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PHY Abstraction Layer
(Updated 2008-04-08)

Purpose

 Most network devices consist of set of registers which provide an interface
 to a MAC layer, which communicates with the physical connection through a
 PHY.  The PHY concerns itself with negotiating link parameters with the link
 partner on the other side of the network connection (typically, an ethernet
 cable), and provides a register interface to allow drivers to determine what
 settings were chosen, and to configure what settings are allowed.

 While these devices are distinct from the network devices, and conform to a
 standard layout for the registers, it has been common practice to integrate
 the PHY management code with the network driver.  This has resulted in large
 amounts of redundant code.  Also, on embedded systems with multiple (and
 sometimes quite different) ethernet controllers connected to the same 
 management bus, it is difficult to ensure safe use of the bus.

 Since the PHYs are devices, and the management busses through which they are
 accessed are, in fact, busses, the PHY Abstraction Layer treats them as such.
 In doing so, it has these goals:

   1) Increase code-reuse
   2) Increase overall code-maintainability
   3) Speed development time for new network drivers, and for new systems
 
 Basically, this layer is meant to provide an interface to PHY devices which
 allows network driver writers to write as little code as possible, while
 still providing a full feature set.

The MDIO bus

 Most network devices are connected to a PHY by means of a management bus.
 Different devices use different busses (though some share common interfaces).
 In order to take advantage of the PAL, each bus interface needs to be
 registered as a distinct device.

 1) read and write functions must be implemented.  Their prototypes are:

     int write(struct mii_bus *bus, int mii_id, int regnum, u16 value);
     int read(struct mii_bus *bus, int mii_id, int regnum);

   mii_id is the address on the bus for the PHY, and regnum is the register
   number.  These functions are guaranteed not to be called from interrupt
   time, so it is safe for them to block, waiting for an interrupt to signal
   the operation is complete
 
 2) A reset function is optional.  This is used to return the bus to an
   initialized state.

 3) A probe function is needed.  This function should set up anything the bus
   driver needs, setup the mii_bus structure, and register with the PAL using
   mdiobus_register.  Similarly, there's a remove function to undo all of
   that (use mdiobus_unregister).
 
 4) Like any driver, the device_driver structure must be configured, and init
   exit functions are used to register the driver.

 5) The bus must also be declared somewhere as a device, and registered.

 As an example for how one driver implemented an mdio bus driver, see
 drivers/net/ethernet/freescale/fsl_pq_mdio.c and an associated DTS file
 for one of the users. (e.g. "git grep fsl,.*-mdio arch/powerpc/boot/dts/")

(RG)MII/electrical interface considerations

 The Reduced Gigabit Medium Independent Interface (RGMII) is a 12-pin
 electrical signal interface using a synchronous 125Mhz clock signal and several
 data lines. Due to this design decision, a 1.5ns to 2ns delay must be added
 between the clock line (RXC or TXC) and the data lines to let the PHY (clock
 sink) have enough setup and hold times to sample the data lines correctly. The
 PHY library offers different types of PHY_INTERFACE_MODE_RGMII* values to let
 the PHY driver and optionally the MAC driver, implement the required delay. The
 values of phy_interface_t must be understood from the perspective of the PHY
 device itself, leading to the following:

 * PHY_INTERFACE_MODE_RGMII: the PHY is not responsible for inserting any
   internal delay by itself, it assumes that either the Ethernet MAC (if capable
   or the PCB traces) insert the correct 1.5-2ns delay

 * PHY_INTERFACE_MODE_RGMII_TXID: the PHY should insert an internal delay
   for the transmit data lines (TXD[3:0]) processed by the PHY device

 * PHY_INTERFACE_MODE_RGMII_RXID: the PHY should insert an internal delay
   for the receive data lines (RXD[3:0]) processed by the PHY device

 * PHY_INTERFACE_MODE_RGMII_ID: the PHY should insert internal delays for
   both transmit AND receive data lines from/to the PHY device

 Whenever possible, use the PHY side RGMII delay for these reasons:

 * PHY devices may offer sub-nanosecond granularity in how they allow a
   receiver/transmitter side delay (e.g: 0.5, 1.0, 1.5ns) to be specified. Such
   precision may be required to account for differences in PCB trace lengths

 * PHY devices are typically qualified for a large range of applications
   (industrial, medical, automotive...), and they provide a constant and
   reliable delay across temperature/pressure/voltage ranges

