[PATCH net-next v3] net: dsa: mv88e6xxx: control mdio bus-id truncation for long paths

[Josua Mayer]

mv88e6xxx supports multiple mdio buses as children, e.g. to model both
internal and external phys. If the child buses mdio ids are truncated,
they might collide with each other leading to an obscure error from
kobject_add.

The maximum length of bus id is currently defined as 61
(MII_BUS_ID_SIZE). Truncation can occur on platforms with long node
names and multiple levels before the parent bus on which the dsa switch
itself sits, e.g. CN9130 [1].

Compare the return value of snprintf against maximum bus-id length to
detect truncation. In that case write an incrementing marker to the end
to avoid name collisions.
This changes the problematic bus-ids mdio and mdio-external from [1]
to [2].

Truncation at the beginning was considered as a workaround, however that
is still subject to name collisions in sysfs where only the first
characters differ.

[1]
[    8.324631] mv88e6085 f212a200.mdio-mii:04: switch 0x1760 detected: Marvell 88E6176, revision 1
[    8.389516] mv88e6085 f212a200.mdio-mii:04: Truncated bus-id may collide.
[    8.592367] mv88e6085 f212a200.mdio-mii:04: Truncated bus-id may collide.
[    8.623593] sysfs: cannot create duplicate filename '/devices/platform/cp0/cp0:config-space@f2000000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi'
[    8.785480] kobject: kobject_add_internal failed for !cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi with -EEXIST, don't try to register things with the same name in the same directory.
[    8.936514] libphy: mii_bus /cp0/config-space@f2000000/mdio@12a200/ethernet-switch@4/mdi failed to register
[    8.946300] mdio_bus !cp0!config-space@f2000000!mdio@12a200!ethernet-switch@4!mdi: __mdiobus_register: -22
[    8.956003] mv88e6085 f212a200.mdio-mii:04: Cannot register MDIO bus (-22)
[    8.965329] mv88e6085: probe of f212a200.mdio-mii:04 failed with error -22

[2]
/devices/platform/cp0/cp0:config-space@f2000000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@f2000000!mdio@12a200!ethernet-switch...!-0
/devices/platform/cp0/cp0:config-space@f2000000/f212a200.mdio/mdio_bus/f212a200.mdio-mii/f212a200.mdio-mii:04/mdio_bus/!cp0!config-space@f2000000!mdio@12a200!ethernet-switch...!-1

Signed-off-by: Josua Mayer <josua@xxxxxxxxxxxxx>
---


Bcc: Mor Nagli <mor.nagli@xxxxxxxxxxxxx>
---
Changes in v3:
- Added better named variable "len" for snprintf return value to improve
  readability
  (Reported-by: Andrew Lunn <andrew@xxxxxxx>)
- Added short explanation why there are two calls of snprintf in comment
- Link to v2: https://lore.kernel.org/r/20240404-mv88e6xxx-truncate-busid-v2-1-69683f67008b@xxxxxxxxxxxxx

Changes in v2:
- fixed typo in commit message
  (Reportby: Jiri Pirko <jiri@xxxxxxxxxxx>)
- replaced warning message with controlled truncation
- Link to v1: https://lore.kernel.org/r/20240320-mv88e6xxx-truncate-busid-v1-1-cface50b2efb@xxxxxxxxxxxxx
---
 drivers/net/dsa/mv88e6xxx/chip.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 964c7b847fd3..9c996be00362 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -3774,10 +3774,10 @@ static int mv88e6xxx_mdio_register(struct mv88e6xxx_chip *chip,
 				   struct device_node *np,
 				   bool external)
 {
-	static int index;
+	static int index, trunc;
 	struct mv88e6xxx_mdio_bus *mdio_bus;
 	struct mii_bus *bus;
-	int err;
+	int err, len;
 
 	if (external) {
 		mv88e6xxx_reg_lock(chip);
@@ -3803,10 +3803,28 @@ static int mv88e6xxx_mdio_register(struct mv88e6xxx_chip *chip,
 
 	if (np) {
 		bus->name = np->full_name;
-		snprintf(bus->id, MII_BUS_ID_SIZE, "%pOF", np);
+		len = snprintf(bus->id, MII_BUS_ID_SIZE, "%pOF", np);
 	} else {
 		bus->name = "mv88e6xxx SMI";
-		snprintf(bus->id, MII_BUS_ID_SIZE, "mv88e6xxx-%d", index++);
+		len = snprintf(bus->id, MII_BUS_ID_SIZE, "mv88e6xxx-%d", index++);
+	}
+	if (len < 0) {
+		return len;
+	} else if (len >= MII_BUS_ID_SIZE) {
+		/* If generated bus id is truncated, names in sysfs
+		 * may collide.
+		 * Generate a special numeric suffix. If it fits
+		 * within MII_BUS_ID_SIZE, insert at the end to mark
+		 * truncation and avoid name collisions.
+		 */
+		len = snprintf(NULL, 0, "...!-%d", trunc);
+		if (len < 0)
+			return err;
+		else if (len >= MII_BUS_ID_SIZE)
+			return -ENOBUFS;
+
+		snprintf(bus->id + MII_BUS_ID_SIZE - err - 1,
+			 MII_BUS_ID_SIZE - err, "...!-%d", trunc++);
 	}
 
 	bus->read = mv88e6xxx_mdio_read;

---
base-commit: 173e7622ccb3f46834bd4176ed363f435e142942
change-id: 20240320-mv88e6xxx-truncate-busid-34a1d2769bbf

Sincerely,
-- 
Josua Mayer <josua@xxxxxxxxxxxxx>