reset: allow using reset_control_reset with shared reset

Some SoCs (for example Amlogic GXBB) implement a reset controller which
only supports a reset pulse (triggered via reset_control_reset). At the
same time multiple devices (in case of the Amlogic GXBB SoC both USB
PHYs) are sharing the same reset line.

This patch allows using reset_control_reset also for shared resets.
There are limitations though:
reset_control_reset can only be used if reset_control_assert was not
used yet.
reset_control_assert can only be used if reset_control_reset was not
used yet.
For shared resets the reset is only triggered once for the lifetime of
the reset_control instance (the reset can be triggered again if all
consumers of that specific reset_control are gone, as the reset
framework will free the reset_control instance in that case).

Signed-off-by: Martin Blumenstingl <[e-mail]>
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/reset/core.c

···
  * @refcnt: Number of gets of this reset_control
  * @shared: Is this a shared (1), or an exclusive (0) reset_control?
  * @deassert_cnt: Number of times this reset line has been deasserted
+ * @triggered_count: Number of times this reset line has been reset. Currently
+ *                   only used for shared resets, which means that the value
+ *                   will be either 0 or 1.
  */
 struct reset_control {
 	struct reset_controller_dev *rcdev;
···
 	unsigned int refcnt;
 	int shared;
 	atomic_t deassert_count;
+	atomic_t triggered_count;
 };
 
 /**
···
  * reset_control_reset - reset the controlled device
  * @rstc: reset controller
  *
- * Calling this on a shared reset controller is an error.
+ * On a shared reset line the actual reset pulse is only triggered once for the
+ * lifetime of the reset_control instance: for all but the first caller this is
+ * a no-op.
+ * Consumers must not use reset_control_(de)assert on shared reset lines when
+ * reset_control_reset has been used.
  */
 int reset_control_reset(struct reset_control *rstc)
 {
-	if (WARN_ON(IS_ERR_OR_NULL(rstc)) ||
-	    WARN_ON(rstc->shared))
+	int ret;
+
+	if (WARN_ON(IS_ERR_OR_NULL(rstc)))
 		return -EINVAL;
 
-	if (rstc->rcdev->ops->reset)
-		return rstc->rcdev->ops->reset(rstc->rcdev, rstc->id);
+	if (!rstc->rcdev->ops->reset)
+		return -ENOTSUPP;
 
-	return -ENOTSUPP;
+	if (rstc->shared) {
+		if (WARN_ON(atomic_read(&rstc->deassert_count) != 0))
+			return -EINVAL;
+
+		if (atomic_inc_return(&rstc->triggered_count) != 1)
+			return 0;
+	}
+
+	ret = rstc->rcdev->ops->reset(rstc->rcdev, rstc->id);
+	if (rstc->shared && !ret)
+		atomic_dec(&rstc->triggered_count);
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(reset_control_reset);
 
···
  *
  * For shared reset controls a driver cannot expect the hw's registers and
  * internal state to be reset, but must be prepared for this to happen.
+ * Consumers must not use reset_control_reset on shared reset lines when
+ * reset_control_(de)assert has been used.
  */
 int reset_control_assert(struct reset_control *rstc)
 {
···
 		return -ENOTSUPP;
 
 	if (rstc->shared) {
+		if (WARN_ON(atomic_read(&rstc->triggered_count) != 0))
+			return -EINVAL;
+
 		if (WARN_ON(atomic_read(&rstc->deassert_count) == 0))
 			return -EINVAL;
 
···
  * @rstc: reset controller
  *
  * After calling this function, the reset is guaranteed to be deasserted.
+ * Consumers must not use reset_control_reset on shared reset lines when
+ * reset_control_(de)assert has been used.
  */
 int reset_control_deassert(struct reset_control *rstc)
 {
···
 		return -ENOTSUPP;
 
 	if (rstc->shared) {
+		if (WARN_ON(atomic_read(&rstc->triggered_count) != 0))
+			return -EINVAL;
+
 		if (atomic_inc_return(&rstc->deassert_count) != 1)
 			return 0;
 	}