[PATCH RFC] hwspinlock: Don't take software spinlock before hwspinlock

[Lina Iyer <lina.iyer@linaro.org>]

Some uses of the hwspinlock could be that one entity acquires the lock
and the other entity releases the lock. This allows for a serialized
traversal path from the locking entity to the other.

For example, the cpuidle entry from Linux to the firmware to power down
the core, can be serialized across the context switch by locking the
hwspinlock in Linux and releasing it in the firmware.

Do not force the caller of __hwspin_trylock() to acquire a kernel
spinlock before acquiring the hwspinlock.

Cc: Jeffrey Hugo <jhugo@codeaurora.org>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Suman Anna <s-anna@ti.com>
Cc: Andy Gross <agross@codeaurora.org>
Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
---
 drivers/hwspinlock/hwspinlock_core.c | 56 ++++++++++++++++++++----------------
 include/linux/hwspinlock.h           |  1 +
 2 files changed, 32 insertions(+), 25 deletions(-)

diff --git a/drivers/hwspinlock/hwspinlock_core.c b/drivers/hwspinlock/hwspinlock_core.c
index 461a0d7..bdc59f2 100644
--- a/drivers/hwspinlock/hwspinlock_core.c
+++ b/drivers/hwspinlock/hwspinlock_core.c
@@ -105,30 +105,34 @@ int __hwspin_trylock(struct hwspinlock *hwlock, int mode, unsigned long *flags)
 	 *    problems with hwspinlock usage (e.g. scheduler checks like
 	 *    'scheduling while atomic' etc.)
 	 */
-	if (mode == HWLOCK_IRQSTATE)
-		ret = spin_trylock_irqsave(&hwlock->lock, *flags);
-	else if (mode == HWLOCK_IRQ)
-		ret = spin_trylock_irq(&hwlock->lock);
-	else
-		ret = spin_trylock(&hwlock->lock);
+	if (mode != HWLOCK_NOLOCK) {
+		if (mode == HWLOCK_IRQSTATE)
+			ret = spin_trylock_irqsave(&hwlock->lock, *flags);
+		else if (mode == HWLOCK_IRQ)
+			ret = spin_trylock_irq(&hwlock->lock);
+		else
+			ret = spin_trylock(&hwlock->lock);
 
-	/* is lock already taken by another context on the local cpu ? */
-	if (!ret)
-		return -EBUSY;
+		/* is lock already taken by another context on the local cpu? */
+		if (!ret)
+			return -EBUSY;
+	}
 
 	/* try to take the hwspinlock device */
 	ret = hwlock->bank->ops->trylock(hwlock);
 
-	/* if hwlock is already taken, undo spin_trylock_* and exit */
-	if (!ret) {
-		if (mode == HWLOCK_IRQSTATE)
-			spin_unlock_irqrestore(&hwlock->lock, *flags);
-		else if (mode == HWLOCK_IRQ)
-			spin_unlock_irq(&hwlock->lock);
-		else
-			spin_unlock(&hwlock->lock);
+	if (mode != HWLOCK_NOLOCK) {
+		/* if hwlock is already taken, undo spin_trylock_* and exit */
+		if (!ret) {
+			if (mode == HWLOCK_IRQSTATE)
+				spin_unlock_irqrestore(&hwlock->lock, *flags);
+			else if (mode == HWLOCK_IRQ)
+				spin_unlock_irq(&hwlock->lock);
+			else
+				spin_unlock(&hwlock->lock);
 
-		return -EBUSY;
+			return -EBUSY;
+		}
 	}
 
 	/*
@@ -247,13 +251,15 @@ void __hwspin_unlock(struct hwspinlock *hwlock, int mode, unsigned long *flags)
 
 	hwlock->bank->ops->unlock(hwlock);
 
-	/* Undo the spin_trylock{_irq, _irqsave} called while locking */
-	if (mode == HWLOCK_IRQSTATE)
-		spin_unlock_irqrestore(&hwlock->lock, *flags);
-	else if (mode == HWLOCK_IRQ)
-		spin_unlock_irq(&hwlock->lock);
-	else
-		spin_unlock(&hwlock->lock);
+	if (mode != HWLOCK_NOLOCK) {
+		/* Undo the spin_trylock{_irq, _irqsave} called while locking */
+		if (mode == HWLOCK_IRQSTATE)
+			spin_unlock_irqrestore(&hwlock->lock, *flags);
+		else if (mode == HWLOCK_IRQ)
+			spin_unlock_irq(&hwlock->lock);
+		else
+			spin_unlock(&hwlock->lock);
+	}
 }
 EXPORT_SYMBOL_GPL(__hwspin_unlock);
 
diff --git a/include/linux/hwspinlock.h b/include/linux/hwspinlock.h
index 3343298..219b333 100644
--- a/include/linux/hwspinlock.h
+++ b/include/linux/hwspinlock.h
@@ -24,6 +24,7 @@
 /* hwspinlock mode argument */
 #define HWLOCK_IRQSTATE	0x01	/* Disable interrupts, save state */
 #define HWLOCK_IRQ	0x02	/* Disable interrupts, don't save state */
+#define HWLOCK_NOLOCK	0xFF	/* Dont take any lock */
 
 struct device;
 struct hwspinlock;
-- 
2.1.4