 * PHY device drivers in PHYLIB being reusable by nature, being able to
   configure correctly a specified delay enables more designs with similar delay
   requirements to be operate correctly

 For cases where the PHY is not capable of providing this delay, but the
 Ethernet MAC driver is capable of doing so, the correct phy_interface_t value
 should be PHY_INTERFACE_MODE_RGMII, and the Ethernet MAC driver should be
 configured correctly in order to provide the required transmit and/or receive
 side delay from the perspective of the PHY device. Conversely, if the Ethernet
 MAC driver looks at the phy_interface_t value, for any other mode but
 PHY_INTERFACE_MODE_RGMII, it should make sure that the MAC-level delays are
 disabled.

 In case neither the Ethernet MAC, nor the PHY are capable of providing the
 required delays, as defined per the RGMII standard, several options may be
 available:

 * Some SoCs may offer a pin pad/mux/controller capable of configuring a given
   set of pins'strength, delays, and voltage; and it may be a suitable
   option to insert the expected 2ns RGMII delay.

 * Modifying the PCB design to include a fixed delay (e.g: using a specifically
   designed serpentine), which may not require software configuration at all.

Common problems with RGMII delay mismatch

 When there is a RGMII delay mismatch between the Ethernet MAC and the PHY, this
 will most likely result in the clock and data line signals to be unstable when
 the PHY or MAC take a snapshot of these signals to translate them into logical
 1 or 0 states and reconstruct the data being transmitted/received. Typical
 symptoms include:

 * Transmission/reception partially works, and there is frequent or occasional
   packet loss observed

 * Ethernet MAC may report some or all packets ingressing with a FCS/CRC error,
   or just discard them all

 * Switching to lower speeds such as 10/100Mbits/sec makes the problem go away
   (since there is enough setup/hold time in that case)


Connecting to a PHY

 Sometime during startup, the network driver needs to establish a connection
 between the PHY device, and the network device.  At this time, the PHY's bus
 and drivers need to all have been loaded, so it is ready for the connection.
 At this point, there are several ways to connect to the PHY:

 1) The PAL handles everything, and only calls the network driver when
   the link state changes, so it can react.

 2) The PAL handles everything except interrupts (usually because the
   controller has the interrupt registers).

 3) The PAL handles everything, but checks in with the driver every second,
   allowing the network driver to react first to any changes before the PAL
   does.
 
 4) The PAL serves only as a library of functions, with the network device
   manually calling functions to update status, and configure the PHY


Letting the PHY Abstraction Layer do Everything

 If you choose option 1 (The hope is that every driver can, but to still be
 useful to drivers that can't), connecting to the PHY is simple:

 First, you need a function to react to changes in the link state.  This
 function follows this protocol:

   static void adjust_link(struct net_device *dev);
 
 Next, you need to know the device name of the PHY connected to this device. 
 The name will look something like, "0:00", where the first number is the
 bus id, and the second is the PHY's address on that bus.  Typically,
 the bus is responsible for making its ID unique.
 
 Now, to connect, just call this function:
 
   phydev = phy_connect(dev, phy_name, &adjust_link, interface);

 phydev is a pointer to the phy_device structure which represents the PHY.  If
 phy_connect is successful, it will return the pointer.  dev, here, is the
 pointer to your net_device.  Once done, this function will have started the
 PHY's software state machine, and registered for the PHY's interrupt, if it
 has one.  The phydev structure will be populated with information about the
 current state, though the PHY will not yet be truly operational at this
 point.

 PHY-specific flags should be set in phydev->dev_flags prior to the call
 to phy_connect() such that the underlying PHY driver can check for flags
 and perform specific operations based on them.
 This is useful if the system has put hardware restrictions on
 the PHY/controller, of which the PHY needs to be aware.

 interface is a u32 which specifies the connection type used
 between the controller and the PHY.  Examples are GMII, MII,
 RGMII, and SGMII.  For a full list, see include/linux/phy.h

 Now just make sure that phydev->supported and phydev->advertising have any
 values pruned from them which don't make sense for your controller (a 10/100
 controller may be connected to a gigabit capable PHY, so you would need to
 mask off SUPPORTED_1000baseT*).  See include/linux/ethtool.h for definitions
 for these bitfields. Note that you should not SET any bits, except the
 SUPPORTED_Pause and SUPPORTED_AsymPause bits (see below), or the PHY may get
 put into an unsupported state.

