[PATCH 1/5] clk: allow reentrant calls into the clk framework

[Mike Turquette <mturquette@linaro.org>]

Reentrancy into the clock framework from the clk.h api is highly
desirable.  This feature is necessary for clocks that are prepared and
unprepared via i2c_transfer (which includes many PMICs and discrete
audio chips) and it is also necessary for performing dynamic voltage &
frequency scaling via clock rate-change notifiers.

This patch implements reentrancy by adding a global atomic_t which
tracks the context of the current caller.  Context in this case is the
return value from get_current().  The clk.h api implementations are
modified to first see if the relevant global lock is already held and if
so compare the global context (set by whoever is holding the lock)
against their own context (via a call to get_current()).  If the two
match then this function is a nested call from the one already holding
the lock and we procede.  If the context does not match then procede to
call mutex_lock and busy-wait for the existing task to complete.

Thus this patch set does not increase concurrency for unrelated calls
into the clock framework.  Instead it simply allows reentrancy by the
single task which is currently holding the global clock framework lock.

Thanks to Rajagoapl Venkat for the original idea to use get_current()
and to David Brown for the suggestion to replace my previous rwlock
scheme with atomic operations during code review at ELC 2013.

Signed-off-by: Mike Turquette <mturquette@linaro.org>
Cc: Rajagopal Venkat <rajagopal.venkat@linaro.org>
Cc: David Brown <davidb@codeaurora.org>
---
 drivers/clk/clk.c |  254 ++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 185 insertions(+), 69 deletions(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index fabbfe1..b7d6a0a 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -19,9 +19,11 @@
 #include <linux/of.h>
 #include <linux/device.h>
 #include <linux/init.h>
+#include <linux/sched.h>
 
 static DEFINE_SPINLOCK(enable_lock);
 static DEFINE_MUTEX(prepare_lock);
+static atomic_t context;
 
 static HLIST_HEAD(clk_root_list);
 static HLIST_HEAD(clk_orphan_list);
@@ -433,27 +435,6 @@ unsigned int __clk_get_prepare_count(struct clk *clk)
 	return !clk ? 0 : clk->prepare_count;
 }
 
-unsigned long __clk_get_rate(struct clk *clk)
-{
-	unsigned long ret;
-
-	if (!clk) {
-		ret = 0;
-		goto out;
-	}
-
-	ret = clk->rate;
-
-	if (clk->flags & CLK_IS_ROOT)
-		goto out;
-
-	if (!clk->parent)
-		ret = 0;
-
-out:
-	return ret;
-}
-
 unsigned long __clk_get_flags(struct clk *clk)
 {
 	return !clk ? 0 : clk->flags;
@@ -524,6 +505,35 @@ struct clk *__clk_lookup(const char *name)
 	return NULL;
 }
 
+/***  locking & reentrancy ***/
+
+static void clk_fwk_lock(void)
+{
+	/* hold the framework-wide lock, context == NULL */
+	mutex_lock(&prepare_lock);
+
+	/* set context for any reentrant calls */
+	atomic_set(&context, (int) get_current());
+}
+
+static void clk_fwk_unlock(void)
+{
+	/* clear the context */
+	atomic_set(&context, 0);
+
+	/* release the framework-wide lock, context == NULL */
+	mutex_unlock(&prepare_lock);
+}
+
+static bool clk_is_reentrant(void)
+{
+	if (mutex_is_locked(&prepare_lock))
+		if ((void *) atomic_read(&context) == get_current())
+			return true;
+
+	return false;
+}
+
 /***        clk api        ***/
 
 void __clk_unprepare(struct clk *clk)
@@ -558,9 +568,15 @@ void __clk_unprepare(struct clk *clk)
  */
 void clk_unprepare(struct clk *clk)
 {
-	mutex_lock(&prepare_lock);
+	/* re-enter if call is from the same context */
+	if (clk_is_reentrant()) {
+		__clk_unprepare(clk);
+		return;
+	}
+
+	clk_fwk_lock();
 	__clk_unprepare(clk);
-	mutex_unlock(&prepare_lock);
+	clk_fwk_unlock();
 }
 EXPORT_SYMBOL_GPL(clk_unprepare);
 
@@ -606,10 +622,16 @@ int clk_prepare(struct clk *clk)
 {
 	int ret;
 
-	mutex_lock(&prepare_lock);
-	ret = __clk_prepare(clk);
-	mutex_unlock(&prepare_lock);
+	/* re-enter if call is from the same context */
+	if (clk_is_reentrant()) {
+		ret = __clk_prepare(clk);
+		goto out;
+	}
 
