[PATCH v3 0/5] gpiolib: cdev: relocate debounce_period_us

[PATCH v3 0/5] gpiolib: cdev: relocate debounce_period_us

[Kent Gibson]

This series contains minor improvements to gpiolib-cdev.

The banner change is relocating the debounce_period_us from gpiolib's
struct gpio_desc to cdev's struct line.  Patch 1 stores the field
locally in cdev.  Patch 2 removes the now unused field from gpiolib.

Patch 3 is somewhat related and removes a FIXME from
gpio_desc_to_lineinfo().  The FIXME relates to a race condition in
the calculation of the used flag, but I would assert that from
the userspace perspective the read operation itself is inherently racy.
The line being reported as unused in the info provides no guarantee -
it just an indicator that requesting the line is likely to succeed -
assuming the line is not otherwise requested in the meantime.
Given the overall operation is racy, trying to stamp out an unlikely
race within the operation is pointless. Accept it as a possibility
that has negligible side-effects and reduce the number of locks held
simultaneously and the duration that the gpio_lock is held.

Patches 1 and 3 introduce usage of guard() and scoped_guard() to cdev.
Patch 4 replaces any remaining discrete lock/unlock calls around
critical sections with guard() or scoped_guard().

Patch 5 is unrelated to debounce or info, but addresses Andy's
recent lamentation that the linereq get/set values functions are
confusing and under documented.
Figured I may as well add that while I was in there.

Changes v2 -> v3:
 - reorder patches to move full adoption of guard()/scoped_guard() to
   patch 4.
 - use guard() rather than scoped_guard() where the scope extends to the
   end of the function.
 - split supinfo into supinfo_tree and supinfo_lock (patch 1).
 - rename flags to dflags in gpio_desc_to_lineinfo() (patch 3).

Changes v1 -> v2:
 (changes are to patch 2 unless otherwise noted)
 - adopt scoped_guard() for critical sections, inserting patch 1 and
   updating patch 2 and 4.
 - move rb_node field to beginning of struct line.
 - merge struct supinfo into supinfo var declaration.
 - move rb_tree field to beginning of struct supinfo.
 - replace pr_warn() with WARN().
 - drop explicit int to bool conversion in line_is_supplemental().
 - use continue to bypass cleanup in linereq_free().
 - fix typo in commit message (patch 4)

Kent Gibson (5):
  gpiolib: cdev: relocate debounce_period_us from struct gpio_desc
  gpiolib: remove debounce_period_us from struct gpio_desc
  gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()
  gpiolib: cdev: fully adopt guard() and scoped_guard()
  gpiolib: cdev: improve documentation of get/set values

 drivers/gpio/gpiolib-cdev.c | 393 ++++++++++++++++++++++--------------
 drivers/gpio/gpiolib.c      |   3 -
 drivers/gpio/gpiolib.h      |   5 -
 3 files changed, 246 insertions(+), 155 deletions(-)

--
2.39.2

[PATCH v3 1/5] gpiolib: cdev: relocate debounce_period_us from struct gpio_desc

[Kent Gibson]

Store the debounce period for a requested line locally, rather than in
the debounce_period_us field in the gpiolib struct gpio_desc.

Add a global tree of lines containing supplemental line information
to make the debounce period available to be reported by the
GPIO_V2_GET_LINEINFO_IOCTL and the line change notifier.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib-cdev.c | 152 ++++++++++++++++++++++++++++++------
 1 file changed, 130 insertions(+), 22 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 02ffda6c1e51..5ba900e5461a 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -21,6 +21,7 @@
 #include <linux/mutex.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/poll.h>
+#include <linux/rbtree.h>
 #include <linux/seq_file.h>
 #include <linux/spinlock.h>
 #include <linux/timekeeping.h>
@@ -461,6 +462,7 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
 
 /**
  * struct line - contains the state of a requested line
+ * @node: to store the object in supinfo_tree if supplemental
  * @desc: the GPIO descriptor for this line.
  * @req: the corresponding line request
  * @irq: the interrupt triggered in response to events on this GPIO
@@ -473,6 +475,7 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
  * @line_seqno: the seqno for the current edge event in the sequence of
  * events for this line.
  * @work: the worker that implements software debouncing
+ * @debounce_period_us: the debounce period in microseconds
  * @sw_debounced: flag indicating if the software debouncer is active
  * @level: the current debounced physical level of the line
  * @hdesc: the Hardware Timestamp Engine (HTE) descriptor
@@ -481,6 +484,7 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
  * @last_seqno: the last sequence number before debounce period expires
  */
 struct line {
+	struct rb_node node;
 	struct gpio_desc *desc;
 	/*
 	 * -- edge detector specific fields --
@@ -514,6 +518,15 @@ struct line {
 	 * -- debouncer specific fields --
 	 */
 	struct delayed_work work;
+	/*
+	 * debounce_period_us is accessed by debounce_irq_handler() and
+	 * process_hw_ts() which are disabled when modified by
+	 * debounce_setup(), edge_detector_setup() or edge_detector_stop()
+	 * or can live with a stale version when updated by
+	 * edge_detector_update().
+	 * The modifying functions are themselves mutually exclusive.
+	 */
+	unsigned int debounce_period_us;
 	/*
 	 * sw_debounce is accessed by linereq_set_config(), which is the
 	 * only setter, and linereq_get_values(), which can live with a
@@ -546,6 +559,17 @@ struct line {
 #endif /* CONFIG_HTE */
 };
 
