iio: Add output buffer support

Currently IIO only supports buffer mode for capture devices like ADCs. Add
support for buffered mode for output devices like DACs.

The output buffer implementation is analogous to the input buffer
implementation. Instead of using read() to get data from the buffer write()
is used to copy data into the buffer.

poll() with POLLOUT will wakeup if there is space available.

Drivers can remove data from a buffer using iio_pop_from_buffer(), the
function can e.g. called from a trigger handler to write the data to
hardware.

A buffer can only be either a output buffer or an input, but not both. So,
for a device that has an ADC and DAC path, this will mean 2 IIO buffers
(one for each direction).

The direction of the buffer is decided by the new direction field of the
iio_buffer struct and should be set after allocating and before registering
it.

Co-developed-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Co-developed-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Mihail Chindris <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/iio_core.h

···
 				 struct poll_table_struct *wait);
 ssize_t iio_buffer_read_wrapper(struct file *filp, char __user *buf,
 				size_t n, loff_t *f_ps);
+ssize_t iio_buffer_write_wrapper(struct file *filp, const char __user *buf,
+				 size_t n, loff_t *f_ps);
 
 int iio_buffers_alloc_sysfs_and_mask(struct iio_dev *indio_dev);
 void iio_buffers_free_sysfs_and_mask(struct iio_dev *indio_dev);
 
 #define iio_buffer_poll_addr (&iio_buffer_poll_wrapper)
 #define iio_buffer_read_outer_addr (&iio_buffer_read_wrapper)
+#define iio_buffer_write_outer_addr (&iio_buffer_write_wrapper)
 
 void iio_disable_all_buffers(struct iio_dev *indio_dev);
 void iio_buffer_wakeup_poll(struct iio_dev *indio_dev);
···
 
 #define iio_buffer_poll_addr NULL
 #define iio_buffer_read_outer_addr NULL
+#define iio_buffer_write_outer_addr NULL
 
 static inline int iio_buffers_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
 {

drivers/iio/industrialio-buffer.c

···
 	if (!rb || !rb->access->read)
 		return -EINVAL;
 
+	if (rb->direction != IIO_BUFFER_DIRECTION_IN)
+		return -EPERM;
+
 	datum_size = rb->bytes_per_datum;
 
 	/*
···
 	return ret;
 }
 
+static size_t iio_buffer_space_available(struct iio_buffer *buf)
+{
+	if (buf->access->space_available)
+		return buf->access->space_available(buf);
+
+	return SIZE_MAX;
+}
+
+static ssize_t iio_buffer_write(struct file *filp, const char __user *buf,
+				size_t n, loff_t *f_ps)
+{
+	struct iio_dev_buffer_pair *ib = filp->private_data;
+	struct iio_buffer *rb = ib->buffer;
+	struct iio_dev *indio_dev = ib->indio_dev;
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	int ret;
+	size_t written;
+
+	if (!indio_dev->info)
+		return -ENODEV;
+
+	if (!rb || !rb->access->write)
+		return -EINVAL;
+
+	if (rb->direction != IIO_BUFFER_DIRECTION_OUT)
+		return -EPERM;
+
+	written = 0;
+	add_wait_queue(&rb->pollq, &wait);
+	do {
+		if (indio_dev->info == NULL)
+			return -ENODEV;
+
+		if (!iio_buffer_space_available(rb)) {
+			if (signal_pending(current)) {
+				ret = -ERESTARTSYS;
+				break;
+			}
+
+			wait_woken(&wait, TASK_INTERRUPTIBLE,
+					MAX_SCHEDULE_TIMEOUT);
+			continue;
+		}
+
+		ret = rb->access->write(rb, n - written, buf + written);
+		if (ret == 0 && (filp->f_flags & O_NONBLOCK))
+			ret = -EAGAIN;
+
+		if (ret > 0) {
+			written += ret;
+			if (written != n && !(filp->f_flags & O_NONBLOCK))
+				continue;
+		}
+	} while (ret == 0);
+	remove_wait_queue(&rb->pollq, &wait);
+
+	return ret < 0 ? ret : n;
+}
+
 /**
  * iio_buffer_poll() - poll the buffer to find out if it has data
  * @filp:	File structure pointer for device access
···
 		return 0;
 
 	poll_wait(filp, &rb->pollq, wait);
-	if (iio_buffer_ready(indio_dev, rb, rb->watermark, 0))
-		return EPOLLIN | EPOLLRDNORM;
+
+	switch (rb->direction) {
+	case IIO_BUFFER_DIRECTION_IN:
+		if (iio_buffer_ready(indio_dev, rb, rb->watermark, 0))
+			return EPOLLIN | EPOLLRDNORM;
+		break;
+	case IIO_BUFFER_DIRECTION_OUT:
+		if (iio_buffer_space_available(rb))
+			return EPOLLOUT | EPOLLWRNORM;
+		break;
+	}
+
 	return 0;
 }
 
···
 		return -EBUSY;
 
 	return iio_buffer_read(filp, buf, n, f_ps);
+}
+
+ssize_t iio_buffer_write_wrapper(struct file *filp, const char __user *buf,
+				 size_t n, loff_t *f_ps)
+{
+	struct iio_dev_buffer_pair *ib = filp->private_data;
+	struct iio_buffer *rb = ib->buffer;
+
+	/* check if buffer was opened through new API */
+	if (test_bit(IIO_BUSY_BIT_POS, &rb->flags))
+		return -EBUSY;
+
+	return iio_buffer_write(filp, buf, n, f_ps);
 }
 
