[PATCH net-next] net: dsa: microchip: ksz9477: follow errata sheet when applying fixups

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

The errata sheets for both ksz9477 and ksz9567 begin with

  IMPORTANT NOTE

  Multiple errata workarounds in this document call for changing PHY
  registers for each PHY port. PHY registers 0x0 to 0x1F are in the
  address range 0xN100 to 0xN13F, while indirect (MMD) PHY registers
  are accessed via the PHY MMD Setup Register and the PHY MMD Data
  Register.

  Before configuring the PHY MMD registers, it is necessary to set the
  PHY to 100 Mbps speed with auto-negotiation disabled by writing to
  register 0xN100-0xN101. After writing the MMD registers, and after
  all errata workarounds that involve PHY register settings, write
  register 0xN100-0xN101 again to enable and restart auto-negotiation.

Without that explicit auto-neg restart, we do sometimes have problems
establishing link.

Rather than writing back the hardcoded 0x1340 value the errata sheet
suggests (which likely just corresponds to the most common strap
configuration), restore the original value, setting the
PORT_AUTO_NEG_RESTART bit if PORT_AUTO_NEG_ENABLE is set.

Fixes: 1fc33199185d ("net: dsa: microchip: Add PHY errata workarounds")
Cc: stable@vger.kernel.org
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
While I do believe this is a fix, I don't think it's post-rc7
material, hence targeting net-next with cc stable.

 drivers/net/dsa/microchip/ksz9477.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/net/dsa/microchip/ksz9477.c b/drivers/net/dsa/microchip/ksz9477.c
index bf13d47c26cf..9a712ea71ee7 100644
--- a/drivers/net/dsa/microchip/ksz9477.c
+++ b/drivers/net/dsa/microchip/ksz9477.c
@@ -902,6 +902,16 @@ static void ksz9477_port_mmd_write(struct ksz_device *dev, int port,
 
 static void ksz9477_phy_errata_setup(struct ksz_device *dev, int port)
 {
+	u16 cr;
+
+	/* Errata document says the PHY must be configured to 100Mbps
+	 * with auto-neg disabled before configuring the PHY MMD
+	 * registers.
+	 */
+	ksz_pread16(dev, port, REG_PORT_PHY_CTRL, &cr);
+	ksz_pwrite16(dev, port, REG_PORT_PHY_CTRL,
+		     PORT_SPEED_100MBIT | PORT_FULL_DUPLEX);
+
 	/* Apply PHY settings to address errata listed in
 	 * KSZ9477, KSZ9897, KSZ9896, KSZ9567, KSZ8565
 	 * Silicon Errata and Data Sheet Clarification documents:
@@ -943,6 +953,13 @@ static void ksz9477_phy_errata_setup(struct ksz_device *dev, int port)
 	ksz9477_port_mmd_write(dev, port, 0x1c, 0x1d, 0xe7ff);
 	ksz9477_port_mmd_write(dev, port, 0x1c, 0x1e, 0xefff);
 	ksz9477_port_mmd_write(dev, port, 0x1c, 0x20, 0xeeee);
+
+	/* Restore PHY CTRL register, restart auto-negotiation if
+	 * enabled in the original value.
+	 */
+	if (cr & PORT_AUTO_NEG_ENABLE)
+		cr |= PORT_AUTO_NEG_RESTART;
+	ksz_pwrite16(dev, port, REG_PORT_PHY_CTRL, cr);
 }
 
 void ksz9477_get_caps(struct ksz_device *dev, int port,
-- 
2.37.2