+/*
+ * a rbtree of the struct lines containing supplemental info.
+ * Used to populate gpio_v2_line_info with cdev specific fields not contained
+ * in the struct gpio_desc.
+ * A line is determined to contain supplemental information by
+ * line_is_supplemental().
+ */
+static struct rb_root supinfo_tree = RB_ROOT;
+/* covers supinfo_tree */
+DEFINE_SPINLOCK(supinfo_lock);
+
 /**
  * struct linereq - contains the state of a userspace line request
  * @gdev: the GPIO device the line request pertains to
@@ -575,6 +599,94 @@ struct linereq {
 	struct line lines[] __counted_by(num_lines);
 };
 
+static void supinfo_insert(struct line *line)
+{
+	struct rb_node **new = &(supinfo_tree.rb_node), *parent = NULL;
+	struct line *entry;
+
+	guard(spinlock)(&supinfo_lock);
+
+	while (*new) {
+		entry = container_of(*new, struct line, node);
+
+		parent = *new;
+		if (line->desc < entry->desc) {
+			new = &((*new)->rb_left);
+		} else if (line->desc > entry->desc) {
+			new = &((*new)->rb_right);
+		} else {
+			/* this should never happen */
+			WARN(1, "duplicate line inserted");
+			return;
+		}
+	}
+
+	rb_link_node(&line->node, parent, new);
+	rb_insert_color(&line->node, &supinfo_tree);
+}
+
+static void supinfo_erase(struct line *line)
+{
+	guard(spinlock)(&supinfo_lock);
+
+	rb_erase(&line->node, &supinfo_tree);
+}
+
+static struct line *supinfo_find(struct gpio_desc *desc)
+{
+	struct rb_node *node = supinfo_tree.rb_node;
+	struct line *line;
+
+	while (node) {
+		line = container_of(node, struct line, node);
+		if (desc < line->desc)
+			node = node->rb_left;
+		else if (desc > line->desc)
+			node = node->rb_right;
+		else
+			return line;
+	}
+	return NULL;
+}
+
+static void supinfo_to_lineinfo(struct gpio_desc *desc,
+				struct gpio_v2_line_info *info)
+{
+	struct gpio_v2_line_attribute *attr;
+	struct line *line;
+
+	guard(spinlock)(&supinfo_lock);
+
+	line = supinfo_find(desc);
+	if (line) {
+		attr = &info->attrs[info->num_attrs];
+		attr->id = GPIO_V2_LINE_ATTR_ID_DEBOUNCE;
+		attr->debounce_period_us = READ_ONCE(line->debounce_period_us);
+		info->num_attrs++;
+	}
+}
+
+static inline bool line_is_supplemental(struct line *line)
+{
+	return READ_ONCE(line->debounce_period_us);
+}
+
+static void line_set_debounce_period(struct line *line,
+				     unsigned int debounce_period_us)
+{
+	bool was_suppl = line_is_supplemental(line);
+
+	WRITE_ONCE(line->debounce_period_us, debounce_period_us);
+
+	if (line_is_supplemental(line) == was_suppl)
+		return;
+
+	if (was_suppl)
+		supinfo_erase(line);
+	else
+		supinfo_insert(line);
+}
+
 #define GPIO_V2_LINE_BIAS_FLAGS \
 	(GPIO_V2_LINE_FLAG_BIAS_PULL_UP | \
 	 GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN | \
@@ -723,7 +835,7 @@ static enum hte_return process_hw_ts(struct hte_ts_data *ts, void *p)
 		line->total_discard_seq++;
 		line->last_seqno = ts->seq;
 		mod_delayed_work(system_wq, &line->work,
-		  usecs_to_jiffies(READ_ONCE(line->desc->debounce_period_us)));
+		  usecs_to_jiffies(READ_ONCE(line->debounce_period_us)));
 	} else {
 		if (unlikely(ts->seq < line->line_seqno))
 			return HTE_CB_HANDLED;
@@ -864,7 +976,7 @@ static irqreturn_t debounce_irq_handler(int irq, void *p)
 	struct line *line = p;
 
 	mod_delayed_work(system_wq, &line->work,
-		usecs_to_jiffies(READ_ONCE(line->desc->debounce_period_us)));
+		usecs_to_jiffies(READ_ONCE(line->debounce_period_us)));
 
