iio: kfifo-buffer: Add output buffer support

Add output buffer support to the kfifo buffer implementation.

The implementation is straight forward and mostly just wraps the kfifo
API to provide the required operations.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Mihail Chindris <mihail.chindris@analog.com>
Link: https://lore.kernel.org/r/20211007080035.2531-3-mihail.chindris@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/buffer/kfifo_buf.c

@@ -138,10 +138,60 @@ static void iio_kfifo_buffer_release(struct iio_buffer *buffer)
 	kfree(kf);
 }
 
+static size_t iio_kfifo_buf_space_available(struct iio_buffer *r)
+{
+	struct iio_kfifo *kf = iio_to_kfifo(r);
+	size_t avail;
+
+	mutex_lock(&kf->user_lock);
+	avail = kfifo_avail(&kf->kf);
+	mutex_unlock(&kf->user_lock);
+
+	return avail;
+}
+
+static int iio_kfifo_remove_from(struct iio_buffer *r, void *data)
+{
+	int ret;
+	struct iio_kfifo *kf = iio_to_kfifo(r);
+
+	if (kfifo_size(&kf->kf) < 1)
+		return -EBUSY;
+
+	ret = kfifo_out(&kf->kf, data, 1);
+	if (ret != 1)
+		return -EBUSY;
+
+	wake_up_interruptible_poll(&r->pollq, EPOLLOUT | EPOLLWRNORM);
+
+	return 0;
+}
+
+static int iio_kfifo_write(struct iio_buffer *r, size_t n,
+	const char __user *buf)
+{
+	struct iio_kfifo *kf = iio_to_kfifo(r);
+	int ret, copied;
+
+	mutex_lock(&kf->user_lock);
+	if (!kfifo_initialized(&kf->kf) || n < kfifo_esize(&kf->kf))
+		ret = -EINVAL;
+	else
+		ret = kfifo_from_user(&kf->kf, buf, n, &copied);
+	mutex_unlock(&kf->user_lock);
+	if (ret)
+		return ret;
+
+	return copied;
+}
+
 static const struct iio_buffer_access_funcs kfifo_access_funcs = {
 	.store_to = &iio_store_to_kfifo,
 	.read = &iio_read_kfifo,
 	.data_available = iio_kfifo_buf_data_available,
+	.remove_from = &iio_kfifo_remove_from,
+	.write = &iio_kfifo_write,
+	.space_available = &iio_kfifo_buf_space_available,
 	.request_update = &iio_request_update_kfifo,
 	.set_bytes_per_datum = &iio_set_bytes_per_datum_kfifo,
 	.set_length = &iio_set_length_kfifo,