#ifndef _LINUX_VIRTIO_H
  #define _LINUX_VIRTIO_H
  /* Everything a virtio driver needs to work with any particular virtio
   * implementation. */
  #include <linux/types.h>
  #include <linux/scatterlist.h>
  #include <linux/spinlock.h>
  #include <linux/device.h>
  #include <linux/mod_devicetable.h>

  #include <linux/gfp.h>

  
  /**
   * virtqueue - a queue to register buffers for sending or receiving.

   * @list: the chain of virtqueues for this device

   * @callback: the function to call when buffers are consumed (can be NULL).

   * @name: the name of this virtqueue (mainly for debugging)

   * @vdev: the virtio device this queue was created for.

   * @priv: a pointer for the virtqueue implementation to use.
   */

  struct virtqueue {
  	struct list_head list;

  	void (*callback)(struct virtqueue *vq);

  	const char *name;

  	struct virtio_device *vdev;

  	void *priv;
  };
  
  /**

   * operations for virtqueue
   * virtqueue_add_buf: expose buffer to other end

   *	vq: the struct virtqueue we're talking about.
   *	sg: the description of the buffer(s).
   *	out_num: the number of sg readable by other side
   *	in_num: the number of sg which are writable (after readable ones)
   *	data: the token identifying the buffer.

   *	gfp: how to do memory allocations (if necessary).

   *      Returns remaining capacity of queue (sg segments) or a negative error.

   * virtqueue_kick: update after add_buf

   *	vq: the struct virtqueue
   *	After one or more add_buf calls, invoke this to kick the other side.

   * virtqueue_get_buf: get the next used buffer

   *	vq: the struct virtqueue we're talking about.
   *	len: the length written into the buffer
   *	Returns NULL or the "data" token handed to add_buf.

   * virtqueue_disable_cb: disable callbacks

   *	vq: the struct virtqueue we're talking about.

   *	Note that this is not necessarily synchronous, hence unreliable and only
   *	useful as an optimization.

   * virtqueue_enable_cb: restart callbacks after disable_cb.

   *	vq: the struct virtqueue we're talking about.

   *	This re-enables callbacks; it returns "false" if there are pending
   *	buffers in the queue, to detect a possible race between the driver
   *	checking for more work, and enabling callbacks.

   * virtqueue_enable_cb_delayed: restart callbacks after disable_cb.
   *	vq: the struct virtqueue we're talking about.
   *	This re-enables callbacks but hints to the other side to delay
   *	interrupts until most of the available buffers have been processed;
   *	it returns "false" if there are many pending buffers in the queue,
   *	to detect a possible race between the driver checking for more work,
   *	and enabling callbacks.

   * virtqueue_detach_unused_buf: detach first unused buffer

   * 	vq: the struct virtqueue we're talking about.
   * 	Returns NULL or the "data" token handed to add_buf

   * virtqueue_get_vring_size: return the size of the virtqueue's vring
   *	vq: the struct virtqueue containing the vring of interest.
   *	Returns the size of the vring.

   *
   * Locking rules are straightforward: the driver is responsible for

   * locking.  No two operations may be invoked simultaneously, with the exception

   * of virtqueue_disable_cb.

   *
   * All operations can be called in any context.
   */

  int virtqueue_add_buf_gfp(struct virtqueue *vq,
  			  struct scatterlist sg[],
  			  unsigned int out_num,
  			  unsigned int in_num,
  			  void *data,
  			  gfp_t gfp);
  
  static inline int virtqueue_add_buf(struct virtqueue *vq,
  				    struct scatterlist sg[],
  				    unsigned int out_num,
  				    unsigned int in_num,
  				    void *data)
  {
  	return virtqueue_add_buf_gfp(vq, sg, out_num, in_num, data, GFP_ATOMIC);
  }

  void virtqueue_kick(struct virtqueue *vq);

  void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);

  void virtqueue_disable_cb(struct virtqueue *vq);

  bool virtqueue_enable_cb(struct virtqueue *vq);

  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);

  void *virtqueue_detach_unused_buf(struct virtqueue *vq);

  unsigned int virtqueue_get_vring_size(struct virtqueue *vq);

  /**
   * virtio_device - representation of a device using virtio
   * @index: unique position on the virtio bus
   * @dev: underlying device.
   * @id: the device type identification (used to match it with a driver).
   * @config: the configuration ops for this device.

   * @vqs: the list of virtqueues for this device.

   * @features: the features supported by both driver and device.

   * @priv: private pointer for the driver's use.
   */

  struct virtio_device {

  	int index;
  	struct device dev;
  	struct virtio_device_id id;
  	struct virtio_config_ops *config;

  	struct list_head vqs;

  	/* Note that this is a Linux set_bit-style bitmap. */
  	unsigned long features[1];

  	void *priv;
  };

  #define dev_to_virtio(dev) container_of(dev, struct virtio_device, dev)

  int register_virtio_device(struct virtio_device *dev);
  void unregister_virtio_device(struct virtio_device *dev);
  
  /**
   * virtio_driver - operations for a virtio I/O driver
   * @driver: underlying device driver (populate name and owner).
   * @id_table: the ids serviced by this driver.

   * @feature_table: an array of feature numbers supported by this driver.

   * @feature_table_size: number of entries in the feature table array.

   * @probe: the function to call when a device is found.  Returns 0 or -errno.

   * @remove: the function to call when a device is removed.

   * @config_changed: optional function to call when the device configuration
   *    changes; may be called in interrupt context.

   */
  struct virtio_driver {
  	struct device_driver driver;
  	const struct virtio_device_id *id_table;

  	const unsigned int *feature_table;
  	unsigned int feature_table_size;

  	int (*probe)(struct virtio_device *dev);
  	void (*remove)(struct virtio_device *dev);

  	void (*config_changed)(struct virtio_device *dev);

  };
  
  int register_virtio_driver(struct virtio_driver *drv);
  void unregister_virtio_driver(struct virtio_driver *drv);
  #endif /* _LINUX_VIRTIO_H */