 	return IRQ_HANDLED;
 }
@@ -946,7 +1058,7 @@ static int debounce_setup(struct line *line, unsigned int debounce_period_us)
 	/* try hardware */
 	ret = gpiod_set_debounce(line->desc, debounce_period_us);
 	if (!ret) {
-		WRITE_ONCE(line->desc->debounce_period_us, debounce_period_us);
+		line_set_debounce_period(line, debounce_period_us);
 		return ret;
 	}
 	if (ret != -ENOTSUPP)
@@ -1025,8 +1137,7 @@ static void edge_detector_stop(struct line *line)
 	cancel_delayed_work_sync(&line->work);
 	WRITE_ONCE(line->sw_debounced, 0);
 	WRITE_ONCE(line->edflags, 0);
-	if (line->desc)
-		WRITE_ONCE(line->desc->debounce_period_us, 0);
+	line_set_debounce_period(line, 0);
 	/* do not change line->level - see comment in debounced_value() */
 }
 
@@ -1051,7 +1162,7 @@ static int edge_detector_setup(struct line *line,
 		ret = debounce_setup(line, debounce_period_us);
 		if (ret)
 			return ret;
-		WRITE_ONCE(line->desc->debounce_period_us, debounce_period_us);
+		line_set_debounce_period(line, debounce_period_us);
 	}
 
 	/* detection disabled or sw debouncer will provide edge detection */
@@ -1093,12 +1204,12 @@ static int edge_detector_update(struct line *line,
 			gpio_v2_line_config_debounce_period(lc, line_idx);
 
 	if ((active_edflags == edflags) &&
-	    (READ_ONCE(line->desc->debounce_period_us) == debounce_period_us))
+	    (READ_ONCE(line->debounce_period_us) == debounce_period_us))
 		return 0;
 
 	/* sw debounced and still will be...*/
 	if (debounce_period_us && READ_ONCE(line->sw_debounced)) {
-		WRITE_ONCE(line->desc->debounce_period_us, debounce_period_us);
+		line_set_debounce_period(line, debounce_period_us);
 		return 0;
 	}
 
@@ -1573,6 +1684,7 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
 
 static void linereq_free(struct linereq *lr)
 {
+	struct line *line;
 	unsigned int i;
 
 	if (lr->device_unregistered_nb.notifier_call)
@@ -1580,10 +1692,14 @@ static void linereq_free(struct linereq *lr)
 						   &lr->device_unregistered_nb);
 
 	for (i = 0; i < lr->num_lines; i++) {
-		if (lr->lines[i].desc) {
-			edge_detector_stop(&lr->lines[i]);
-			gpiod_free(lr->lines[i].desc);
-		}
+		line = &lr->lines[i];
+		if (!line->desc)
+			continue;
+
+		edge_detector_stop(line);
+		if (line_is_supplemental(line))
+			supinfo_erase(line);
+		gpiod_free(line->desc);
 	}
 	kfifo_free(&lr->events);
 	kfree(lr->label);
@@ -2274,8 +2390,6 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 	struct gpio_chip *gc = desc->gdev->chip;
 	bool ok_for_pinctrl;
 	unsigned long flags;
-	u32 debounce_period_us;
-	unsigned int num_attrs = 0;
 
 	memset(info, 0, sizeof(*info));
 	info->offset = gpio_chip_hwgpio(desc);
@@ -2341,14 +2455,6 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 	else if (test_bit(FLAG_EVENT_CLOCK_HTE, &desc->flags))
 		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE;
 
-	debounce_period_us = READ_ONCE(desc->debounce_period_us);
-	if (debounce_period_us) {
-		info->attrs[num_attrs].id = GPIO_V2_LINE_ATTR_ID_DEBOUNCE;
-		info->attrs[num_attrs].debounce_period_us = debounce_period_us;
-		num_attrs++;
-	}
-	info->num_attrs = num_attrs;
-
 	spin_unlock_irqrestore(&gpio_lock, flags);
 }
 
@@ -2455,6 +2561,7 @@ static int lineinfo_get(struct gpio_chardev_data *cdev, void __user *ip,
 			return -EBUSY;
 	}
 	gpio_desc_to_lineinfo(desc, &lineinfo);
+	supinfo_to_lineinfo(desc, &lineinfo);
 
 	if (copy_to_user(ip, &lineinfo, sizeof(lineinfo))) {
 		if (watch)
@@ -2545,6 +2652,7 @@ static int lineinfo_changed_notify(struct notifier_block *nb,
 	chg.event_type = action;
 	chg.timestamp_ns = ktime_get_ns();
 	gpio_desc_to_lineinfo(desc, &chg.info);
+	supinfo_to_lineinfo(desc, &chg.info);
 
 	ret = kfifo_in_spinlocked(&cdev->events, &chg, 1, &cdev->wait.lock);
 	if (ret)
-- 
2.39.2

