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diff --git a/Documentation/networking/phy.rst b/Documentation/networking/phy.rst new file mode 100644 index 000000000000..0dd90d7df5ec --- /dev/null +++ b/Documentation/networking/phy.rst @@ -0,0 +1,447 @@ +===================== +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 |