Date:   Thu, 21 Feb 2019 16:28:58 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] reset: add acquired/released state for exclusive
 reset controls

On Thu, Feb 21, 2019 at 04:25:53PM +0100, Thierry Reding wrote:
> From: Philipp Zabel <p.zabel@...gutronix.de>
> 
> There are cases where a driver needs explicit control over a reset line
> that is exclusively conneted to its device, but this control has to be
> temporarily handed over to the power domain controller to handle reset
> requirements during power transitions.
> Allow multiple exclusive reset controls to be requested in 'released'
> state for the same physical reset line, only one of which can be
> acquired at the same time.
> 
> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
>  drivers/reset/core.c  | 139 ++++++++++++++++++++++++++++++++++++++----
>  include/linux/reset.h |  93 ++++++++++++++++++++++------
>  2 files changed, 200 insertions(+), 32 deletions(-)

Hi Philipp,

the bulk of this is unchanged relative to what you had posted
originally. I squashed in the few things that we had already discussed
earlier (EINVAL -> EPERM) and a couple of minor fixes for issues that I
found while working with this.

Attached is my fixup patch which contains all the changes I made on top
of your version and that I squashed into this.

Thierry

--- >8 ---
commit aa618d0b63eec676d9ea8db91a4c5fdc9330fc6b
Author: Thierry Reding <treding@...dia.com>
Date:   Mon Feb 18 11:32:46 2019 +0100

    fixup! reset: add acquired/released state for exclusive reset controls

diff --git a/drivers/reset/core.c b/drivers/reset/core.c
index c6a7a4474142..1e8a42b16f23 100644
--- a/drivers/reset/core.c
+++ b/drivers/reset/core.c
@@ -65,6 +65,17 @@ struct reset_control_array {
 	struct reset_control *rstc[];
 };
 
+static const char *rcdev_name(struct reset_controller_dev *rcdev)
+{
+	if (rcdev->dev)
+		return dev_name(rcdev->dev);
+
+	if (rcdev->of_node)
+		return rcdev->of_node->full_name;
+
+	return NULL;
+}
+
 /**
  * of_reset_simple_xlate - translate reset_spec to the reset line number
  * @rcdev: a pointer to the reset controller device
@@ -276,7 +287,7 @@ int reset_control_reset(struct reset_control *rstc)
 			return 0;
 	} else {
 		if (!rstc->acquired)
-			return -EINVAL;
+			return -EPERM;
 	}
 
 	ret = rstc->rcdev->ops->reset(rstc->rcdev, rstc->id);
@@ -340,8 +351,11 @@ int reset_control_assert(struct reset_control *rstc)
 		if (!rstc->rcdev->ops->assert)
 			return -ENOTSUPP;
 
-		if (!rstc->acquired)
-			return -EINVAL;
+		if (!rstc->acquired) {
+			WARN(1, "reset %s (ID: %u) is not acquired\n",
+			     rcdev_name(rstc->rcdev), rstc->id);
+			return -EPERM;
+		}
 	}
 
 	return rstc->rcdev->ops->assert(rstc->rcdev, rstc->id);
@@ -378,8 +392,11 @@ int reset_control_deassert(struct reset_control *rstc)
 		if (atomic_inc_return(&rstc->deassert_count) != 1)
 			return 0;
 	} else {
-		if (!rstc->acquired)
-			return -EINVAL;
+		if (!rstc->acquired) {
+			WARN(1, "reset %s (ID: %u) is not acquired\n",
+			     rcdev_name(rstc->rcdev), rstc->id);
+			return -EPERM;
+		}
 	}
 
 	/*
@@ -417,15 +434,43 @@ int reset_control_status(struct reset_control *rstc)
 }
 EXPORT_SYMBOL_GPL(reset_control_status);
 
+/**
+ * reset_control_acquire() - acquires a reset control for exclusive use
+ * @rstc: reset control
+ *
+ * This is used to explicitly acquire a reset control for exclusive use. Note
+ * that exclusive resets are requested as acquired by default. In order for a
+ * second consumer to be able to control the reset, the first consumer has to
+ * release it first. Typically the easiest way to achieve this is to call the
+ * reset_control_get_exclusive_released() to obtain an instance of the reset
+ * control. Such reset controls are not acquired by default.
+ *
+ * Consumers implementing shared access to an exclusive reset need to follow
+ * a specific protocol in order to work together. Before consumers can change
+ * a reset they must acquire exclusive access using reset_control_acquire().
+ * After they are done operating the reset, they must release exclusive access
+ * with a call to reset_control_release(). Consumers are not granted exclusive
+ * access to the reset as long as another consumer hasn't released a reset.
+ *
+ * See also: reset_control_release()
+ */
 int reset_control_acquire(struct reset_control *rstc)
 {
 	struct reset_control *rc;
 
-	if (!rstc || rstc->acquired)
+	if (!rstc)
 		return 0;
 
+	if (WARN_ON(IS_ERR(rstc)))
+		return -EINVAL;
+
 	mutex_lock(&reset_list_mutex);
 
+	if (rstc->acquired) {
+		mutex_unlock(&reset_list_mutex);
+		return 0;
+	}
+
 	list_for_each_entry(rc, &rstc->rcdev->reset_control_head, list) {
 		if (rstc != rc && rstc->id == rc->id) {
 			if (rc->acquired) {
@@ -435,13 +480,28 @@ int reset_control_acquire(struct reset_control *rstc)
 		}
 	}
 
+	rstc->acquired = true;
+
 	mutex_unlock(&reset_list_mutex);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(reset_control_acquire);
 
+/**
+ * reset_control_release() - releases exclusive access to a reset control
+ * @rstc: reset control
+ *
+ * Releases exclusive access right to a reset control previously obtained by a
+ * call to reset_control_acquire(). Until a consumer calls this function, no
+ * other consumers will be granted exclusive access.
+ *
+ * See also: reset_control_acquire()
+ */
 void reset_control_release(struct reset_control *rstc)
 {
+	if (!rstc || WARN_ON(IS_ERR(rstc)))
+		return;
+
 	rstc->acquired = false;
 }
 EXPORT_SYMBOL_GPL(reset_control_release);

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