[PATCH v3 2/5] gpiolib: remove debounce_period_us from struct gpio_desc

[Kent Gibson]

cdev is the only user of the debounce_period_us field in
struct gpio_desc, and it no longer uses it, so remove it.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib.c | 3 ---
 drivers/gpio/gpiolib.h | 5 -----
 2 files changed, 8 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 4e190be75dc2..ca2216621619 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -2344,9 +2344,6 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
 		clear_bit(FLAG_IS_HOGGED, &desc->flags);
 #ifdef CONFIG_OF_DYNAMIC
 		desc->hog = NULL;
-#endif
-#ifdef CONFIG_GPIO_CDEV
-		WRITE_ONCE(desc->debounce_period_us, 0);
 #endif
 		ret = true;
 	}
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index 3ccacf3c1288..a4a2520b5f31 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -147,7 +147,6 @@ void gpiod_line_state_notify(struct gpio_desc *desc, unsigned long action);
  * @label:		Name of the consumer
  * @name:		Line name
  * @hog:		Pointer to the device node that hogs this line (if any)
- * @debounce_period_us:	Debounce period in microseconds
  *
  * These are obtained using gpiod_get() and are preferable to the old
  * integer-based handles.
@@ -185,10 +184,6 @@ struct gpio_desc {
 #ifdef CONFIG_OF_DYNAMIC
 	struct device_node	*hog;
 #endif
-#ifdef CONFIG_GPIO_CDEV
-	/* debounce period in microseconds */
-	unsigned int		debounce_period_us;
-#endif
 };
 
 #define gpiod_not_found(desc)		(IS_ERR(desc) && PTR_ERR(desc) == -ENOENT)
-- 
2.39.2

[PATCH v3 3/5] gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()

[Kent Gibson]

Reduce the time holding the gpio_lock by snapshotting the desc flags,
rather than testing them individually while holding the lock.

Accept that the calculation of the used field is inherently racy, and
only check the availability of the line from pinctrl if other checks
pass, so avoiding the check for lines that are otherwise in use.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib-cdev.c | 74 ++++++++++++++++++-------------------
 1 file changed, 36 insertions(+), 38 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 5ba900e5461a..c140bcd63361 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -2388,74 +2388,72 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 				  struct gpio_v2_line_info *info)
 {
 	struct gpio_chip *gc = desc->gdev->chip;
-	bool ok_for_pinctrl;
-	unsigned long flags;
+	unsigned long dflags;
 
 	memset(info, 0, sizeof(*info));
 	info->offset = gpio_chip_hwgpio(desc);
 
-	/*
-	 * This function takes a mutex so we must check this before taking
-	 * the spinlock.
-	 *
-	 * FIXME: find a non-racy way to retrieve this information. Maybe a
-	 * lock common to both frameworks?
-	 */
-	ok_for_pinctrl = pinctrl_gpio_can_use_line(gc, info->offset);
+	scoped_guard(spinlock_irqsave, &gpio_lock) {
+		if (desc->name)
+			strscpy(info->name, desc->name, sizeof(info->name));
 
-	spin_lock_irqsave(&gpio_lock, flags);
+		if (desc->label)
+			strscpy(info->consumer, desc->label,
+				sizeof(info->consumer));
 
-	if (desc->name)
-		strscpy(info->name, desc->name, sizeof(info->name));
-
-	if (desc->label)
-		strscpy(info->consumer, desc->label, sizeof(info->consumer));
+		dflags = READ_ONCE(desc->flags);
+	}
 
 	/*
-	 * Userspace only need to know that the kernel is using this GPIO so
-	 * it can't use it.
+	 * Userspace only need know that the kernel is using this GPIO so it
+	 * can't use it.
+	 * The calculation of the used flag is slightly racy, as it may read
+	 * desc, gc and pinctrl state without a lock covering all three at
+	 * once.  Worst case if the line is in transition and the calculation
+	 * is inconsistent then it looks to the user like they performed the
+	 * read on the other side of the transition - but that can always
+	 * happen.
+	 * The definitive test that a line is available to userspace is to
+	 * request it.
 	 */
-	info->flags = 0;
-	if (test_bit(FLAG_REQUESTED, &desc->flags) ||
-	    test_bit(FLAG_IS_HOGGED, &desc->flags) ||
-	    test_bit(FLAG_USED_AS_IRQ, &desc->flags) ||
-	    test_bit(FLAG_EXPORT, &desc->flags) ||
-	    test_bit(FLAG_SYSFS, &desc->flags) ||
+	if (test_bit(FLAG_REQUESTED, &dflags) ||
+	    test_bit(FLAG_IS_HOGGED, &dflags) ||
+	    test_bit(FLAG_USED_AS_IRQ, &dflags) ||
+	    test_bit(FLAG_EXPORT, &dflags) ||
+	    test_bit(FLAG_SYSFS, &dflags) ||
 	    !gpiochip_line_is_valid(gc, info->offset) ||
-	    !ok_for_pinctrl)
+	    !pinctrl_gpio_can_use_line(gc, info->offset))
 		info->flags |= GPIO_V2_LINE_FLAG_USED;
 
-	if (test_bit(FLAG_IS_OUT, &desc->flags))
+	if (test_bit(FLAG_IS_OUT, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OUTPUT;
 	else
 		info->flags |= GPIO_V2_LINE_FLAG_INPUT;
 
