[PATCH RFC v2] regmap: maple: Provide lockdep (sub)class for maple tree's internal lock

[Cristian Ciocaltea]

In some cases when using the maple tree register cache, the lockdep
validator might complain about invalid deadlocks:

[7.131886]  Possible interrupt unsafe locking scenario:

[7.131890]        CPU0                    CPU1
[7.131893]        ----                    ----
[7.131896]   lock(&mt->ma_lock);
[7.131904]                                local_irq_disable();
[7.131907]                                lock(rockchip_drm_vop2:3114:(&vop2_regmap_config)->lock);
[7.131916]                                lock(&mt->ma_lock);
[7.131925]   <Interrupt>
[7.131928]     lock(rockchip_drm_vop2:3114:(&vop2_regmap_config)->lock);
[7.131936]
                *** DEADLOCK ***

[7.131939] no locks held by swapper/0/0.
[7.131944]
               the shortest dependencies between 2nd lock and 1st lock:
[7.131950]  -> (&mt->ma_lock){+.+.}-{2:2} {
[7.131966]     HARDIRQ-ON-W at:
[7.131973]                       lock_acquire+0x200/0x330
[7.131986]                       _raw_spin_lock+0x50/0x70
[7.131998]                       regcache_maple_write+0x68/0xe0
[7.132010]                       regcache_write+0x6c/0x90
[7.132019]                       _regmap_read+0x19c/0x1d0
[7.132029]                       _regmap_update_bits+0xc0/0x148
[7.132038]                       regmap_update_bits_base+0x6c/0xa8
[7.132048]                       rk8xx_probe+0x22c/0x3d8
[7.132057]                       rk8xx_spi_probe+0x74/0x88
[7.132065]                       spi_probe+0xa8/0xe0

[...]

[7.132675]   }
[7.132678]   ... key      at: [<ffff800082943c20>] __key.0+0x0/0x10
[7.132691]   ... acquired at:
[7.132695]    _raw_spin_lock+0x50/0x70
[7.132704]    regcache_maple_write+0x68/0xe0
[7.132714]    regcache_write+0x6c/0x90
[7.132724]    _regmap_read+0x19c/0x1d0
[7.132732]    _regmap_update_bits+0xc0/0x148
[7.132741]    regmap_field_update_bits_base+0x74/0xb8
[7.132751]    vop2_plane_atomic_update+0x480/0x14d8 [rockchipdrm]
[7.132820]    drm_atomic_helper_commit_planes+0x1a0/0x320 [drm_kms_helper]

[...]

[7.135112] -> (rockchip_drm_vop2:3114:(&vop2_regmap_config)->lock){-...}-{2:2} {
[7.135130]    IN-HARDIRQ-W at:
[7.135136]                     lock_acquire+0x200/0x330
[7.135147]                     _raw_spin_lock_irqsave+0x6c/0x98
[7.135157]                     regmap_lock_spinlock+0x20/0x40
[7.135166]                     regmap_read+0x44/0x90
[7.135175]                     vop2_isr+0x90/0x290 [rockchipdrm]
[7.135225]                     __handle_irq_event_percpu+0x124/0x2d0

In the example above, the validator seems to get the scope of
dependencies wrong, since the regmap instance used in rk8xx-spi driver
has nothing to do with the instance from vop2.

Improve validation by sharing the regmap's lockdep class with the maple
tree's internal lock, while also providing a subclass for the latter.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>
---
Changes in v2:
- Rebased onto next-20241031 to fix the conflicts reported by Mark
- Link to v1: https://lore.kernel.org/r/20241029-regmap-maple-lockdep-fix-v1-1-7589e57390bd@xxxxxxxxxxxxx
---
 drivers/base/regmap/internal.h       | 1 +
 drivers/base/regmap/regcache-maple.c | 3 +++
 drivers/base/regmap/regmap.c         | 1 +
 3 files changed, 5 insertions(+)

diff --git a/drivers/base/regmap/internal.h b/drivers/base/regmap/internal.h
index 83acccdc1008976de681396a300b78b701bbfa3c..bdb450436cbc53031c3d921f9f6067d26eca2549 100644
--- a/drivers/base/regmap/internal.h
+++ b/drivers/base/regmap/internal.h
@@ -59,6 +59,7 @@ struct regmap {
 			unsigned long raw_spinlock_flags;
 		};
 	};
+	struct lock_class_key *lock_key;
 	regmap_lock lock;
 	regmap_unlock unlock;
 	void *lock_arg; /* This is passed to lock/unlock functions */
diff --git a/drivers/base/regmap/regcache-maple.c b/drivers/base/regmap/regcache-maple.c
index 8d27d3653ea3e71e7b134cd3e47be385860e7375..23da7b31d7153450e1b16528be47216d7fbdd872 100644
--- a/drivers/base/regmap/regcache-maple.c
+++ b/drivers/base/regmap/regcache-maple.c
@@ -355,6 +355,9 @@ static int regcache_maple_init(struct regmap *map)
 
 	mt_init(mt);
 
+	if (!mt_external_lock(mt) && map->lock_key)
+		lockdep_set_class_and_subclass(&mt->ma_lock, map->lock_key, 1);
+
 	if (!map->num_reg_defaults)
 		return 0;
 
diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
index 4ded93687c1f0ab3584947c61398cd6333b2e72c..53131a7ede0a6aad54bc85e970124f6b166a8010 100644
--- a/drivers/base/regmap/regmap.c
+++ b/drivers/base/regmap/regmap.c
@@ -745,6 +745,7 @@ struct regmap *__regmap_init(struct device *dev,
 						   lock_key, lock_name);
 		}
 		map->lock_arg = map;
+		map->lock_key = lock_key;
 	}
 
 	/*

---
base-commit: f9f24ca362a4d84dd8aeb4b8f3ec28cb6c43dd06
change-id: 20241029-regmap-maple-lockdep-fix-f368ce6e7363