[PATCH] phy: use per-PHY lockdep keys

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

If the PHY driver uses another PHY internally (e.g. in case of eUSB2,
repeaters are represented as PHYs), then it would trigger the following
lockdep splat because all PHYs use a single static lockdep key and thus
lockdep can not identify whether there is a dependency or not and
reports a false positive.

Make PHY subsystem use dynamic lockdep keys, assigning each driver a
separate key. This way lockdep can correctly identify dependency graph
between mutexes.

 ============================================
 WARNING: possible recursive locking detected
 6.15.0-rc7-next-20250522-12896-g3932f283970c #3455 Not tainted
 --------------------------------------------
 kworker/u51:0/78 is trying to acquire lock:
 ffff0008116554f0 (&phy->mutex){+.+.}-{4:4}, at: phy_init+0x4c/0x12c

 but task is already holding lock:
 ffff000813c10cf0 (&phy->mutex){+.+.}-{4:4}, at: phy_init+0x4c/0x12c

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&phy->mutex);
   lock(&phy->mutex);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 4 locks held by kworker/u51:0/78:
  #0: ffff000800010948 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x18c/0x5ec
  #1: ffff80008036bdb0 (deferred_probe_work){+.+.}-{0:0}, at: process_one_work+0x1b4/0x5ec
  #2: ffff0008094ac8f8 (&dev->mutex){....}-{4:4}, at: __device_attach+0x38/0x188
  #3: ffff000813c10cf0 (&phy->mutex){+.+.}-{4:4}, at: phy_init+0x4c/0x12c

 stack backtrace:
 CPU: 0 UID: 0 PID: 78 Comm: kworker/u51:0 Not tainted 6.15.0-rc7-next-20250522-12896-g3932f283970c #3455 PREEMPT
 Hardware name: Qualcomm CRD, BIOS 6.0.240904.BOOT.MXF.2.4-00528.1-HAMOA-1 09/ 4/2024
 Workqueue: events_unbound deferred_probe_work_func
 Call trace:
  show_stack+0x18/0x24 (C)
  dump_stack_lvl+0x90/0xd0
  dump_stack+0x18/0x24
  print_deadlock_bug+0x258/0x348
  __lock_acquire+0x10fc/0x1f84
  lock_acquire+0x1c8/0x338
  __mutex_lock+0xb8/0x59c
  mutex_lock_nested+0x24/0x30
  phy_init+0x4c/0x12c
  snps_eusb2_hsphy_init+0x54/0x1a0
  phy_init+0xe0/0x12c
  dwc3_core_init+0x450/0x10b4
  dwc3_core_probe+0xce4/0x15fc
  dwc3_probe+0x64/0xb0
  platform_probe+0x68/0xc4
  really_probe+0xbc/0x298
  __driver_probe_device+0x78/0x12c
  driver_probe_device+0x3c/0x160
  __device_attach_driver+0xb8/0x138
  bus_for_each_drv+0x84/0xe0
  __device_attach+0x9c/0x188
  device_initial_probe+0x14/0x20
  bus_probe_device+0xac/0xb0
  deferred_probe_work_func+0x8c/0xc8
  process_one_work+0x208/0x5ec
  worker_thread+0x1c0/0x368
  kthread+0x14c/0x20c
  ret_from_fork+0x10/0x20

Fixes: 3584f6392f09 ("phy: qcom: phy-qcom-snps-eusb2: Add support for eUSB2 repeater")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
---
Note: I've used a Fixes tag pointing to the commit which actually
started using nested PHYs. If you think that it's incorrect, I'm fine
with dropping it.

Note2: I've tried using mutex_lock_nested, however that didn't play
well. We can not store nest level in the struct phy (as it can be used
by different drivers), so using mutex_lock_nested() would require us to
change and wrap all PHY APIs which take a lock internally. Using dynamic
lockdep keys looks like a more ellegant solution, especially granted
that there is no extra impact if lockdep is disabled.
---
 drivers/phy/phy-core.c  | 5 ++++-
 include/linux/phy/phy.h | 2 ++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/phy/phy-core.c b/drivers/phy/phy-core.c
index 8e2daea81666bf8a76d9c936c1a16d6318105c91..04a5a34e7a950ae94fae915673c25d476fc071c1 100644
--- a/drivers/phy/phy-core.c
+++ b/drivers/phy/phy-core.c
@@ -994,7 +994,8 @@ struct phy *phy_create(struct device *dev, struct device_node *node,
 	}
 
 	device_initialize(&phy->dev);
-	mutex_init(&phy->mutex);
+	lockdep_register_key(&phy->lockdep_key);
+	mutex_init_with_key(&phy->mutex, &phy->lockdep_key);
 
 	phy->dev.class = &phy_class;
 	phy->dev.parent = dev;
@@ -1259,6 +1260,8 @@ static void phy_release(struct device *dev)
 	dev_vdbg(dev, "releasing '%s'\n", dev_name(dev));
 	debugfs_remove_recursive(phy->debugfs);
 	regulator_put(phy->pwr);
+	mutex_destroy(&phy->mutex);
+	lockdep_unregister_key(&phy->lockdep_key);
 	ida_free(&phy_ida, phy->id);
 	kfree(phy);
 }
diff --git a/include/linux/phy/phy.h b/include/linux/phy/phy.h
index 437769e061b7030105c9ea4e9b0da9d32b6fa158..13add0c2c40721fe9ca3f0350d13c035cd25af45 100644
--- a/include/linux/phy/phy.h
+++ b/include/linux/phy/phy.h
@@ -154,6 +154,7 @@ struct phy_attrs {
  * @id: id of the phy device
  * @ops: function pointers for performing phy operations
  * @mutex: mutex to protect phy_ops
+ * @lockdep_key: lockdep information for this mutex
  * @init_count: used to protect when the PHY is used by multiple consumers
  * @power_count: used to protect when the PHY is used by multiple consumers
  * @attrs: used to specify PHY specific attributes
@@ -165,6 +166,7 @@ struct phy {
 	int			id;
 	const struct phy_ops	*ops;
 	struct mutex		mutex;
+	struct lock_class_key	lockdep_key;
 	int			init_count;
 	int			power_count;
 	struct phy_attrs	attrs;

---
base-commit: 460178e842c7a1e48a06df684c66eb5fd630bcf7
change-id: 20250528-phy-subinit-42c988a12b6c

Best regards,
-- 
Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>


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