-	if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
+	if (test_bit(FLAG_ACTIVE_LOW, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_ACTIVE_LOW;
 
-	if (test_bit(FLAG_OPEN_DRAIN, &desc->flags))
+	if (test_bit(FLAG_OPEN_DRAIN, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OPEN_DRAIN;
-	if (test_bit(FLAG_OPEN_SOURCE, &desc->flags))
+	if (test_bit(FLAG_OPEN_SOURCE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OPEN_SOURCE;
 
-	if (test_bit(FLAG_BIAS_DISABLE, &desc->flags))
+	if (test_bit(FLAG_BIAS_DISABLE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_DISABLED;
-	if (test_bit(FLAG_PULL_DOWN, &desc->flags))
+	if (test_bit(FLAG_PULL_DOWN, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN;
-	if (test_bit(FLAG_PULL_UP, &desc->flags))
+	if (test_bit(FLAG_PULL_UP, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_UP;
 
-	if (test_bit(FLAG_EDGE_RISING, &desc->flags))
+	if (test_bit(FLAG_EDGE_RISING, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_RISING;
-	if (test_bit(FLAG_EDGE_FALLING, &desc->flags))
+	if (test_bit(FLAG_EDGE_FALLING, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_FALLING;
 
-	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &desc->flags))
+	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME;
-	else if (test_bit(FLAG_EVENT_CLOCK_HTE, &desc->flags))
+	else if (test_bit(FLAG_EVENT_CLOCK_HTE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE;
-
-	spin_unlock_irqrestore(&gpio_lock, flags);
 }
 
 struct gpio_chardev_data {
-- 
2.39.2

[PATCH v3 4/5] gpiolib: cdev: fully adopt guard() and scoped_guard()

[Kent Gibson]

Use guard() or scoped_guard() for critical sections rather than
discrete lock/unlock pairs.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib-cdev.c | 143 ++++++++++++++----------------------
 1 file changed, 57 insertions(+), 86 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index c140bcd63361..b956664f8649 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -737,13 +737,13 @@ static void linereq_put_event(struct linereq *lr,
 {
 	bool overflow = false;
 
-	spin_lock(&lr->wait.lock);
-	if (kfifo_is_full(&lr->events)) {
-		overflow = true;
-		kfifo_skip(&lr->events);
+	scoped_guard(spinlock, &lr->wait.lock) {
+		if (kfifo_is_full(&lr->events)) {
+			overflow = true;
+			kfifo_skip(&lr->events);
+		}
+		kfifo_in(&lr->events, le, 1);
 	}
-	kfifo_in(&lr->events, le, 1);
-	spin_unlock(&lr->wait.lock);
 	if (!overflow)
 		wake_up_poll(&lr->wait, EPOLLIN);
 	else
@@ -1476,18 +1476,13 @@ static long linereq_set_values_unlocked(struct linereq *lr,
 static long linereq_set_values(struct linereq *lr, void __user *ip)
 {
 	struct gpio_v2_line_values lv;
-	int ret;
 
 	if (copy_from_user(&lv, ip, sizeof(lv)))
 		return -EFAULT;
 
-	mutex_lock(&lr->config_mutex);
-
-	ret = linereq_set_values_unlocked(lr, &lv);
-
-	mutex_unlock(&lr->config_mutex);
+	guard(mutex)(&lr->config_mutex);
 
-	return ret;
+	return linereq_set_values_unlocked(lr, &lv);
 }
 
 static long linereq_set_config_unlocked(struct linereq *lr,
@@ -1545,13 +1540,9 @@ static long linereq_set_config(struct linereq *lr, void __user *ip)
 	if (ret)
 		return ret;
 
-	mutex_lock(&lr->config_mutex);
-
-	ret = linereq_set_config_unlocked(lr, &lc);
+	guard(mutex)(&lr->config_mutex);
 
-	mutex_unlock(&lr->config_mutex);
-
-	return ret;
+	return linereq_set_config_unlocked(lr, &lc);
 }
 
 static long linereq_ioctl_unlocked(struct file *file, unsigned int cmd,
@@ -1633,28 +1624,22 @@ static ssize_t linereq_read_unlocked(struct file *file, char __user *buf,
 		return -EINVAL;
 
 	do {
-		spin_lock(&lr->wait.lock);
-		if (kfifo_is_empty(&lr->events)) {
-			if (bytes_read) {
-				spin_unlock(&lr->wait.lock);
-				return bytes_read;
+		scoped_guard(spinlock, &lr->wait.lock) {
+			if (kfifo_is_empty(&lr->events)) {
+				if (bytes_read)
+					return bytes_read;
+
+				if (file->f_flags & O_NONBLOCK)
+					return -EAGAIN;
+
+				ret = wait_event_interruptible_locked(lr->wait,
+						!kfifo_is_empty(&lr->events));
+				if (ret)
+					return ret;
 			}
 
