gpiolib: add support for changing value polarity in sysfs

Drivers may use gpiolib sysfs as part of their public user space
interface. The GPIO number and polarity might change from board to
board. The gpio_export_link() call can be used to hide the GPIO number
from user space. Add support for also hiding the GPIO line polarity
changes from user space.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/gpio.txt

@@ -531,6 +531,13 @@ and have the following read/write attributes:
 		This file exists only if the pin can be configured as an
 		interrupt generating input pin.
 
+	"active_low" ... reads as either 0 (false) or 1 (true).  Write
+		any nonzero value to invert the value attribute both
+		for reading and writing.  Existing and subsequent
+		poll(2) support configuration via the edge attribute
+		for "rising" and "falling" edges will follow this
+		setting.
+
 GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
 controller implementing GPIOs starting at #42) and have the following
 read-only attributes:
@@ -566,6 +573,8 @@ requested using gpio_request():
 	int gpio_export_link(struct device *dev, const char *name,
 		unsigned gpio)
 
+	/* change the polarity of a GPIO node in sysfs */
+	int gpio_sysfs_set_active_low(unsigned gpio, int value);
 
 After a kernel driver requests a GPIO, it may only be made available in
 the sysfs interface by gpio_export().  The driver can control whether the
@@ -580,3 +589,9 @@ After the GPIO has been exported, gpio_export_link() allows creating
 symlinks from elsewhere in sysfs to the GPIO sysfs node.  Drivers can
 use this to provide the interface under their own device in sysfs with
 a descriptive name.
+
+Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
+differences between boards from user space.  This only affects the
+sysfs interface.  Polarity change can be done both before and after
+gpio_export(), and previously enabled poll(2) support for either
+rising or falling edge will be reconfigured to follow this setting.

drivers/gpio/gpiolib.c

@@ -53,6 +53,7 @@ struct gpio_desc {
 #define FLAG_SYSFS	4	/* exported via /sys/class/gpio/control */
 #define FLAG_TRIG_FALL	5	/* trigger on falling edge */
 #define FLAG_TRIG_RISE	6	/* trigger on rising edge */
+#define FLAG_ACTIVE_LOW	7	/* sysfs value has active low */
 
 #define PDESC_ID_SHIFT	16	/* add new flags before this one */
 
@@ -210,6 +211,11 @@ static DEFINE_MUTEX(sysfs_lock);
  *      * configures behavior of poll(2) on /value
  *      * available only if pin can generate IRQs on input
  *      * is read/write as "none", "falling", "rising", or "both"
+ *   /active_low
+ *      * configures polarity of /value
+ *      * is read/write as zero/nonzero
+ *      * also affects existing and subsequent "falling" and "rising"
+ *        /edge configuration
  */
 
 static ssize_t gpio_direction_show(struct device *dev,
@@ -255,7 +261,7 @@ static ssize_t gpio_direction_store(struct device *dev,
 	return status ? : size;
 }
 
-static const DEVICE_ATTR(direction, 0644,
+static /* const */ DEVICE_ATTR(direction, 0644,
 		gpio_direction_show, gpio_direction_store);
 
 static ssize_t gpio_value_show(struct device *dev,
@@ -267,10 +273,17 @@ static ssize_t gpio_value_show(struct device *dev,
 
 	mutex_lock(&sysfs_lock);
 
-	if (!test_bit(FLAG_EXPORT, &desc->flags))
+	if (!test_bit(FLAG_EXPORT, &desc->flags)) {
 		status = -EIO;
-	else
-		status = sprintf(buf, "%d\n", !!gpio_get_value_cansleep(gpio));
+	} else {
+		int value;
+
+		value = !!gpio_get_value_cansleep(gpio);
+		if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
+			value = !value;
+
+		status = sprintf(buf, "%d\n", value);
+	}
 
 	mutex_unlock(&sysfs_lock);
 	return status;
@@ -294,6 +307,8 @@ static ssize_t gpio_value_store(struct device *dev,
 
 		status = strict_strtol(buf, 0, &value);
 		if (status == 0) {
+			if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
+				value = !value;
 			gpio_set_value_cansleep(gpio, value != 0);
 			status = size;
 		}
@@ -303,7 +318,7 @@ static ssize_t gpio_value_store(struct device *dev,
 	return status;
 }
 
