Revert "net: phy: Uniform PHY driver access"

This reverts commit 3ac8eed, which did
more than it said on the box, and not only it replaced to_phy_driver
with phydev->drv, but it also removed the "!drv" check, without actually
explaining why that is fine.

That patch in fact breaks suspend/resume on any system which has PHY
devices with no drivers bound.

The stack trace is:

Unable to handle kernel NULL pointer dereference at virtual address 00000000000000e8
pc : mdio_bus_phy_suspend+0xd8/0xec
lr : dpm_run_callback+0x38/0x90
Call trace:
 mdio_bus_phy_suspend+0xd8/0xec
 dpm_run_callback+0x38/0x90
 __device_suspend+0x108/0x3cc
 dpm_suspend+0x140/0x210
 dpm_suspend_start+0x7c/0xa0
 suspend_devices_and_enter+0x13c/0x540
 pm_suspend+0x2a4/0x330

Examples why that assumption is not fine:

- There is an MDIO bus with a PHY device that doesn't have a specific
  PHY driver loaded, because mdiobus_register() automatically creates a
  PHY device for it but there is no specific PHY driver in the system.
  Normally under those circumstances, the generic PHY driver will be
  bound lazily to it (at phy_attach_direct time). But some Ethernet
  drivers attach to their PHY at .ndo_open time. Until then it, the
  to-be-driven-by-genphy PHY device will not have a driver. The blamed
  patch amounts to saying "you need to open all net devices before the
  system can suspend, to avoid the NULL pointer dereference".

- There is any raw MDIO device which has 'plausible' values in the PHY
  ID registers 2 and 3, which is located on an MDIO bus whose driver
  does not set bus->phy_mask = ~0 (which prevents auto-scanning of PHY
  devices). An example could be a MAC's internal MDIO bus with PCS
  devices on it, for serial links such as SGMII. PHY devices will get
  created for those PCSes too, due to that MDIO bus auto-scanning, and
  although those PHY devices are not used, they do not bother anybody
  either. PCS devices are usually managed in Linux as raw MDIO devices.
  Nonetheless, they do not have a PHY driver, nor does anybody attempt
  to connect to them (because they are not a PHY), and therefore this
  patch breaks that.

The goal itself of the patch is questionable, so I am going for a
straight revert. to_phy_driver does not seem to have a need to be
replaced by phydev->drv, in fact that might even trigger code paths
which were not given too deep of a thought.

For instance:

phy_probe populates phydev->drv at the beginning, but does not clean it
up on any error (including EPROBE_DEFER). So if the phydev driver
requests probe deferral, phydev->drv will remain populated despite there
being no driver bound.

If a system suspend starts in between the initial probe deferral request
and the subsequent probe retry, we will be calling the phydev->drv->suspend
method, but _before_ any phydev->drv->probe call has succeeded.

That is to say, if the phydev->drv is allocating any driver-private data
structure in ->probe, it pretty much expects that data structure to be
available in ->suspend. But it may not. That is a pretty insane
environment to present to PHY drivers.

In the code structure before the blamed patch, mdio_bus_phy_may_suspend
would just say "no, don't suspend" to any PHY device which does not have
a driver pointer _in_the_device_structure_ (not the phydev->drv). That
would essentially ensure that ->suspend will never get called for a
device that has not yet successfully completed probe. This is the code
structure the patch is returning to, via the revert.

Fixes: 3ac8eed ("net: phy: Uniform PHY driver access")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210914140515.2311548-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phy_device.c

@@ -233,9 +233,11 @@ static DEFINE_MUTEX(phy_fixup_lock);
 
 static bool mdio_bus_phy_may_suspend(struct phy_device *phydev)
 {
+	struct device_driver *drv = phydev->mdio.dev.driver;
+	struct phy_driver *phydrv = to_phy_driver(drv);
 	struct net_device *netdev = phydev->attached_dev;
 
-	if (!phydev->drv->suspend)
+	if (!drv || !phydrv->suspend)
 		return false;
 
 	/* PHY not attached? May suspend if the PHY has not already been