 __poll_t iio_buffer_poll_wrapper(struct file *filp,
···
 		wake_up(&buffer->pollq);
 	}
 }
+
+int iio_pop_from_buffer(struct iio_buffer *buffer, void *data)
+{
+	if (!buffer || !buffer->access || !buffer->access->remove_from)
+		return -EINVAL;
+
+	return buffer->access->remove_from(buffer, data);
+}
+EXPORT_SYMBOL_GPL(iio_pop_from_buffer);
 
 void iio_buffer_init(struct iio_buffer *buffer)
 {
···
 	if (insert_buffer == remove_buffer)
 		return 0;
 
+	if (insert_buffer &&
+	    (insert_buffer->direction == IIO_BUFFER_DIRECTION_OUT))
+		return -EINVAL;
+
 	mutex_lock(&iio_dev_opaque->info_exist_lock);
 	mutex_lock(&indio_dev->mlock);
 
···
 	return sysfs_emit(buf, "%zu\n", iio_buffer_data_available(buffer));
 }
 
+static ssize_t direction_show(struct device *dev,
+			      struct device_attribute *attr,
+			      char *buf)
+{
+	struct iio_buffer *buffer = to_iio_dev_attr(attr)->buffer;
+
+	switch (buffer->direction) {
+	case IIO_BUFFER_DIRECTION_IN:
+		return sprintf(buf, "in\n");
+	case IIO_BUFFER_DIRECTION_OUT:
+		return sprintf(buf, "out\n");
+	default:
+		return -EINVAL;
+	}
+}
+
 static DEVICE_ATTR(length, S_IRUGO | S_IWUSR, iio_buffer_read_length,
 		   iio_buffer_write_length);
 static struct device_attribute dev_attr_length_ro = __ATTR(length,
···
 	S_IRUGO, iio_buffer_show_watermark, NULL);
 static DEVICE_ATTR(data_available, S_IRUGO,
 		iio_dma_show_data_available, NULL);
+static DEVICE_ATTR_RO(direction);
 
+/*
+ * When adding new attributes here, put the at the end, at least until
+ * the code that handles the length/length_ro & watermark/watermark_ro
+ * assignments gets cleaned up. Otherwise these can create some weird
+ * duplicate attributes errors under some setups.
+ */
 static struct attribute *iio_buffer_attrs[] = {
 	&dev_attr_length.attr,
 	&dev_attr_enable.attr,
 	&dev_attr_watermark.attr,
 	&dev_attr_data_available.attr,
+	&dev_attr_direction.attr,
 };
 
 #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)
···
 	.owner = THIS_MODULE,
 	.llseek = noop_llseek,
 	.read = iio_buffer_read,
+	.write = iio_buffer_write,
 	.poll = iio_buffer_poll,
 	.release = iio_buffer_chrdev_release,
 };

drivers/iio/industrialio-core.c

···
 	.owner = THIS_MODULE,
 	.llseek = noop_llseek,
 	.read = iio_buffer_read_outer_addr,
+	.write = iio_buffer_write_outer_addr,
 	.poll = iio_buffer_poll_addr,
 	.unlocked_ioctl = iio_ioctl,
 	.compat_ioctl = compat_ptr_ioctl,

include/linux/iio/buffer.h

···
 
 struct iio_buffer;
 
+enum iio_buffer_direction {
+	IIO_BUFFER_DIRECTION_IN,
+	IIO_BUFFER_DIRECTION_OUT,
+};
+
 int iio_push_to_buffers(struct iio_dev *indio_dev, const void *data);
+
+int iio_pop_from_buffer(struct iio_buffer *buffer, void *data);
 
 /**
  * iio_push_to_buffers_with_timestamp() - push data and timestamp to buffers

include/linux/iio/buffer_impl.h

···
 #ifdef CONFIG_IIO_BUFFER
 
 #include <uapi/linux/iio/buffer.h>
+#include <linux/iio/buffer.h>
 
 struct iio_dev;
 struct iio_buffer;
···
  * @read:		try to get a specified number of bytes (must exist)
  * @data_available:	indicates how much data is available for reading from
  *			the buffer.
+ * @remove_from:	remove scan from buffer. Drivers should calls this to
+ *			remove a scan from a buffer.
+ * @write:		try to write a number of bytes
+ * @space_available:	returns the amount of bytes available in a buffer
  * @request_update:	if a parameter change has been marked, update underlying
  *			storage.
  * @set_bytes_per_datum:set number of bytes per datum
···
 	int (*store_to)(struct iio_buffer *buffer, const void *data);
 	int (*read)(struct iio_buffer *buffer, size_t n, char __user *buf);
 	size_t (*data_available)(struct iio_buffer *buffer);
+	int (*remove_from)(struct iio_buffer *buffer, void *data);
+	int (*write)(struct iio_buffer *buffer, size_t n, const char __user *buf);
+	size_t (*space_available)(struct iio_buffer *buffer);
 
 	int (*request_update)(struct iio_buffer *buffer);
 
···
 
 	/**  @bytes_per_datum: Size of individual datum including timestamp. */
 	size_t bytes_per_datum;
+
+	/* @direction: Direction of the data stream (in/out). */
+	enum iio_buffer_direction direction;
 
 	/**
 	 * @access: Buffer access functions associated with the