[PATCH net v12 3/4] net: phy: take the interrupt back from the bus on detach

[Aleksei Sviridkin <f@lex.la>]

When a PHY's driver is a module that is not loaded yet when the MAC
connects, the PHY gets the generic driver first. phy_probe() then
replaces phydev->irq with PHY_POLL, because genphy has no interrupt
support. Nothing puts the number back, so after the real driver binds
the PHY is polled for the rest of the uptime.

Seen on a Keenetic KN-1012: the Airoha EN8811H behind an MT7531 port
has its driver on the root filesystem. Its devicetree interrupt maps to
irq 15, and after the real driver binds phydev->irq reads -1.

Restore the number when the PHY detaches. It comes from the bus table,
mdiobus->irq[], where the PHY got it at creation. Do it only when
phy_attach_direct() bound the generic driver, since that is the
substitution being undone; otherwise the field belongs to whoever wrote
it. Do it before device_release_driver(), because after the release a
probing driver can write the same field.

The store is ordered before the release rather than locked against it:
device_release_driver() takes the device lock itself. A MAC that sets
phydev->irq before phy_start(), as phy.rst describes, is not affected,
since the restore runs on detach, between connections.

Tested on the KN-1012 with a 6.18 distribution kernel: phydev->irq
reads 15 after the real driver binds, and -1 without this patch.

Fixes: 00db8189d984 ("This patch adds a PHY Abstraction Layer to the Linux Kernel, enabling ethernet drivers to remain as ignorant as is reasonable of the connected PHY's design and operation details.")
Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>
---

Notes:
    v12: shorter commit message, no code change.
    
    The PHY module was made to load after the root filesystem, and the
    distribution's own late-PHY patch was removed. The value was read with a
    debug-only module parameter; the three switch ports read 79, 80 and 81
    in both runs.

 drivers/net/phy/phy_device.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 94b2e85e00a3..a9c71a286118 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c
@@ -1965,6 +1965,8 @@ void phy_detach(struct phy_device *phydev)
 	 * real driver could be loaded
 	 */
 	if (phydev->is_genphy_driven) {
+		/* The release below lets phy_probe() write this field. */
+		phydev->irq = phydev->mdio.bus->irq[phydev->mdio.addr];
 		device_release_driver(&phydev->mdio.dev);
 		phydev->is_genphy_driven = 0;
 	}
-- 
2.53.0