An introduction to the videobuf layer
  Jonathan Corbet <corbet@lwn.net>
  Current as of 2.6.33
  
  The videobuf layer functions as a sort of glue layer between a V4L2 driver
  and user space.  It handles the allocation and management of buffers for
  the storage of video frames.  There is a set of functions which can be used
  to implement many of the standard POSIX I/O system calls, including read(),
  poll(), and, happily, mmap().  Another set of functions can be used to
  implement the bulk of the V4L2 ioctl() calls related to streaming I/O,
  including buffer allocation, queueing and dequeueing, and streaming
  control.  Using videobuf imposes a few design decisions on the driver
  author, but the payback comes in the form of reduced code in the driver and
  a consistent implementation of the V4L2 user-space API.
  
  Buffer types
  
  Not all video devices use the same kind of buffers.  In fact, there are (at
  least) three common variations:
  
   - Buffers which are scattered in both the physical and (kernel) virtual
     address spaces.  (Almost) all user-space buffers are like this, but it
     makes great sense to allocate kernel-space buffers this way as well when
     it is possible.  Unfortunately, it is not always possible; working with
     this kind of buffer normally requires hardware which can do
     scatter/gather DMA operations.
  
   - Buffers which are physically scattered, but which are virtually
     contiguous; buffers allocated with vmalloc(), in other words.  These
     buffers are just as hard to use for DMA operations, but they can be
     useful in situations where DMA is not available but virtually-contiguous
     buffers are convenient.
  
   - Buffers which are physically contiguous.  Allocation of this kind of
     buffer can be unreliable on fragmented systems, but simpler DMA
     controllers cannot deal with anything else.
  
  Videobuf can work with all three types of buffers, but the driver author
  must pick one at the outset and design the driver around that decision.
  
  [It's worth noting that there's a fourth kind of buffer: "overlay" buffers
  which are located within the system's video memory.  The overlay
  functionality is considered to be deprecated for most use, but it still
  shows up occasionally in system-on-chip drivers where the performance
  benefits merit the use of this technique.  Overlay buffers can be handled
  as a form of scattered buffer, but there are very few implementations in
  the kernel and a description of this technique is currently beyond the
  scope of this document.]
  
  Data structures, callbacks, and initialization
  
  Depending on which type of buffers are being used, the driver should
  include one of the following files:
  
      <media/videobuf-dma-sg.h>		/* Physically scattered */
      <media/videobuf-vmalloc.h>		/* vmalloc() buffers	*/
      <media/videobuf-dma-contig.h>	/* Physically contiguous */
  
  The driver's data structure describing a V4L2 device should include a
  struct videobuf_queue instance for the management of the buffer queue,
  along with a list_head for the queue of available buffers.  There will also
  need to be an interrupt-safe spinlock which is used to protect (at least)
  the queue.
  
  The next step is to write four simple callbacks to help videobuf deal with
  the management of buffers:
  
      struct videobuf_queue_ops {
  	int (*buf_setup)(struct videobuf_queue *q,
  			 unsigned int *count, unsigned int *size);
  	int (*buf_prepare)(struct videobuf_queue *q,
  			   struct videobuf_buffer *vb,
  			   enum v4l2_field field);
  	void (*buf_queue)(struct videobuf_queue *q,
  			  struct videobuf_buffer *vb);
  	void (*buf_release)(struct videobuf_queue *q,
  			    struct videobuf_buffer *vb);
      };
  
  buf_setup() is called early in the I/O process, when streaming is being
  initiated; its purpose is to tell videobuf about the I/O stream.  The count
  parameter will be a suggested number of buffers to use; the driver should
  check it for rationality and adjust it if need be.  As a practical rule, a
  minimum of two buffers are needed for proper streaming, and there is
  usually a maximum (which cannot exceed 32) which makes sense for each
  device.  The size parameter should be set to the expected (maximum) size
  for each frame of data.
  