 Lastly, once the controller is ready to handle network traffic, you call
 phy_start(phydev).  This tells the PAL that you are ready, and configures the
 PHY to connect to the network.  If you want to handle your own interrupts,
 just set phydev->irq to PHY_IGNORE_INTERRUPT before you call phy_start.
 Similarly, if you don't want to use interrupts, set phydev->irq to PHY_POLL.

 When you want to disconnect from the network (even if just briefly), you call
 phy_stop(phydev).

Pause frames / flow control

 The PHY does not participate directly in flow control/pause frames except by
 making sure that the SUPPORTED_Pause and SUPPORTED_AsymPause bits are set in
 MII_ADVERTISE to indicate towards the link partner that the Ethernet MAC
 controller supports such a thing. Since flow control/pause frames generation
 involves the Ethernet MAC driver, it is recommended that this driver takes care
 of properly indicating advertisement and support for such features by setting
 the SUPPORTED_Pause and SUPPORTED_AsymPause bits accordingly. This can be done
 either before or after phy_connect() and/or as a result of implementing the
 ethtool::set_pauseparam feature.


Keeping Close Tabs on the PAL

 It is possible that the PAL's built-in state machine needs a little help to
 keep your network device and the PHY properly in sync.  If so, you can
 register a helper function when connecting to the PHY, which will be called
 every second before the state machine reacts to any changes.  To do this, you
 need to manually call phy_attach() and phy_prepare_link(), and then call
 phy_start_machine() with the second argument set to point to your special
 handler.

 Currently there are no examples of how to use this functionality, and testing
 on it has been limited because the author does not have any drivers which use
 it (they all use option 1).  So Caveat Emptor.

Doing it all yourself

 There's a remote chance that the PAL's built-in state machine cannot track
 the complex interactions between the PHY and your network device.  If this is
 so, you can simply call phy_attach(), and not call phy_start_machine or
 phy_prepare_link().  This will mean that phydev->state is entirely yours to
 handle (phy_start and phy_stop toggle between some of the states, so you
 might need to avoid them).

 An effort has been made to make sure that useful functionality can be
 accessed without the state-machine running, and most of these functions are
 descended from functions which did not interact with a complex state-machine.
 However, again, no effort has been made so far to test running without the
 state machine, so tryer beware.

 Here is a brief rundown of the functions:

 int phy_read(struct phy_device *phydev, u16 regnum);
 int phy_write(struct phy_device *phydev, u16 regnum, u16 val);

   Simple read/write primitives.  They invoke the bus's read/write function
   pointers.

 void phy_print_status(struct phy_device *phydev);
 
   A convenience function to print out the PHY status neatly.

 int phy_start_interrupts(struct phy_device *phydev);
 int phy_stop_interrupts(struct phy_device *phydev);

   Requests the IRQ for the PHY interrupts, then enables them for
   start, or disables then frees them for stop.

 struct phy_device * phy_attach(struct net_device *dev, const char *phy_id,
		 phy_interface_t interface);

   Attaches a network device to a particular PHY, binding the PHY to a generic
   driver if none was found during bus initialization.

 int phy_start_aneg(struct phy_device *phydev);
   
   Using variables inside the phydev structure, either configures advertising
   and resets autonegotiation, or disables autonegotiation, and configures
   forced settings.

 static inline int phy_read_status(struct phy_device *phydev);

   Fills the phydev structure with up-to-date information about the current
   settings in the PHY.

 int phy_ethtool_sset(struct phy_device *phydev, struct ethtool_cmd *cmd);

   Ethtool convenience functions.

 int phy_mii_ioctl(struct phy_device *phydev,
                 struct mii_ioctl_data *mii_data, int cmd);

   The MII ioctl.  Note that this function will completely screw up the state
   machine if you write registers like BMCR, BMSR, ADVERTISE, etc.  Best to
   use this only to write registers which are not standard, and don't set off
   a renegotiation.


PHY Device Drivers

 With the PHY Abstraction Layer, adding support for new PHYs is
 quite easy.  In some cases, no work is required at all!  However,
 many PHYs require a little hand-holding to get up-and-running.

Generic PHY driver

 If the desired PHY doesn't have any errata, quirks, or special
 features you want to support, then it may be best to not add
 support, and let the PHY Abstraction Layer's Generic PHY Driver
 do all of the work.  

