Merge branch 'phy-doc-improvements'

Florian Fainelli says:

====================
Documentation: net: phy: Improve documentation

This patch series addresses discussions and feedback that was recently received
on the mailing-list in the area of: flow control/pause frames, interpretation of
phy_interface_t and finally add some links to useful standards documents.

Changes in v3:

- add Timur's feedback into patch 3

Changes in v2:

- clarify a few things in the RGMII section, add a paragraph about common issues
  with RGMII delay mismatches
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/phy.txt

@@ -65,6 +65,83 @@ The MDIO bus
  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
@@ -127,8 +204,9 @@ Letting the PHY Abstraction Layer do Everything
  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, or the PHY may
- get put into an unsupported state.
+ 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
@@ -139,6 +217,19 @@ Letting the PHY Abstraction Layer do Everything
  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
@@ -251,39 +342,8 @@ Writing a PHY driver
  PHY_BASIC_FEATURES, but you can look in include/mii.h for other
  features.
 
- Each driver consists of a number of function pointers:
-
-   soft_reset: perform a PHY software reset
-   config_init: configures PHY into a sane state after a reset.
-     For instance, a Davicom PHY requires descrambling disabled.
-   probe: Allocate phy->priv, optionally refuse to bind.
-   PHY may not have been reset or had fixups run yet.
-   suspend/resume: power management
-   config_aneg: Changes the speed/duplex/negotiation settings
-   aneg_done: Determines the auto-negotiation result
-   read_status: Reads the current speed/duplex/negotiation settings
-   ack_interrupt: Clear a pending interrupt
-   did_interrupt: Checks if the PHY generated an interrupt
-   config_intr: Enable or disable interrupts
-   remove: Does any driver take-down
-   ts_info: Queries about the HW timestamping status
-   match_phy_device: used for Clause 45 capable PHYs to match devices
-   in package and ensure they are compatible
-   hwtstamp: Set the PHY HW timestamping configuration
-   rxtstamp: Requests a receive timestamp at the PHY level for a 'skb'
-   txtsamp: Requests a transmit timestamp at the PHY level for a 'skb'
-   set_wol: Enable Wake-on-LAN at the PHY level
-   get_wol: Get the Wake-on-LAN status at the PHY level
-   link_change_notify: called to inform the core is about to change the
-   link state, can be used to work around bogus PHY between state changes
-   read_mmd_indirect: Read PHY MMD indirect register
-   write_mmd_indirect: Write PHY MMD indirect register
-   module_info: Get the size and type of an EEPROM contained in an plug-in
-   module
-   module_eeprom: Get EEPROM information of a plug-in module
-   get_sset_count: Get number of strings sets that get_strings will count
-   get_strings: Get strings from requested objects (statistics)
-   get_stats: Get the extended statistics from the PHY device
+ 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
@@ -347,3 +407,13 @@ Board Fixups
  The stubs set one of the two matching criteria, and set the other one to
  match anything.
 
+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