@@ -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