[PATCH net-next 2/2] net: pcs: xpcs: select operating mode for 10G-baseR capable PCS

[Alex Elder <elder@riscstar.com>]

From: Daniel Thompson <daniel@riscstar.com>

Currently the XPCS found on Toshiba TC9564 is unable to operate at
2500base-X and slower with a PHY connected using SGMII/2500base-X
(in our case a Qualcomm QCA8081).

The problem arises because this XPCS supports 10Gbase-R. That means
that the reset value of SR_XS_PCS_CTRL2:PCS_TYPE_SEL (0) is valid and
this suppresses the modal switching based on bit 13 of SR_PMA_CTRL1
or SR_XS_PCS_CTRL1.

A fix for this behaviour is implemented by txgbe_xpcs_switch_mode()
as part of the quirks for WangXun devices.

Rather than introduce another quirk for TC9564 let's attempt so solve
this generically by setting SR_XS_PCS_CTRL2:PCS_TYPE_SEL to a reserved
value when we detect the right combination of phy interface and XPCS
feature support.

The generic strategy adopted requires the default value of PCS_TYPE_SEL
to be 0 on devices that support 10Gbase-R. Based on TC9564 documentation
and the logic already implemented for WangXun I believe this is likely
to be the case for currently supported XPCS devices. Sadly I don't have
access to generic XPCS docs to confirm. However I think the benefits
of avoiding a cargo culted quirk outweighs the risk of regression.

Signed-off-by: Daniel Thompson <daniel@riscstar.com>
Co-developed-by: Alex Elder <elder@riscstar.com>
Signed-off-by: Alex Elder <elder@riscstar.com>
---
 drivers/net/pcs/pcs-xpcs.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
index 49abf5ea6f5cc..5316cf9c31f19 100644
--- a/drivers/net/pcs/pcs-xpcs.c
+++ b/drivers/net/pcs/pcs-xpcs.c
@@ -705,10 +705,47 @@ static void xpcs_get_interfaces(struct dw_xpcs *xpcs, unsigned long *interfaces)
 static int xpcs_switch_interface_mode(struct dw_xpcs *xpcs,
 				      phy_interface_t interface)
 {
+	int mdio_stat2, ret;
+
 	/* Wangxun provides a full alternative implementation to handle quirks */
 	if (xpcs->info.pma == WX_TXGBE_XPCS_PMA_10G_ID)
 		return txgbe_xpcs_switch_mode(xpcs, interface);
 
+	mdio_stat2 = xpcs_read(xpcs, MDIO_MMD_PCS, MDIO_STAT2);
+	if (mdio_stat2 < 0)
+		return mdio_stat2;
+
+	/*
+	 * If this XPCS supports 10Gbase-R then that will be the default
+	 * operating mode. There are several interface modes where this default
+	 * is unhelpful. Change the operating mode for interfaces were we know
+	 * the default is wrong, and restore the default otherwise.
+	 */
+	if (mdio_stat2 & MDIO_PCS_STAT2_10GBR) {
+		switch (interface) {
+		case PHY_INTERFACE_MODE_SGMII:
+		case PHY_INTERFACE_MODE_1000BASEX:
+		case PHY_INTERFACE_MODE_2500BASEX:
+			/*
+			 * Why are we writing MDIO_PCS_CTRL2_TYPE + 1? We want
+			 * the modal behaviour that comes when we pick a
+			 * reserved value. XPCS allocates extra bits to this
+			 * field and allocates values from 15 down so
+			 * MDIO_PCS_CTRL2_TYPE + 1 is the value likely to be
+			 * allocated last (and hopefully never).
+			 */
+			ret = xpcs_write(xpcs, MDIO_MMD_PCS, MDIO_CTRL2,
+					 MDIO_PCS_CTRL2_TYPE + 1);
+			break;
+		default:
+			ret = xpcs_write(xpcs, MDIO_MMD_PCS, MDIO_CTRL2,
+					 MDIO_PCS_CTRL2_10GBR);
+			break;
+		}
+		if (ret < 0)
+			return ret;
+	}
+
 	xpcs->interface = interface;
 
 	return 0;
-- 
2.53.0