clk: add clk accuracy retrieval support

The clock accuracy is expressed in ppb (parts per billion) and represents
the possible clock drift.
Say you have a clock (e.g. an oscillator) which provides a fixed clock of
20MHz with an accuracy of +- 20Hz. This accuracy expressed in ppb is
20Hz/20MHz = 1000 ppb (or 1 ppm).

Clock users may need the clock accuracy information in order to choose
the best clock (the one with the best accuracy) across several available
clocks.

This patch adds clk accuracy retrieval support for common clk framework by
means of a new function called clk_get_accuracy.
This function returns the given clock accuracy expressed in ppb.

In order to get the clock accuracy, this implementation adds one callback
called recalc_accuracy to the clk_ops structure.
This callback is given the parent clock accuracy (if the clock is not a
root clock) and should recalculate the given clock accuracy.

This callback is optional and may be implemented if the clock is not
a perfect clock (accuracy != 0 ppb).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

Documentation/clk.txt

@@ -77,6 +77,8 @@ the operations defined in clk.h:
 		int		(*set_parent)(struct clk_hw *hw, u8 index);
 		u8		(*get_parent)(struct clk_hw *hw);
 		int		(*set_rate)(struct clk_hw *hw, unsigned long);
+		unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
+						   unsigned long parent_accuracy);
 		void		(*init)(struct clk_hw *hw);
 	};
 
@@ -202,6 +204,8 @@ optional or must be evaluated on a case-by-case basis.
 .set_parent     |      |             | n             | y           | n    |
 .get_parent     |      |             | n             | y           | n    |
                 |      |             |               |             |      |
+.recalc_accuracy|      |             |               |             |      |
+                |      |             |               |             |      |
 .init           |      |             |               |             |      |
                 -----------------------------------------------------------
 [1] either one of round_rate or determine_rate is required.

drivers/clk/clk.c

@@ -104,10 +104,11 @@ static void clk_summary_show_one(struct seq_file *s, struct clk *c, int level)
 	if (!c)
 		return;
 
-	seq_printf(s, "%*s%-*s %-11d %-12d %-10lu",
+	seq_printf(s, "%*s%-*s %-11d %-12d %-10lu %-11lu",
 		   level * 3 + 1, "",
 		   30 - level * 3, c->name,
-		   c->enable_count, c->prepare_count, clk_get_rate(c));
+		   c->enable_count, c->prepare_count, clk_get_rate(c),
+		   clk_get_accuracy(c));
 	seq_printf(s, "\n");
 }
 
@@ -129,8 +130,8 @@ static int clk_summary_show(struct seq_file *s, void *data)
 {
 	struct clk *c;
 
-	seq_printf(s, "   clock                        enable_cnt  prepare_cnt  rate\n");
-	seq_printf(s, "---------------------------------------------------------------------\n");
+	seq_printf(s, "   clock                        enable_cnt  prepare_cnt  rate        accuracy\n");
+	seq_printf(s, "---------------------------------------------------------------------------------\n");
 
 	clk_prepare_lock();
 
@@ -167,6 +168,7 @@ static void clk_dump_one(struct seq_file *s, struct clk *c, int level)
 	seq_printf(s, "\"enable_count\": %d,", c->enable_count);
 	seq_printf(s, "\"prepare_count\": %d,", c->prepare_count);
 	seq_printf(s, "\"rate\": %lu", clk_get_rate(c));
+	seq_printf(s, "\"accuracy\": %lu", clk_get_accuracy(c));
 }
 
 static void clk_dump_subtree(struct seq_file *s, struct clk *c, int level)
@@ -248,6 +250,11 @@ static int clk_debug_create_one(struct clk *clk, struct dentry *pdentry)
 	if (!d)
 		goto err_out;
 