-			if (file->f_flags & O_NONBLOCK) {
-				spin_unlock(&lr->wait.lock);
-				return -EAGAIN;
-			}
-
-			ret = wait_event_interruptible_locked(lr->wait,
-					!kfifo_is_empty(&lr->events));
-			if (ret) {
-				spin_unlock(&lr->wait.lock);
-				return ret;
-			}
+			ret = kfifo_out(&lr->events, &le, 1);
 		}
-
-		ret = kfifo_out(&lr->events, &le, 1);
-		spin_unlock(&lr->wait.lock);
 		if (ret != 1) {
 			/*
 			 * This should never happen - we were holding the
@@ -2004,28 +1989,22 @@ static ssize_t lineevent_read_unlocked(struct file *file, char __user *buf,
 		return -EINVAL;
 
 	do {
-		spin_lock(&le->wait.lock);
-		if (kfifo_is_empty(&le->events)) {
-			if (bytes_read) {
-				spin_unlock(&le->wait.lock);
-				return bytes_read;
-			}
-
-			if (file->f_flags & O_NONBLOCK) {
-				spin_unlock(&le->wait.lock);
-				return -EAGAIN;
+		scoped_guard(spinlock, &le->wait.lock) {
+			if (kfifo_is_empty(&le->events)) {
+				if (bytes_read)
+					return bytes_read;
+
+				if (file->f_flags & O_NONBLOCK)
+					return -EAGAIN;
+
+				ret = wait_event_interruptible_locked(le->wait,
+						!kfifo_is_empty(&le->events));
+				if (ret)
+					return ret;
 			}
 
-			ret = wait_event_interruptible_locked(le->wait,
-					!kfifo_is_empty(&le->events));
-			if (ret) {
-				spin_unlock(&le->wait.lock);
-				return ret;
-			}
+			ret = kfifo_out(&le->events, &ge, 1);
 		}
-
-		ret = kfifo_out(&le->events, &ge, 1);
-		spin_unlock(&le->wait.lock);
 		if (ret != 1) {
 			/*
 			 * This should never happen - we were holding the lock
@@ -2719,38 +2698,30 @@ static ssize_t lineinfo_watch_read_unlocked(struct file *file, char __user *buf,
 #endif
 
 	do {
-		spin_lock(&cdev->wait.lock);
-		if (kfifo_is_empty(&cdev->events)) {
-			if (bytes_read) {
-				spin_unlock(&cdev->wait.lock);
-				return bytes_read;
-			}
-
-			if (file->f_flags & O_NONBLOCK) {
-				spin_unlock(&cdev->wait.lock);
-				return -EAGAIN;
-			}
-
-			ret = wait_event_interruptible_locked(cdev->wait,
-					!kfifo_is_empty(&cdev->events));
-			if (ret) {
-				spin_unlock(&cdev->wait.lock);
-				return ret;
+		scoped_guard(spinlock, &cdev->wait.lock) {
+			if (kfifo_is_empty(&cdev->events)) {
+				if (bytes_read)
+					return bytes_read;
+
+				if (file->f_flags & O_NONBLOCK)
+					return -EAGAIN;
+
+				ret = wait_event_interruptible_locked(cdev->wait,
+						!kfifo_is_empty(&cdev->events));
+				if (ret)
+					return ret;
 			}
-		}
 #ifdef CONFIG_GPIO_CDEV_V1
-		/* must be after kfifo check so watch_abi_version is set */
-		if (atomic_read(&cdev->watch_abi_version) == 2)
-			event_size = sizeof(struct gpio_v2_line_info_changed);
-		else
-			event_size = sizeof(struct gpioline_info_changed);
-		if (count < event_size) {
-			spin_unlock(&cdev->wait.lock);
-			return -EINVAL;
-		}
+			/* must be after kfifo check so watch_abi_version is set */
+			if (atomic_read(&cdev->watch_abi_version) == 2)
+				event_size = sizeof(struct gpio_v2_line_info_changed);
+			else
+				event_size = sizeof(struct gpioline_info_changed);
+			if (count < event_size)
+				return -EINVAL;
 #endif
-		ret = kfifo_out(&cdev->events, &event, 1);
-		spin_unlock(&cdev->wait.lock);
+			ret = kfifo_out(&cdev->events, &event, 1);
+		}
 		if (ret != 1) {
 			ret = -EIO;
 			break;
-- 
2.39.2

[PATCH v3 5/5] gpiolib: cdev: improve documentation of get/set values

[Kent Gibson]

Add documentation of the algorithm used to perform scatter/gather
of the requested lines and values in linereq_get_values() and
linereq_set_values_unlocked() to improve maintainability.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
---
 drivers/gpio/gpiolib-cdev.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index b956664f8649..37794de691f3 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -1383,9 +1383,18 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 	if (copy_from_user(&lv, ip, sizeof(lv)))
 		return -EFAULT;
 
