cxl/acpi: Add root device lockdep validation

The CXL "root" device, ACPI0017, is an attach point for coordinating
platform level CXL resources and is the parent device for a CXL port
topology tree. As such it has distinct locking rules relative to other
CXL subsystem objects, but because it is an ACPI device the lock class
is established well before it is given to the cxl_acpi driver.

However, the lockdep API does support changing the lock class "live" for
situations like this. Add a device_lock_set_class() helper that a driver
can use in ->probe() to set a custom lock class, and
device_lock_reset_class() to return to the default "no validate" class
before the custom lock class key goes out of scope after ->remove().

Note the helpers are all macros to support dead code elimination in the
CONFIG_PROVE_LOCKING=n case, however device_set_lock_class() still needs
#ifdef CONFIG_PROVE_LOCKING since lockdep_match_class() explicitly does
not have a helper in the CONFIG_PROVE_LOCKING=n case (see comment in
lockdep.h). The lockdep API needs 2 small tweaks to prevent "unused"
warnings for the @key argument to lock_set_class(), and a new
lock_set_novalidate_class() is added to supplement
lockdep_set_novalidate_class() in the cases where the lock class is
converted while the lock is held.

Suggested-by: Peter Zijlstra <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Alison Schofield <[e-mail]>
Cc: Vishal Verma <[e-mail]>
Cc: Ben Widawsky <[e-mail]>
Cc: Jonathan Cameron <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Ira Weiny <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/cxl/acpi.c

···
 	return 1;
 }
 
+static struct lock_class_key cxl_root_key;
+
+static void cxl_acpi_lock_reset_class(void *dev)
+{
+	device_lock_reset_class(dev);
+}
+
 static int cxl_acpi_probe(struct platform_device *pdev)
 {
 	int rc;
···
 	struct device *host = &pdev->dev;
 	struct acpi_device *adev = ACPI_COMPANION(host);
 	struct cxl_cfmws_context ctx;
+
+	device_lock_set_class(&pdev->dev, &cxl_root_key);
+	rc = devm_add_action_or_reset(&pdev->dev, cxl_acpi_lock_reset_class,
+				      &pdev->dev);
+	if (rc)
+		return rc;
 
 	root_port = devm_cxl_add_port(host, host, CXL_RESOURCE_NONE, NULL);
 	if (IS_ERR(root_port))

include/linux/device.h

···
 	return dev->bus && dev->bus->offline && dev->bus->online;
 }
 
+#define __device_lock_set_class(dev, name, key)                        \
+do {                                                                   \
+	struct device *__d2 __maybe_unused = dev;                      \
+	lock_set_class(&__d2->mutex.dep_map, name, key, 0, _THIS_IP_); \
+} while (0)
+
+/**
+ * device_lock_set_class - Specify a temporary lock class while a device
+ *			   is attached to a driver
+ * @dev: device to modify
+ * @key: lock class key data
+ *
+ * This must be called with the device_lock() already held, for example
+ * from driver ->probe(). Take care to only override the default
+ * lockdep_no_validate class.
+ */
+#ifdef CONFIG_LOCKDEP
+#define device_lock_set_class(dev, key)                                    \
+do {                                                                       \
+	struct device *__d = dev;                                          \
+	dev_WARN_ONCE(__d, !lockdep_match_class(&__d->mutex,               \
+						&__lockdep_no_validate__), \
+		 "overriding existing custom lock class\n");               \
+	__device_lock_set_class(__d, #key, key);                           \
+} while (0)
+#else
+#define device_lock_set_class(dev, key) __device_lock_set_class(dev, #key, key)
+#endif
+
+/**
+ * device_lock_reset_class - Return a device to the default lockdep novalidate state
+ * @dev: device to modify
+ *
+ * This must be called with the device_lock() already held, for example
+ * from driver ->remove().
+ */
+#define device_lock_reset_class(dev) \
+do { \
+	struct device *__d __maybe_unused = dev;                       \
+	lock_set_novalidate_class(&__d->mutex.dep_map, "&dev->mutex",  \
+				  _THIS_IP_);                          \
+} while (0)
+
 void lock_device_hotplug(void);
 void unlock_device_hotplug(void);
 int lock_device_hotplug_sysfs(void);

include/linux/lockdep.h

···
 			   struct lock_class_key *key, unsigned int subclass,
 			   unsigned long ip);
 
+#define lock_set_novalidate_class(l, n, i) \
+	lock_set_class(l, n, &__lockdep_no_validate__, 0, i)
+
 static inline void lock_set_subclass(struct lockdep_map *lock,
 		unsigned int subclass, unsigned long ip)
 {
···
 # define lock_acquire(l, s, t, r, c, n, i)	do { } while (0)
 # define lock_release(l, i)			do { } while (0)
 # define lock_downgrade(l, i)			do { } while (0)
-# define lock_set_class(l, n, k, s, i)		do { } while (0)
+# define lock_set_class(l, n, key, s, i)	do { (void)(key); } while (0)
+# define lock_set_novalidate_class(l, n, i)	do { } while (0)
 # define lock_set_subclass(l, s, i)		do { } while (0)
 # define lockdep_init()				do { } while (0)
 # define lockdep_init_map_type(lock, name, key, sub, inner, outer, type) \