+	d = debugfs_create_u32("clk_accuracy", S_IRUGO, clk->dentry,
+			(u32 *)&clk->accuracy);
+	if (!d)
+		goto err_out;
+
 	d = debugfs_create_x32("clk_flags", S_IRUGO, clk->dentry,
 			(u32 *)&clk->flags);
 	if (!d)
@@ -603,6 +610,14 @@ out:
 	return ret;
 }
 
+unsigned long __clk_get_accuracy(struct clk *clk)
+{
+	if (!clk)
+		return 0;
+
+	return clk->accuracy;
+}
+
 unsigned long __clk_get_flags(struct clk *clk)
 {
 	return !clk ? 0 : clk->flags;
@@ -1016,6 +1031,59 @@ static int __clk_notify(struct clk *clk, unsigned long msg,
 	return ret;
 }
 
+/**
+ * __clk_recalc_accuracies
+ * @clk: first clk in the subtree
+ *
+ * Walks the subtree of clks starting with clk and recalculates accuracies as
+ * it goes.  Note that if a clk does not implement the .recalc_accuracy
+ * callback then it is assumed that the clock will take on the accuracy of it's
+ * parent.
+ *
+ * Caller must hold prepare_lock.
+ */
+static void __clk_recalc_accuracies(struct clk *clk)
+{
+	unsigned long parent_accuracy = 0;
+	struct clk *child;
+
+	if (clk->parent)
+		parent_accuracy = clk->parent->accuracy;
+
+	if (clk->ops->recalc_accuracy)
+		clk->accuracy = clk->ops->recalc_accuracy(clk->hw,
+							  parent_accuracy);
+	else
+		clk->accuracy = parent_accuracy;
+
+	hlist_for_each_entry(child, &clk->children, child_node)
+		__clk_recalc_accuracies(child);
+}
+
+/**
+ * clk_get_accuracy - return the accuracy of clk
+ * @clk: the clk whose accuracy is being returned
+ *
+ * Simply returns the cached accuracy of the clk, unless
+ * CLK_GET_ACCURACY_NOCACHE flag is set, which means a recalc_rate will be
+ * issued.
+ * If clk is NULL then returns 0.
+ */
+long clk_get_accuracy(struct clk *clk)
+{
+	unsigned long accuracy;
+
+	clk_prepare_lock();
+	if (clk && (clk->flags & CLK_GET_ACCURACY_NOCACHE))
+		__clk_recalc_accuracies(clk);
+
+	accuracy = __clk_get_accuracy(clk);
+	clk_prepare_unlock();
+
+	return accuracy;
+}
+EXPORT_SYMBOL_GPL(clk_get_accuracy);
+
 /**
  * __clk_recalc_rates
  * @clk: first clk in the subtree
@@ -1552,6 +1620,7 @@ void __clk_reparent(struct clk *clk, struct clk *new_parent)
 {
 	clk_reparent(clk, new_parent);
 	clk_debug_reparent(clk, new_parent);
+	__clk_recalc_accuracies(clk);
 	__clk_recalc_rates(clk, POST_RATE_CHANGE);
 }
 
@@ -1622,11 +1691,13 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
 	/* do the re-parent */
 	ret = __clk_set_parent(clk, parent, p_index);
 
-	/* propagate rate recalculation accordingly */
-	if (ret)
+	/* propagate rate an accuracy recalculation accordingly */
+	if (ret) {
 		__clk_recalc_rates(clk, ABORT_RATE_CHANGE);
-	else
+	} else {
 		__clk_recalc_rates(clk, POST_RATE_CHANGE);
+		__clk_recalc_accuracies(clk);
+	}
 
 out:
 	clk_prepare_unlock();
@@ -1730,6 +1801,21 @@ int __clk_init(struct device *dev, struct clk *clk)
 	else
 		hlist_add_head(&clk->child_node, &clk_orphan_list);
 