+	/*
+	 * gpiod_get_array_value_complex() requires compacted desc and val
+	 * arrays, rather than the sparse ones in lv.
+	 * Calculation of num_get and construction of the desc array is
+	 * optimized to avoid allocation for the desc array for the common
+	 * num_get == 1 case.
+	 */
+	/* scan requested lines to calculate the subset to get */
 	for (num_get = 0, i = 0; i < lr->num_lines; i++) {
 		if (lv.mask & BIT_ULL(i)) {
 			num_get++;
+			/* capture desc for the num_get == 1 case */
 			descs = &lr->lines[i].desc;
 		}
 	}
@@ -1394,6 +1403,7 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 		return -EINVAL;
 
 	if (num_get != 1) {
+		/* build compacted desc array */
 		descs = kmalloc_array(num_get, sizeof(*descs), GFP_KERNEL);
 		if (!descs)
 			return -ENOMEM;
@@ -1414,6 +1424,7 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
 
 	lv.bits = 0;
 	for (didx = 0, i = 0; i < lr->num_lines; i++) {
+		/* unpack compacted vals for the response */
 		if (lv.mask & BIT_ULL(i)) {
 			if (lr->lines[i].sw_debounced)
 				val = debounced_value(&lr->lines[i]);
@@ -1439,14 +1450,25 @@ static long linereq_set_values_unlocked(struct linereq *lr,
 	unsigned int i, didx, num_set;
 	int ret;
 
+	/*
+	 * gpiod_set_array_value_complex() requires compacted desc and val
+	 * arrays, rather than the sparse ones in lv.
+	 * Calculation of num_set and construction of the descs and vals arrays
+	 * is optimized to minimize scanning the lv->mask, and to avoid
+	 * allocation for the desc array for the common num_set == 1 case.
+	 */
 	bitmap_zero(vals, GPIO_V2_LINES_MAX);
+	/* scan requested lines to determine the subset to be set */
 	for (num_set = 0, i = 0; i < lr->num_lines; i++) {
 		if (lv->mask & BIT_ULL(i)) {
+			/* setting inputs is not allowed */
 			if (!test_bit(FLAG_IS_OUT, &lr->lines[i].desc->flags))
 				return -EPERM;
+			/* add to compacted values */
 			if (lv->bits & BIT_ULL(i))
 				__set_bit(num_set, vals);
 			num_set++;
+			/* capture desc for the num_set == 1 case */
 			descs = &lr->lines[i].desc;
 		}
 	}
@@ -1454,7 +1476,7 @@ static long linereq_set_values_unlocked(struct linereq *lr,
 		return -EINVAL;
 
 	if (num_set != 1) {
-		/* build compacted desc array and values */
+		/* build compacted desc array */
 		descs = kmalloc_array(num_set, sizeof(*descs), GFP_KERNEL);
 		if (!descs)
 			return -ENOMEM;
-- 
2.39.2

Re: [PATCH v3 1/5] gpiolib: cdev: relocate debounce_period_us from struct gpio_desc

[Andy Shevchenko]

On Fri, Dec 15, 2023 at 10:38:01AM +0800, Kent Gibson wrote:
> Store the debounce period for a requested line locally, rather than in
> the debounce_period_us field in the gpiolib struct gpio_desc.
> 
> Add a global tree of lines containing supplemental line information
> to make the debounce period available to be reported by the
> GPIO_V2_GET_LINEINFO_IOCTL and the line change notifier.

LGTM, a few minor comments below.

...

> +/*

(now you can have a kernel doc :-)

> + * a rbtree of the struct lines containing supplemental info.
> + * Used to populate gpio_v2_line_info with cdev specific fields not contained
> + * in the struct gpio_desc.
> + * A line is determined to contain supplemental information by
> + * line_is_supplemental().
> + */
> +static struct rb_root supinfo_tree = RB_ROOT;
> +/* covers supinfo_tree */
> +DEFINE_SPINLOCK(supinfo_lock);

Shouldn't this also be static?

...

> +	guard(spinlock)(&supinfo_lock);

+ cleanup.h ?

...

> +static void supinfo_to_lineinfo(struct gpio_desc *desc,
> +				struct gpio_v2_line_info *info)
> +{
> +	struct gpio_v2_line_attribute *attr;
> +	struct line *line;
> +
> +	guard(spinlock)(&supinfo_lock);
> +
> +	line = supinfo_find(desc);
> +	if (line) {

	if (!line)
		return;

?

> +		attr = &info->attrs[info->num_attrs];
> +		attr->id = GPIO_V2_LINE_ATTR_ID_DEBOUNCE;
> +		attr->debounce_period_us = READ_ONCE(line->debounce_period_us);
> +		info->num_attrs++;
> +	}
> +}

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v3 3/5] gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()

[Andy Shevchenko]

On Fri, Dec 15, 2023 at 10:38:03AM +0800, Kent Gibson wrote:
> Reduce the time holding the gpio_lock by snapshotting the desc flags,
> rather than testing them individually while holding the lock.
> 
> Accept that the calculation of the used field is inherently racy, and
> only check the availability of the line from pinctrl if other checks
> pass, so avoiding the check for lines that are otherwise in use.