+	clk_fwk_lock();
+	ret = __clk_prepare(clk);
+	clk_fwk_unlock();
+out:
 	return ret;
 }
 EXPORT_SYMBOL_GPL(clk_prepare);
@@ -650,8 +672,27 @@ void clk_disable(struct clk *clk)
 {
 	unsigned long flags;
 
+	/* must check both the global spinlock and the global mutex */
+	if (spin_is_locked(&enable_lock) || mutex_is_locked(&prepare_lock)) {
+		if ((void *) atomic_read(&context) == get_current()) {
+			__clk_disable(clk);
+			return;
+		}
+	}
+
+	/* hold the framework-wide lock, context == NULL */
 	spin_lock_irqsave(&enable_lock, flags);
+
+	/* set context for any reentrant calls */
+	atomic_set(&context, (int) get_current());
+
+	/* disable the clock(s) */
 	__clk_disable(clk);
+
+	/* clear the context */
+	atomic_set(&context, 0);
+
+	/* release the framework-wide lock, context == NULL */
 	spin_unlock_irqrestore(&enable_lock, flags);
 }
 EXPORT_SYMBOL_GPL(clk_disable);
@@ -703,10 +744,29 @@ int clk_enable(struct clk *clk)
 	unsigned long flags;
 	int ret;
 
+	/* this call re-enters if it is from the same context */
+	if (spin_is_locked(&enable_lock) || mutex_is_locked(&prepare_lock)) {
+		if ((void *) atomic_read(&context) == get_current()) {
+			ret = __clk_enable(clk);
+			goto out;
+		}
+	}
+
+	/* hold the framework-wide lock, context == NULL */
 	spin_lock_irqsave(&enable_lock, flags);
+
+	/* set context for any reentrant calls */
+	atomic_set(&context, (int) get_current());
+
+	/* enable the clock(s) */
 	ret = __clk_enable(clk);
-	spin_unlock_irqrestore(&enable_lock, flags);
 
+	/* clear the context */
+	atomic_set(&context, 0);
+
+	/* release the framework-wide lock, context == NULL */
+	spin_unlock_irqrestore(&enable_lock, flags);
+out:
 	return ret;
 }
 EXPORT_SYMBOL_GPL(clk_enable);
@@ -750,10 +810,17 @@ long clk_round_rate(struct clk *clk, unsigned long rate)
 {
 	unsigned long ret;
 
-	mutex_lock(&prepare_lock);
+	/* this call re-enters if it is from the same context */
+	if (clk_is_reentrant()) {
+		ret = __clk_round_rate(clk, rate);
+		goto out;
+	}
+
+	clk_fwk_lock();
 	ret = __clk_round_rate(clk, rate);
-	mutex_unlock(&prepare_lock);
+	clk_fwk_unlock();
 
+out:
 	return ret;
 }
 EXPORT_SYMBOL_GPL(clk_round_rate);
@@ -836,6 +903,30 @@ static void __clk_recalc_rates(struct clk *clk, unsigned long msg)
 		__clk_recalc_rates(child, msg);
 }
 
+unsigned long __clk_get_rate(struct clk *clk)
+{
+	unsigned long ret;
+
+	if (!clk) {
+		ret = 0;
+		goto out;
+	}
+
+	if (clk->flags & CLK_GET_RATE_NOCACHE)
+		__clk_recalc_rates(clk, 0);
+
+	ret = clk->rate;
+
+	if (clk->flags & CLK_IS_ROOT)
+		goto out;
+
+	if (!clk->parent)
+		ret = 0;
+
+out:
+	return ret;
+}
+
 /**
  * clk_get_rate - return the rate of clk
  * @clk: the clk whose rate is being returned
@@ -848,14 +939,22 @@ unsigned long clk_get_rate(struct clk *clk)
 {
 	unsigned long rate;
 
-	mutex_lock(&prepare_lock);
+	/*
+	 * FIXME - any locking here seems heavy weight
+	 * can clk->rate be replaced with an atomic_t?
+	 * same logic can likely be applied to prepare_count & enable_count
+	 */
 
-	if (clk && (clk->flags & CLK_GET_RATE_NOCACHE))
-		__clk_recalc_rates(clk, 0);
+	if (clk_is_reentrant()) {
+		rate = __clk_get_rate(clk);
+		goto out;
+	}
 
+	clk_fwk_lock();
 	rate = __clk_get_rate(clk);
-	mutex_unlock(&prepare_lock);
+	clk_fwk_unlock();
 
