[PATCH net-next v4 3/3] net: phy: add a PHY write barrier when disabling interrupts

[Charles Perry <charles.perry@microchip.com>]

MDIO bus controllers are not required to wait for write transactions to
complete before returning as synchronization is often achieved by polling
status bits.

This can cause issues when disabling interrupts since an interrupt could
fire before the interrupt handler is unregistered and there's no status
bit to poll.

Add a phy_write_barrier() function and use it in phy_disable_interrupts()
to fix this issue. The write barrier just reads an MII register and
discards the value, which is enough to guarantee that previous writes have
completed.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
---

Notes:
    Changes in v4:
      - Add this patch (Russell, Andrew)

 drivers/net/phy/phy.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
index 13dd1691886d..2be0b90e9947 100644
--- a/drivers/net/phy/phy.c
+++ b/drivers/net/phy/phy.c
@@ -1368,14 +1368,39 @@ void phy_error(struct phy_device *phydev)
 }
 EXPORT_SYMBOL(phy_error);
 
+/**
+ * phy_write_barrier - ensure the last write completed for this PHY device
+ * @phydev: target phy_device struct
+ *
+ * MDIO bus controllers are not required to wait for write transactions to
+ * complete before returning. Calling this function ensures that the previous
+ * write has completed.
+ */
+static int phy_write_barrier(struct phy_device *phydev)
+{
+	int err;
+
+	err = phy_read(phydev, MII_PHYSID1);
+	if (err < 0)
+		return err;
+
+	return 0;
+}
+
 /**
  * phy_disable_interrupts - Disable the PHY interrupts from the PHY side
  * @phydev: target phy_device struct
  */
 int phy_disable_interrupts(struct phy_device *phydev)
 {
+	int err;
+
 	/* Disable PHY interrupts */
-	return phy_config_interrupt(phydev, PHY_INTERRUPT_DISABLED);
+	err = phy_config_interrupt(phydev, PHY_INTERRUPT_DISABLED);
+	if (err)
+		return err;
+
+	return phy_write_barrier(phydev);
 }
 
 /**
-- 
2.47.3