| ===================== |
| PHY Abstraction Layer |
| ===================== |
| |
| 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: |
| |
| #. Increase code-reuse |
| #. Increase overall code-maintainability |
| #. 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. |
| |
| #. 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 |
| |
| #. A reset function is optional. This is used to return the bus to an |
| initialized state. |
| |
| #. 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). |
| |
| #. Like any driver, the device_driver structure must be configured, and init |
| exit functions are used to register the driver. |
| |
| #. 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: |
| |
| #. The PAL handles everything, and only calls the network driver when |
| the link state changes, so it can react. |
| |
| #. The PAL handles everything except interrupts (usually because the |
| controller has the interrupt registers). |
| |
| #. 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. |
| |
| #. 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 the MAC interrupt of your network driver |
| also handles PHY status changes, just set phydev->irq to PHY_IGNORE_INTERRUPT |
| before you call phy_start and use phy_mac_interrupt() from the network |
| driver. If you don't want to use interrupts, set phydev->irq to PHY_POLL. |
| phy_start() enables the PHY interrupts (if applicable) and starts the |
| phylib state machine. |
| |
| When you want to disconnect from the network (even if just briefly), you call |
| phy_stop(phydev). This function also stops the phylib state machine and |
| disables PHY interrupts. |
| |
| 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. |
| :: |
| |
| void phy_request_interrupt(struct phy_device *phydev); |
| |
| Requests the IRQ for the PHY interrupts. |
| :: |
| |
| 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 |