+out:
 	return rate;
 }
 EXPORT_SYMBOL_GPL(clk_get_rate);
@@ -1036,6 +1135,39 @@ static void clk_change_rate(struct clk *clk)
 		clk_change_rate(child);
 }
 
+int __clk_set_rate(struct clk *clk, unsigned long rate)
+{
+	int ret = 0;
+	struct clk *top, *fail_clk;
+
+	/* bail early if nothing to do */
+	if (rate == clk->rate)
+		return 0;
+
+	if ((clk->flags & CLK_SET_RATE_GATE) && clk->prepare_count) {
+		return -EBUSY;
+	}
+
+	/* calculate new rates and get the topmost changed clock */
+	top = clk_calc_new_rates(clk, rate);
+	if (!top)
+		return -EINVAL;
+
+	/* notify that we are about to change rates */
+	fail_clk = clk_propagate_rate_change(top, PRE_RATE_CHANGE);
+	if (fail_clk) {
+		pr_warn("%s: failed to set %s rate\n", __func__,
+				fail_clk->name);
+		clk_propagate_rate_change(top, ABORT_RATE_CHANGE);
+		return -EBUSY;
+	}
+
+	/* change the rates */
+	clk_change_rate(top);
+
+	return ret;
+}
+
 /**
  * clk_set_rate - specify a new rate for clk
  * @clk: the clk whose rate is being changed
@@ -1059,44 +1191,18 @@ static void clk_change_rate(struct clk *clk)
  */
 int clk_set_rate(struct clk *clk, unsigned long rate)
 {
-	struct clk *top, *fail_clk;
 	int ret = 0;
 
-	/* prevent racing with updates to the clock topology */
-	mutex_lock(&prepare_lock);
-
-	/* bail early if nothing to do */
-	if (rate == clk->rate)
-		goto out;
-
-	if ((clk->flags & CLK_SET_RATE_GATE) && clk->prepare_count) {
-		ret = -EBUSY;
-		goto out;
-	}
-
-	/* calculate new rates and get the topmost changed clock */
-	top = clk_calc_new_rates(clk, rate);
-	if (!top) {
-		ret = -EINVAL;
-		goto out;
-	}
-
-	/* notify that we are about to change rates */
-	fail_clk = clk_propagate_rate_change(top, PRE_RATE_CHANGE);
-	if (fail_clk) {
-		pr_warn("%s: failed to set %s rate\n", __func__,
-				fail_clk->name);
-		clk_propagate_rate_change(top, ABORT_RATE_CHANGE);
-		ret = -EBUSY;
+	if (clk_is_reentrant()) {
+		ret = __clk_set_rate(clk, rate);
 		goto out;
 	}
 
-	/* change the rates */
-	clk_change_rate(top);
+	clk_fwk_lock();
+	ret = __clk_set_rate(clk, rate);
+	clk_fwk_unlock();
 
 out:
-	mutex_unlock(&prepare_lock);
-
 	return ret;
 }
 EXPORT_SYMBOL_GPL(clk_set_rate);
@@ -1111,10 +1217,16 @@ struct clk *clk_get_parent(struct clk *clk)
 {
 	struct clk *parent;
 
-	mutex_lock(&prepare_lock);
+	if (clk_is_reentrant()) {
+		parent = __clk_get_parent(clk);
+		goto out;
+	}
+
+	clk_fwk_lock();
 	parent = __clk_get_parent(clk);
-	mutex_unlock(&prepare_lock);
+	clk_fwk_unlock();
 
+out:
 	return parent;
 }
 EXPORT_SYMBOL_GPL(clk_get_parent);
@@ -1293,6 +1405,7 @@ out:
 int clk_set_parent(struct clk *clk, struct clk *parent)
 {
 	int ret = 0;
+	bool reenter;
 
 	if (!clk || !clk->ops)
 		return -EINVAL;
@@ -1300,8 +1413,10 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
 	if (!clk->ops->set_parent)
 		return -ENOSYS;
 
-	/* prevent racing with updates to the clock topology */
-	mutex_lock(&prepare_lock);
+	reenter = clk_is_reentrant();
+
+	if (!reenter)
+		clk_fwk_lock();
 
 	if (clk->parent == parent)
 		goto out;
@@ -1330,7 +1445,8 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
 	__clk_reparent(clk, parent);
 
 out:
-	mutex_unlock(&prepare_lock);
+	if (!reenter)
+		clk_fwk_unlock();
 
 	return ret;
 }
-- 
1.7.10.4