[PATCH V3 2/6] PM: runtime: Add pm_runtime_if_active guard and conditional variant

[Golla Nagendra <nagendra.golla@amd.com>]

Add a pm_runtime_if_active base guard and its _try conditional variant
to pm_runtime.h for drivers that need to conditionally acquire a runtime
PM reference only when the device is already active.

The base guard must not be used directly via guard()/scoped_guard()
because pm_runtime_get_if_active() only acquires a reference when it
returns 1; the destructor unconditionally calls pm_runtime_put(), which
would underflow usage_count on a suspended or RPM-disabled device. The
base guard exists solely to back the DEFINE_GUARD_COND _try variant.

The _try variant (used via PM_RUNTIME_ACQUIRE_IF_ACTIVE) checks the
return value and only runs the destructor when the reference was
actually acquired. This is useful in interrupt handlers where the
device may be runtime-suspended and MMIO accesses must be avoided.

Signed-off-by: Golla Nagendra <nagendra.golla@amd.com>
---
Changes in V3:
- New patch: add PM_RUNTIME_ACQUIRE_IF_ACTIVE() guard and backing
  infrastructure to pm_runtime.h so that drivers
  can use a structured guard instead of an open-coded
  pm_runtime_get_if_active()/pm_runtime_put() pair
---
 include/linux/pm_runtime.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
index 64921b10ac74..bc6de97e6111 100644
--- a/include/linux/pm_runtime.h
+++ b/include/linux/pm_runtime.h
@@ -609,6 +609,13 @@ DEFINE_GUARD(pm_runtime_active, struct device *,
 	     pm_runtime_get_sync(_T), pm_runtime_put(_T));
 DEFINE_GUARD(pm_runtime_active_auto, struct device *,
 	     pm_runtime_get_sync(_T), pm_runtime_put_autosuspend(_T));
+/*
+ * Do not use directly -- the destructor calls pm_runtime_put()
+ * unconditionally, which underflows if no reference was acquired.
+ * Use only via the _try variant below.
+ */
+DEFINE_GUARD(pm_runtime_if_active, struct device *,
+	     pm_runtime_get_if_active(_T), pm_runtime_put(_T));
 /*
  * Use the following guards with ACQUIRE()/ACQUIRE_ERR().
  *
@@ -624,6 +631,8 @@ DEFINE_GUARD_COND(pm_runtime_active_auto, _try,
 		  pm_runtime_get_active(_T, RPM_TRANSPARENT), _RET == 0)
 DEFINE_GUARD_COND(pm_runtime_active_auto, _try_enabled,
 		  pm_runtime_resume_and_get(_T), _RET == 0)
+DEFINE_GUARD_COND(pm_runtime_if_active, _try,
+		  pm_runtime_get_if_active(_T) ?: -EAGAIN, _RET == 1)
 
 /* ACQUIRE() wrapper macros for the guards defined above. */
 
@@ -639,6 +648,9 @@ DEFINE_GUARD_COND(pm_runtime_active_auto, _try_enabled,
 #define PM_RUNTIME_ACQUIRE_IF_ENABLED_AUTOSUSPEND(_dev, _var)	\
 	ACQUIRE(pm_runtime_active_auto_try_enabled, _var)(_dev)
 
+#define PM_RUNTIME_ACQUIRE_IF_ACTIVE(_dev, _var)	\
+	ACQUIRE(pm_runtime_if_active_try, _var)(_dev)
+
 /*
  * ACQUIRE_ERR() wrapper macro for guard pm_runtime_active.
  *
-- 
2.43.7