net: dsa: mv88e6xxx: avoid reg_lock deadlock in mv88e6xxx_setup_port()

In the blamed commit, it was not noticed that one implementation of
chip->info->ops->phylink_get_caps(), called by mv88e6xxx_get_caps(),
may access hardware registers, and in doing so, it takes the
mv88e6xxx_reg_lock(). Namely, this is mv88e6352_phylink_get_caps().

This is a problem because mv88e6xxx_get_caps(), apart from being
a top-level function (method invoked by dsa_switch_ops), is now also
directly called from mv88e6xxx_setup_port(), which runs under the
mv88e6xxx_reg_lock() taken by mv88e6xxx_setup(). Therefore, when running
on mv88e6352, the reg_lock would be acquired a second time and the
system would deadlock on driver probe.

The things that mv88e6xxx_setup() can compete with in terms of register
access with are the IRQ handlers and MDIO bus operations registered by
mv88e6xxx_probe(). So there is a real need to acquire the register lock.

The register lock can, in principle, be dropped and re-acquired pretty
much at will within the driver, as long as no operations that involve
waiting for indirect access to complete (essentially, callers of
mv88e6xxx_smi_direct_wait() and mv88e6xxx_wait_mask()) are interrupted
with the lock released. However, I would guess that in mv88e6xxx_setup(),
the critical section is kept open for such a long time just in order to
optimize away multiple lock/unlock operations on the registers.

We could, in principle, drop the reg_lock right before the
mv88e6xxx_setup_port() -> mv88e6xxx_get_caps() call, and
re-acquire it immediately afterwards. But this would look ugly, because
mv88e6xxx_setup_port() would release a lock which it didn't acquire, but
the caller did.

A cleaner solution to this issue comes from the observation that struct
mv88e6xxxx_ops methods generally assume they are called with the
reg_lock already acquired. Whereas mv88e6352_phylink_get_caps() is more
the exception rather than the norm, in that it acquires the lock itself.

Let's enforce the same locking pattern/convention for
chip->info->ops->phylink_get_caps() as well, and make
mv88e6xxx_get_caps(), the top-level function, acquire the register lock
explicitly, for this one implementation that will access registers for
port 4 to work properly.

This means that mv88e6xxx_setup_port() will no longer call the top-level
function, but the low-level mv88e6xxx_ops method which expects the
correct calling context (register lock held).

Compared to chip->info->ops->phylink_get_caps(), mv88e6xxx_get_caps()
also fixes up the supported_interfaces bitmap for internal ports, since
that can be done generically and does not require per-switch knowledge.
That's code which will no longer execute, however mv88e6xxx_setup_port()
doesn't need that. It just needs to look at the mac_capabilities bitmap.

Fixes: cc1049cc ("net: dsa: mv88e6xxx: fix speed setting for CPU/DSA ports")
Reported-by: Maksim Kiselev <bigunclemax@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Maksim Kiselev <bigunclemax@gmail.com>
Link: https://lore.kernel.org/r/20221214110120.3368472-1-vladimir.oltean@nxp.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -689,13 +689,12 @@ static void mv88e6352_phylink_get_caps(struct mv88e6xxx_chip *chip, int port,
 
 	/* Port 4 supports automedia if the serdes is associated with it. */
 	if (port == 4) {
-		mv88e6xxx_reg_lock(chip);
 		err = mv88e6352_g2_scratch_port_has_serdes(chip, port);
 		if (err < 0)
 			dev_err(chip->dev, "p%d: failed to read scratch\n",
 				port);
 		if (err <= 0)
-			goto unlock;
+			return;
 
 		cmode = mv88e6352_get_port4_serdes_cmode(chip);
 		if (cmode < 0)
@@ -703,8 +702,6 @@ static void mv88e6352_phylink_get_caps(struct mv88e6xxx_chip *chip, int port,
 				port);
 		else
 			mv88e6xxx_translate_cmode(cmode, supported);
-unlock:
-		mv88e6xxx_reg_unlock(chip);
 	}
 }
 
@@ -831,7 +828,9 @@ static void mv88e6xxx_get_caps(struct dsa_switch *ds, int port,
 {
 	struct mv88e6xxx_chip *chip = ds->priv;
 
+	mv88e6xxx_reg_lock(chip);
 	chip->info->ops->phylink_get_caps(chip, port, config);
+	mv88e6xxx_reg_unlock(chip);
 
 	if (mv88e6xxx_phy_is_internal(ds, port)) {
 		__set_bit(PHY_INTERFACE_MODE_INTERNAL,
@@ -3307,7 +3306,7 @@ static int mv88e6xxx_setup_port(struct mv88e6xxx_chip *chip, int port)
 		struct phylink_config pl_config = {};
 		unsigned long caps;
 
-		mv88e6xxx_get_caps(ds, port, &pl_config);
+		chip->info->ops->phylink_get_caps(chip, port, &pl_config);
 
 		caps = pl_config.mac_capabilities;