net: dsa: microchip: ksz8: Add function to configure ports with integrated PHYs

This patch introduces the function 'ksz8_phy_port_link_up' to the
Microchip KSZ8xxx driver. This function is responsible for setting up
flow control and duplex settings for the ports that are integrated with
PHYs.

The KSZ8795 switch supports asymmetric pause control, which can't be
fully utilized since a single bit controls both RX and TX pause. Despite
this, the flow control can be adjusted based on the auto-negotiation
process, taking into account the capabilities of both link partners.

On the other hand, the KSZ8873's PORT_FORCE_FLOW_CTRL bit can be set by
the hardware bootstrap, which ignores the auto-negotiation result.
Therefore, even in auto-negotiation mode, we need to ensure that this
bit is correctly set.

When auto-negotiation isn't in use, we enforce symmetric pause control
for the KSZ8795 switch.

Please note, forcing flow control disable on a port while still
advertising pause support isn't possible. While this scenario
might not be practical or desired, it's important to be aware of this
limitation when working with the KSZ8873 and similar devices.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20231127145101.3039399-3-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: olerem

drivers/net/dsa/microchip/ksz8795.c

@@ -1439,6 +1439,74 @@ void ksz8_config_cpu_port(struct dsa_switch *ds)
 	}
 }
 
+/**
+ * ksz8_phy_port_link_up - Configures ports with integrated PHYs
+ * @dev: The KSZ device instance.
+ * @port: The port number to configure.
+ * @duplex: The desired duplex mode.
+ * @tx_pause: If true, enables transmit pause.
+ * @rx_pause: If true, enables receive pause.
+ *
+ * Description:
+ * The function configures flow control settings for a given port based on the
+ * desired settings and current duplex mode.
+ *
+ * According to the KSZ8873 datasheet, the PORT_FORCE_FLOW_CTRL bit in the
+ * Port Control 2 register (0x1A for Port 1, 0x22 for Port 2, 0x32 for Port 3)
+ * determines how flow control is handled on the port:
+ *    "1 = will always enable full-duplex flow control on the port, regardless
+ *         of AN result.
+ *     0 = full-duplex flow control is enabled based on AN result."
+ *
+ * This means that the flow control behavior depends on the state of this bit:
+ * - If PORT_FORCE_FLOW_CTRL is set to 1, the switch will ignore AN results and
+ *   force flow control on the port.
+ * - If PORT_FORCE_FLOW_CTRL is set to 0, the switch will enable or disable
+ *   flow control based on the AN results.
+ *
+ * However, there is a potential limitation in this configuration. It is
+ * currently not possible to force disable flow control on a port if we still
+ * advertise pause support. While such a configuration is not currently
+ * supported by Linux, and may not make practical sense, it's important to be
+ * aware of this limitation when working with the KSZ8873 and similar devices.
+ */
+static void ksz8_phy_port_link_up(struct ksz_device *dev, int port, int duplex,
+				  bool tx_pause, bool rx_pause)
+{
+	const u16 *regs = dev->info->regs;
+	u8 sctrl = 0;
+
+	/* The KSZ8795 switch differs from the KSZ8873 by supporting
+	 * asymmetric pause control. However, since a single bit is used to
+	 * control both RX and TX pause, we can't enforce asymmetric pause
+	 * control - both TX and RX pause will be either enabled or disabled
+	 * together.
+	 *
+	 * If auto-negotiation is enabled, we usually allow the flow control to
+	 * be determined by the auto-negotiation process based on the
+	 * capabilities of both link partners. However, for KSZ8873, the
+	 * PORT_FORCE_FLOW_CTRL bit may be set by the hardware bootstrap,
+	 * ignoring the auto-negotiation result. Thus, even in auto-negotiation
+	 * mode, we need to ensure that the PORT_FORCE_FLOW_CTRL bit is
+	 * properly cleared.
+	 *
+	 * In the absence of pause auto-negotiation, we will enforce symmetric
+	 * pause control for both variants of switches - KSZ8873 and KSZ8795.
+	 *
+	 * Autoneg Pause Autoneg      rx,tx	PORT_FORCE_FLOW_CTRL
+	 * 1		1		x	0
+	 * 0		1		x	0 (flow control probably disabled)
+	 * x		0		1	1 (flow control force enabled)
+	 * 1		0		0	0 (flow control still depends on
+	 *					   aneg result due to hardware)
+	 * 0		0		0	0 (flow control probably disabled)
+	 */
+	if (dev->ports[port].manual_flow && tx_pause)
+		sctrl |= PORT_FORCE_FLOW_CTRL;
+
+	ksz_prmw8(dev, port, regs[P_STP_CTRL], PORT_FORCE_FLOW_CTRL, sctrl);
+}
+
 /**
  * ksz8_cpu_port_link_up - Configures the CPU port of the switch.
  * @dev: The KSZ device instance.
@@ -1488,6 +1556,8 @@ void ksz8_phylink_mac_link_up(struct ksz_device *dev, int port,
 	 */
 	if (dev->cpu_port == port)
 		ksz8_cpu_port_link_up(dev, speed, duplex, tx_pause, rx_pause);
+	else if (dev->info->internal_phy[port])
+		ksz8_phy_port_link_up(dev, port, duplex, tx_pause, rx_pause);
 }
 
 static int ksz8_handle_global_errata(struct dsa_switch *ds)

drivers/net/dsa/microchip/ksz_common.c

@@ -2967,8 +2967,10 @@ static void ksz_phylink_mac_config(struct dsa_switch *ds, int port,
 {
 	struct ksz_device *dev = ds->priv;
 
-	if (ksz_is_ksz88x3(dev))
+	if (ksz_is_ksz88x3(dev)) {
+		dev->ports[port].manual_flow = !(state->pause & MLO_PAUSE_AN);
 		return;
+	}
 
 	/* Internal PHYs */
 	if (dev->info->internal_phy[port])

drivers/net/dsa/microchip/ksz_common.h

@@ -134,6 +134,7 @@ struct ksz_port {
 	ktime_t tstamp_msg;
 	struct completion tstamp_msg_comp;
 #endif
+	bool manual_flow;
 };
 
 struct ksz_device {