Just wondering if you are using --histogram (diff algo) when preparing
the patches...

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v3 0/5] gpiolib: cdev: relocate debounce_period_us

[Andy Shevchenko]

On Fri, Dec 15, 2023 at 10:38:00AM +0800, Kent Gibson wrote:
> This series contains minor improvements to gpiolib-cdev.
> 
> The banner change is relocating the debounce_period_us from gpiolib's
> struct gpio_desc to cdev's struct line.  Patch 1 stores the field
> locally in cdev.  Patch 2 removes the now unused field from gpiolib.
> 
> Patch 3 is somewhat related and removes a FIXME from
> gpio_desc_to_lineinfo().  The FIXME relates to a race condition in
> the calculation of the used flag, but I would assert that from
> the userspace perspective the read operation itself is inherently racy.
> The line being reported as unused in the info provides no guarantee -
> it just an indicator that requesting the line is likely to succeed -
> assuming the line is not otherwise requested in the meantime.
> Given the overall operation is racy, trying to stamp out an unlikely
> race within the operation is pointless. Accept it as a possibility
> that has negligible side-effects and reduce the number of locks held
> simultaneously and the duration that the gpio_lock is held.
> 
> Patches 1 and 3 introduce usage of guard() and scoped_guard() to cdev.
> Patch 4 replaces any remaining discrete lock/unlock calls around
> critical sections with guard() or scoped_guard().
> 
> Patch 5 is unrelated to debounce or info, but addresses Andy's
> recent lamentation that the linereq get/set values functions are
> confusing and under documented.
> Figured I may as well add that while I was in there.

This version LGTM, few minor comments here and there, but feel free to add

Reviewed-by: Andy Shevchenko <andy@kernel.org>

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v3 3/5] gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()

[Kent Gibson]

On Fri, Dec 15, 2023 at 06:36:23PM +0200, Andy Shevchenko wrote:
> On Fri, Dec 15, 2023 at 10:38:03AM +0800, Kent Gibson wrote:
> > Reduce the time holding the gpio_lock by snapshotting the desc flags,
> > rather than testing them individually while holding the lock.
> >
> > Accept that the calculation of the used field is inherently racy, and
> > only check the availability of the line from pinctrl if other checks
> > pass, so avoiding the check for lines that are otherwise in use.
>
> Just wondering if you are using --histogram (diff algo) when preparing
> the patches...
>

Was just using the default myers.
Only changes patch 4, which it does make slightly more readable, so
will use it for v4.

Cheers,
Kent.

Re: [PATCH v3 1/5] gpiolib: cdev: relocate debounce_period_us from struct gpio_desc

[Kent Gibson]

On Fri, Dec 15, 2023 at 06:35:14PM +0200, Andy Shevchenko wrote:
> On Fri, Dec 15, 2023 at 10:38:01AM +0800, Kent Gibson wrote:
> > Store the debounce period for a requested line locally, rather than in
> > the debounce_period_us field in the gpiolib struct gpio_desc.
> >
> > Add a global tree of lines containing supplemental line information
> > to make the debounce period available to be reported by the
> > GPIO_V2_GET_LINEINFO_IOCTL and the line change notifier.
>
> LGTM, a few minor comments below.
>
> ...
>
> > +/*
>
> (now you can have a kernel doc :-)
>

Maybe you can, but I can't.

> > + * a rbtree of the struct lines containing supplemental info.
> > + * Used to populate gpio_v2_line_info with cdev specific fields not contained
> > + * in the struct gpio_desc.
> > + * A line is determined to contain supplemental information by
> > + * line_is_supplemental().
> > + */
> > +static struct rb_root supinfo_tree = RB_ROOT;
> > +/* covers supinfo_tree */
> > +DEFINE_SPINLOCK(supinfo_lock);
>
> Shouldn't this also be static?
>

Indeed.

> ...
>
> > +	guard(spinlock)(&supinfo_lock);
>
> + cleanup.h ?
>

Bah, I could've sworn I added that in, but it isn't evident in any of
the patches all the way back to v1, so apparently not.

> ...
>
> > +static void supinfo_to_lineinfo(struct gpio_desc *desc,
> > +				struct gpio_v2_line_info *info)
> > +{
> > +	struct gpio_v2_line_attribute *attr;
> > +	struct line *line;
> > +
> > +	guard(spinlock)(&supinfo_lock);
> > +
> > +	line = supinfo_find(desc);
> > +	if (line) {
>
> 	if (!line)
> 		return;
>
> ?

Will do.

>
> > +		attr = &info->attrs[info->num_attrs];
> > +		attr->id = GPIO_V2_LINE_ATTR_ID_DEBOUNCE;
> > +		attr->debounce_period_us = READ_ONCE(line->debounce_period_us);
> > +		info->num_attrs++;
> > +	}
> > +}
>

Cheers,
Kent.

> --
> With Best Regards,
> Andy Shevchenko
>
>