[PATCH] net: dsa: mv88e6xxx: don't dispose of Global2 IRQ mappings from mdiobus code

[Vladimir Oltean]

irq_find_mapping() does not need irq_dispose_mapping(), only
irq_create_mapping() does.

Calling irq_dispose_mapping() from mv88e6xxx_g2_irq_mdio_free() and from
the error path of mv88e6xxx_g2_irq_mdio_setup() effectively means that
the mdiobus logic (for internal PHY interrupts) is disposing of a
hwirq->virq mapping which it is not responsible of (but instead, the
function pair mv88e6xxx_g2_irq_setup() + mv88e6xxx_g2_irq_free() is).

With the current code structure, this isn't such a huge problem, because
mv88e6xxx_g2_irq_mdio_free() is called relatively close to the real
owner of the IRQ mappings:

mv88e6xxx_remove()
-> mv88e6xxx_unregister_switch()
-> mv88e6xxx_mdios_unregister()
   -> mv88e6xxx_g2_irq_mdio_free()
-> mv88e6xxx_g2_irq_free()

and the switch isn't 'live' in any way such that it would be able of
generating interrupts at this point (mv88e6xxx_unregister_switch() has
been called).

However, there is a desire to split mv88e6xxx_mdios_unregister() and
mv88e6xxx_g2_irq_free() such that mv88e6xxx_mdios_unregister() only gets
called from mv88e6xxx_teardown(). This is much more problematic, as can
be seen below.

In a cross-chip scenario (say 3 switches d0032004.mdio-mii:10,
d0032004.mdio-mii:11 and d0032004.mdio-mii:12 which form a single DSA
tree), it is possible to unbind the device driver from a single switch
(say d0032004.mdio-mii:10).

When that happens, mv88e6xxx_remove() will be called for just that one
switch, and this will call mv88e6xxx_unregister_switch() which will tear
down the entire tree (calling mv88e6xxx_teardown() for all 3 switches).

Assuming mv88e6xxx_mdios_unregister() was moved to mv88e6xxx_teardown(),
at this stage, all 3 switches will have called irq_dispose_mapping() on
their mdiobus virqs.

When we bind again the device driver to d0032004.mdio-mii:10,
mv88e6xxx_probe() is called for it, which calls dsa_register_switch().
The DSA tree is now complete again, and mv88e6xxx_setup() is called for
all 3 switches.

Also assuming that mv88e6xxx_mdios_register() is moved to
mv88e6xxx_setup() (the 2 assumptions go together), at this point,
d0032004.mdio-mii:11 and d0032004.mdio-mii:12 don't have an IRQ mapping
for the internal PHYs anymore, as they've disposed of it in
mv88e6xxx_teardown(). Whereas switch d0032004.mdio-mii:10 has re-created
it, because its code path comes from mv88e6xxx_probe().

Simply put, this change prepares the driver to handle the movement of
mv88e6xxx_mdios_register() to mv88e6xxx_setup() for cross-chip DSA trees.

Also, the code being deleted was partially wrong anyway (in a way which
may have hidden this other issue). mv88e6xxx_g2_irq_mdio_setup()
populates bus->irq[] starting with offset chip->info->phy_base_addr, but
the teardown path doesn't apply that offset too. So it disposes of virq
0 for phy = [ 0, phy_base_addr ).

All switch families have phy_base_addr = 0, except for MV88E6141 and
MV88E6341 which have it as 0x10. I guess those families would have
happened to work by mistake in cross-chip scenarios too.

I'm deleting the body of mv88e6xxx_g2_irq_mdio_free() but leaving its
call sites and prototype in place. This is because, if we ever need to
add back some teardown procedure in the future, it will be perhaps
error-prone to deduce the proper call sites again. Whereas like this,
no extra code should get generated, it shouldn't bother anybody.

Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
---
 drivers/net/dsa/mv88e6xxx/global2.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/global2.c b/drivers/net/dsa/mv88e6xxx/global2.c
index ed3b2f88e783..a26546d3d7b5 100644
--- a/drivers/net/dsa/mv88e6xxx/global2.c
+++ b/drivers/net/dsa/mv88e6xxx/global2.c
@@ -1176,31 +1176,19 @@ int mv88e6xxx_g2_irq_setup(struct mv88e6xxx_chip *chip)
 int mv88e6xxx_g2_irq_mdio_setup(struct mv88e6xxx_chip *chip,
 				struct mii_bus *bus)
 {
-	int phy, irq, err, err_phy;
+	int phy, irq;
 
 	for (phy = 0; phy < chip->info->num_internal_phys; phy++) {
 		irq = irq_find_mapping(chip->g2_irq.domain, phy);
-		if (irq < 0) {
-			err = irq;
-			goto out;
-		}
+		if (irq < 0)
+			return irq;
+
 		bus->irq[chip->info->phy_base_addr + phy] = irq;
 	}
 	return 0;
-out:
-	err_phy = phy;
-
-	for (phy = 0; phy < err_phy; phy++)
-		irq_dispose_mapping(bus->irq[phy]);
-
-	return err;
 }
 
 void mv88e6xxx_g2_irq_mdio_free(struct mv88e6xxx_chip *chip,
 				struct mii_bus *bus)
 {
-	int phy;
-
-	for (phy = 0; phy < chip->info->num_internal_phys; phy++)
-		irq_dispose_mapping(bus->irq[phy]);
 }
-- 
2.34.1