to get a reference
  to the PHY in the case of non-dt boot.
  Say we have a single device that implements 3 PHYs that of USB, SATA and PCIe,
  then in the board file the following initialization should be done.
  
  struct phy_consumer consumers[] = {
  	PHY_CONSUMER("dwc3.0", "usb"),
  	PHY_CONSUMER("pcie.0", "pcie"),
  	PHY_CONSUMER("sata.0", "sata"),
  };
  PHY_CONSUMER takes 2 parameters, first is the device name of the controller
  (PHY consumer) and second is the port name.
  
  struct phy_init_data init_data = {
  	.consumers = consumers,
  	.num_consumers = ARRAY_SIZE(consumers),
  };
  
  static const struct platform_device pipe3_phy_dev = {
  	.name = "pipe3-phy",
  	.id = -1,
  	.dev = {
  		.platform_data = {
  			.init_data = &init_data,
  		},
  	},
  };
  
  then, while doing phy_create, the PHY driver should pass this init_data
  	phy_create(dev, ops, pdata->init_data);
  
  and the controller driver (phy consumer) should pass the port name along with
  the device to get a reference to the PHY
  	phy_get(dev, "pcie");
  
  9. DeviceTree Binding
  
  The documentation for PHY dt binding can be found @
  Documentation/devicetree/bindings/phy/phy-bindings.txt