-static /*const*/ DEVICE_ATTR(value, 0644,
+static const DEVICE_ATTR(value, 0644,
 		gpio_value_show, gpio_value_store);
 
 static irqreturn_t gpio_sysfs_irq(int irq, void *priv)
@@ -352,9 +367,11 @@ static int gpio_setup_irq(struct gpio_desc *desc, struct device *dev,
 
 	irq_flags = IRQF_SHARED;
 	if (test_bit(FLAG_TRIG_FALL, &gpio_flags))
-		irq_flags |= IRQF_TRIGGER_FALLING;
+		irq_flags |= test_bit(FLAG_ACTIVE_LOW, &desc->flags) ?
+			IRQF_TRIGGER_RISING : IRQF_TRIGGER_FALLING;
 	if (test_bit(FLAG_TRIG_RISE, &gpio_flags))
-		irq_flags |= IRQF_TRIGGER_RISING;
+		irq_flags |= test_bit(FLAG_ACTIVE_LOW, &desc->flags) ?
+			IRQF_TRIGGER_FALLING : IRQF_TRIGGER_RISING;
 
 	if (!pdesc) {
 		pdesc = kmalloc(sizeof(*pdesc), GFP_KERNEL);
@@ -475,9 +492,79 @@ static ssize_t gpio_edge_store(struct device *dev,
 
 static DEVICE_ATTR(edge, 0644, gpio_edge_show, gpio_edge_store);
 
+static int sysfs_set_active_low(struct gpio_desc *desc, struct device *dev,
+				int value)
+{
+	int			status = 0;
+
+	if (!!test_bit(FLAG_ACTIVE_LOW, &desc->flags) == !!value)
+		return 0;
+
+	if (value)
+		set_bit(FLAG_ACTIVE_LOW, &desc->flags);
+	else
+		clear_bit(FLAG_ACTIVE_LOW, &desc->flags);
+
+	/* reconfigure poll(2) support if enabled on one edge only */
+	if (dev != NULL && (!!test_bit(FLAG_TRIG_RISE, &desc->flags) ^
+				!!test_bit(FLAG_TRIG_FALL, &desc->flags))) {
+		unsigned long trigger_flags = desc->flags & GPIO_TRIGGER_MASK;
+
+		gpio_setup_irq(desc, dev, 0);
+		status = gpio_setup_irq(desc, dev, trigger_flags);
+	}
+
+	return status;
+}
+
+static ssize_t gpio_active_low_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	const struct gpio_desc	*desc = dev_get_drvdata(dev);
+	ssize_t			status;
+
+	mutex_lock(&sysfs_lock);
+
+	if (!test_bit(FLAG_EXPORT, &desc->flags))
+		status = -EIO;
+	else
+		status = sprintf(buf, "%d\n",
+				!!test_bit(FLAG_ACTIVE_LOW, &desc->flags));
+
+	mutex_unlock(&sysfs_lock);
+
+	return status;
+}
+
+static ssize_t gpio_active_low_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t size)
+{
+	struct gpio_desc	*desc = dev_get_drvdata(dev);
+	ssize_t			status;
+
+	mutex_lock(&sysfs_lock);
+
+	if (!test_bit(FLAG_EXPORT, &desc->flags)) {
+		status = -EIO;
+	} else {
+		long		value;
+
+		status = strict_strtol(buf, 0, &value);
+		if (status == 0)
+			status = sysfs_set_active_low(desc, dev, value != 0);
+	}
+
+	mutex_unlock(&sysfs_lock);
+
+	return status ? : size;
+}
+
+static const DEVICE_ATTR(active_low, 0644,
+		gpio_active_low_show, gpio_active_low_store);
+
 static const struct attribute *gpio_attrs[] = {
-	&dev_attr_direction.attr,
 	&dev_attr_value.attr,
+	&dev_attr_active_low.attr,
 	NULL,
 };
 