Writing a PHY driver

 If you do need to write a PHY driver, the first thing to do is
 make sure it can be matched with an appropriate PHY device.
 This is done during bus initialization by reading the device's
 UID (stored in registers 2 and 3), then comparing it to each
 driver's phy_id field by ANDing it with each driver's
 phy_id_mask field.  Also, it needs a name.  Here's an example:

   static struct phy_driver dm9161_driver = {
         .phy_id         = 0x0181b880,
	 .name           = "Davicom DM9161E",
	 .phy_id_mask    = 0x0ffffff0,
	 ...
   }

 Next, you need to specify what features (speed, duplex, autoneg,
 etc) your PHY device and driver support.  Most PHYs support
 PHY_BASIC_FEATURES, but you can look in include/mii.h for other
 features.

 Each driver consists of a number of function pointers, documented
 in include/linux/phy.h under the phy_driver structure.

 Of these, only config_aneg and read_status are required to be
 assigned by the driver code.  The rest are optional.  Also, it is
 preferred to use the generic phy driver's versions of these two
 functions if at all possible: genphy_read_status and
 genphy_config_aneg.  If this is not possible, it is likely that
 you only need to perform some actions before and after invoking
 these functions, and so your functions will wrap the generic
 ones.

 Feel free to look at the Marvell, Cicada, and Davicom drivers in
 drivers/net/phy/ for examples (the lxt and qsemi drivers have
 not been tested as of this writing).

 The PHY's MMD register accesses are handled by the PAL framework
 by default, but can be overridden by a specific PHY driver if
 required. This could be the case if a PHY was released for
 manufacturing before the MMD PHY register definitions were
 standardized by the IEEE. Most modern PHYs will be able to use
 the generic PAL framework for accessing the PHY's MMD registers.
 An example of such usage is for Energy Efficient Ethernet support,
 implemented in the PAL. This support uses the PAL to access MMD
 registers for EEE query and configuration if the PHY supports
 the IEEE standard access mechanisms, or can use the PHY's specific
 access interfaces if overridden by the specific PHY driver. See
 the Micrel driver in drivers/net/phy/ for an example of how this
 can be implemented.

Board Fixups

 Sometimes the specific interaction between the platform and the PHY requires
 special handling.  For instance, to change where the PHY's clock input is,
 or to add a delay to account for latency issues in the data path.  In order
 to support such contingencies, the PHY Layer allows platform code to register
 fixups to be run when the PHY is brought up (or subsequently reset).

 When the PHY Layer brings up a PHY it checks to see if there are any fixups
 registered for it, matching based on UID (contained in the PHY device's phy_id
 field) and the bus identifier (contained in phydev->dev.bus_id).  Both must
 match, however two constants, PHY_ANY_ID and PHY_ANY_UID, are provided as
 wildcards for the bus ID and UID, respectively.

 When a match is found, the PHY layer will invoke the run function associated
 with the fixup.  This function is passed a pointer to the phy_device of
 interest.  It should therefore only operate on that PHY.

 The platform code can either register the fixup using phy_register_fixup():

	int phy_register_fixup(const char *phy_id,
		u32 phy_uid, u32 phy_uid_mask,
		int (*run)(struct phy_device *));

 Or using one of the two stubs, phy_register_fixup_for_uid() and
 phy_register_fixup_for_id():

 int phy_register_fixup_for_uid(u32 phy_uid, u32 phy_uid_mask,
		int (*run)(struct phy_device *));
 int phy_register_fixup_for_id(const char *phy_id,
		int (*run)(struct phy_device *));

 The stubs set one of the two matching criteria, and set the other one to
 match anything.

 When phy_register_fixup() or *_for_uid()/*_for_id() is called at module,
 unregister fixup and free allocate memory are required.

 Call one of following function before unloading module.

 int phy_unregister_fixup(const char *phy_id, u32 phy_uid, u32 phy_uid_mask);
 int phy_unregister_fixup_for_uid(u32 phy_uid, u32 phy_uid_mask);
 int phy_register_fixup_for_id(const char *phy_id);

Standards

 IEEE Standard 802.3: CSMA/CD Access Method and Physical Layer Specifications, Section Two:
 http://standards.ieee.org/getieee802/download/802.3-2008_section2.pdf

 RGMII v1.3:
 http://web.archive.org/web/20160303212629/http://www.hp.com/rnd/pdfs/RGMIIv1_3.pdf

 RGMII v2.0:
 http://web.archive.org/web/20160303171328/http://www.hp.com/rnd/pdfs/RGMIIv2_0_final_hp.pdf