+	/*
+	 * Set clk's accuracy.  The preferred method is to use
+	 * .recalc_accuracy. For simple clocks and lazy developers the default
+	 * fallback is to use the parent's accuracy.  If a clock doesn't have a
+	 * parent (or is orphaned) then accuracy is set to zero (perfect
+	 * clock).
+	 */
+	if (clk->ops->recalc_accuracy)
+		clk->accuracy = clk->ops->recalc_accuracy(clk->hw,
+					__clk_get_accuracy(clk->parent));
+	else if (clk->parent)
+		clk->accuracy = clk->parent->accuracy;
+	else
+		clk->accuracy = 0;
+
 	/*
 	 * Set clk's rate.  The preferred method is to use .recalc_rate.  For
 	 * simple clocks and lazy developers the default fallback is to use the

include/linux/clk-private.h

@@ -41,6 +41,7 @@ struct clk {
 	unsigned long		flags;
 	unsigned int		enable_count;
 	unsigned int		prepare_count;
+	unsigned long		accuracy;
 	struct hlist_head	children;
 	struct hlist_node	child_node;
 	unsigned int		notifier_count;

include/linux/clk-provider.h

@@ -29,6 +29,7 @@
 #define CLK_IS_BASIC		BIT(5) /* Basic clk, can't do a to_clk_foo() */
 #define CLK_GET_RATE_NOCACHE	BIT(6) /* do not use the cached clk rate */
 #define CLK_SET_RATE_NO_REPARENT BIT(7) /* don't re-parent on rate change */
+#define CLK_GET_ACCURACY_NOCACHE BIT(8) /* do not use the cached clk accuracy */
 
 struct clk_hw;
 
@@ -108,6 +109,13 @@ struct clk_hw;
  *		which is likely helpful for most .set_rate implementation.
  *		Returns 0 on success, -EERROR otherwise.
  *
+ * @recalc_accuracy: Recalculate the accuracy of this clock. The clock accuracy
+ *		is expressed in ppb (parts per billion). The parent accuracy is
+ *		an input parameter.
+ *		Returns the calculated accuracy.  Optional - if	this op is not
+ *		set then clock accuracy will be initialized to parent accuracy
+ *		or 0 (perfect clock) if clock has no parent.
+ *
  * The clk_enable/clk_disable and clk_prepare/clk_unprepare pairs allow
  * implementations to split any work between atomic (enable) and sleepable
  * (prepare) contexts.  If enabling a clock requires code that might sleep,
@@ -139,6 +147,8 @@ struct clk_ops {
 	u8		(*get_parent)(struct clk_hw *hw);
 	int		(*set_rate)(struct clk_hw *hw, unsigned long,
 				    unsigned long);
+	unsigned long	(*recalc_accuracy)(struct clk_hw *hw,
+					   unsigned long parent_accuracy);
 	void		(*init)(struct clk_hw *hw);
 };
 
@@ -433,6 +443,7 @@ struct clk *clk_get_parent_by_index(struct clk *clk, u8 index);
 unsigned int __clk_get_enable_count(struct clk *clk);
 unsigned int __clk_get_prepare_count(struct clk *clk);
 unsigned long __clk_get_rate(struct clk *clk);
+unsigned long __clk_get_accuracy(struct clk *clk);
 unsigned long __clk_get_flags(struct clk *clk);
 bool __clk_is_prepared(struct clk *clk);
 bool __clk_is_enabled(struct clk *clk);

include/linux/clk.h

@@ -82,6 +82,23 @@ int clk_notifier_register(struct clk *clk, struct notifier_block *nb);
 
 int clk_notifier_unregister(struct clk *clk, struct notifier_block *nb);
 
+/**
+ * clk_get_accuracy - obtain the clock accuracy in ppb (parts per billion)
+ *		      for a clock source.
+ * @clk: clock source
+ *
+ * This gets the clock source accuracy expressed in ppb.
+ * A perfect clock returns 0.
+ */
+long clk_get_accuracy(struct clk *clk);
+
+#else
+
+static inline long clk_get_accuracy(struct clk *clk)
+{
+	return -ENOTSUPP;
+}
+
 #endif
 
 /**