@@ -662,12 +749,12 @@ int gpio_export(unsigned gpio, bool direction_may_change)
 		dev = device_create(&gpio_class, desc->chip->dev, MKDEV(0, 0),
 				desc, ioname ? ioname : "gpio%d", gpio);
 		if (!IS_ERR(dev)) {
-			if (direction_may_change)
-				status = sysfs_create_group(&dev->kobj,
+			status = sysfs_create_group(&dev->kobj,
 						&gpio_attr_group);
-			else
+
+			if (!status && direction_may_change)
 				status = device_create_file(dev,
-						&dev_attr_value);
+						&dev_attr_direction);
 
 			if (!status && gpio_to_irq(gpio) >= 0
 					&& (direction_may_change
@@ -744,6 +831,55 @@ int gpio_export_link(struct device *dev, const char *name, unsigned gpio)
 }
 EXPORT_SYMBOL_GPL(gpio_export_link);
 
+
+/**
+ * gpio_sysfs_set_active_low - set the polarity of gpio sysfs value
+ * @gpio: gpio to change
+ * @value: non-zero to use active low, i.e. inverted values
+ *
+ * Set the polarity of /sys/class/gpio/gpioN/value sysfs attribute.
+ * The GPIO does not have to be exported yet.  If poll(2) support has
+ * been enabled for either rising or falling edge, it will be
+ * reconfigured to follow the new polarity.
+ *
+ * Returns zero on success, else an error.
+ */
+int gpio_sysfs_set_active_low(unsigned gpio, int value)
+{
+	struct gpio_desc	*desc;
+	struct device		*dev = NULL;
+	int			status = -EINVAL;
+
+	if (!gpio_is_valid(gpio))
+		goto done;
+
+	mutex_lock(&sysfs_lock);
+
+	desc = &gpio_desc[gpio];
+
+	if (test_bit(FLAG_EXPORT, &desc->flags)) {
+		struct device *dev;
+
+		dev = class_find_device(&gpio_class, NULL, desc, match_export);
+		if (dev == NULL) {
+			status = -ENODEV;
+			goto unlock;
+		}
+	}
+
+	status = sysfs_set_active_low(desc, dev, value);
+
+unlock:
+	mutex_unlock(&sysfs_lock);
+
+done:
+	if (status)
+		pr_debug("%s: gpio%d status %d\n", __func__, gpio, status);
+
+	return status;
+}
+EXPORT_SYMBOL_GPL(gpio_sysfs_set_active_low);
+
 /**
  * gpio_unexport - reverse effect of gpio_export()
  * @gpio: gpio to make unavailable
@@ -1094,6 +1230,7 @@ void gpio_free(unsigned gpio)
 		}
 		desc_set_label(desc, NULL);
 		module_put(desc->chip->owner);
+		clear_bit(FLAG_ACTIVE_LOW, &desc->flags);
 		clear_bit(FLAG_REQUESTED, &desc->flags);
 	} else
 		WARN_ON(extra_checks);

include/asm-generic/gpio.h

@@ -145,6 +145,7 @@ extern int __gpio_to_irq(unsigned gpio);
 extern int gpio_export(unsigned gpio, bool direction_may_change);
 extern int gpio_export_link(struct device *dev, const char *name,
 			unsigned gpio);
+extern int gpio_sysfs_set_active_low(unsigned gpio, int value);
 extern void gpio_unexport(unsigned gpio);
 
 #endif	/* CONFIG_GPIO_SYSFS */
@@ -197,6 +198,11 @@ static inline int gpio_export_link(struct device *dev, const char *name,
 	return -ENOSYS;
 }
 
+static inline int gpio_sysfs_set_active_low(unsigned gpio, int value)
+{
+	return -ENOSYS;
+}
+
 static inline void gpio_unexport(unsigned gpio)
 {
 }

include/linux/gpio.h

@@ -99,6 +99,12 @@ static inline int gpio_export_link(struct device *dev, const char *name,
 	return -EINVAL;
 }
 
+static inline int gpio_sysfs_set_active_low(unsigned gpio, int value)
+{
+	/* GPIO can never have been requested */
+	WARN_ON(1);
+	return -EINVAL;
+}
 
 static inline void gpio_unexport(unsigned gpio)
 {