  Each buffer (in the form of a struct videobuf_buffer pointer) will be
  passed to buf_prepare(), which should set the buffer's size, width, height,
  and field fields properly.  If the buffer's state field is
  VIDEOBUF_NEEDS_INIT, the driver should pass it to:
  
      int videobuf_iolock(struct videobuf_queue* q, struct videobuf_buffer *vb,
  			struct v4l2_framebuffer *fbuf);
  
  Among other things, this call will usually allocate memory for the buffer.
  Finally, the buf_prepare() function should set the buffer's state to
  VIDEOBUF_PREPARED.
  
  When a buffer is queued for I/O, it is passed to buf_queue(), which should
  put it onto the driver's list of available buffers and set its state to
  VIDEOBUF_QUEUED.  Note that this function is called with the queue spinlock
  held; if it tries to acquire it as well things will come to a screeching
  halt.  Yes, this is the voice of experience.  Note also that videobuf may
  wait on the first buffer in the queue; placing other buffers in front of it
  could again gum up the works.  So use list_add_tail() to enqueue buffers.
  
  Finally, buf_release() is called when a buffer is no longer intended to be
  used.  The driver should ensure that there is no I/O active on the buffer,
  then pass it to the appropriate free routine(s):
  
      /* Scatter/gather drivers */
      int videobuf_dma_unmap(struct videobuf_queue *q,
  			   struct videobuf_dmabuf *dma);
      int videobuf_dma_free(struct videobuf_dmabuf *dma);
  
      /* vmalloc drivers */
      void videobuf_vmalloc_free (struct videobuf_buffer *buf);
  
      /* Contiguous drivers */
      void videobuf_dma_contig_free(struct videobuf_queue *q,
  				  struct videobuf_buffer *buf);
  
  One way to ensure that a buffer is no longer under I/O is to pass it to:
  
      int videobuf_waiton(struct videobuf_buffer *vb, int non_blocking, int intr);
  
  Here, vb is the buffer, non_blocking indicates whether non-blocking I/O
  should be used (it should be zero in the buf_release() case), and intr
  controls whether an interruptible wait is used.
  
  File operations
  
  At this point, much of the work is done; much of the rest is slipping
  videobuf calls into the implementation of the other driver callbacks.  The
  first step is in the open() function, which must initialize the
  videobuf queue.  The function to use depends on the type of buffer used:
  
      void videobuf_queue_sg_init(struct videobuf_queue *q,
  				struct videobuf_queue_ops *ops,
  				struct device *dev,
  				spinlock_t *irqlock,
  				enum v4l2_buf_type type,
  				enum v4l2_field field,
  				unsigned int msize,
  				void *priv);
  
      void videobuf_queue_vmalloc_init(struct videobuf_queue *q,
  				struct videobuf_queue_ops *ops,
  				struct device *dev,
  				spinlock_t *irqlock,
  				enum v4l2_buf_type type,
  				enum v4l2_field field,
  				unsigned int msize,
  				void *priv);
  
      void videobuf_queue_dma_contig_init(struct videobuf_queue *q,
  				       struct videobuf_queue_ops *ops,
  				       struct device *dev,
  				       spinlock_t *irqlock,
  				       enum v4l2_buf_type type,
  				       enum v4l2_field field,
  				       unsigned int msize,
  				       void *priv);
  
  In each case, the parameters are the same: q is the queue structure for the
  device, ops is the set of callbacks as described above, dev is the device
  structure for this video device, irqlock is an interrupt-safe spinlock to
  protect access to the data structures, type is the buffer type used by the
  device (cameras will use V4L2_BUF_TYPE_VIDEO_CAPTURE, for example), field
  describes which field is being captured (often V4L2_FIELD_NONE for
  progressive devices), msize is the size of any containing structure used
  around struct videobuf_buffer, and priv is a private data pointer which
  shows up in the priv_data field of struct videobuf_queue.  Note that these
  are void functions which, evidently, are immune to failure.
  
  V4L2 capture drivers can be written to support either of two APIs: the
  read() system call and the rather more complicated streaming mechanism.  As
  a general rule, it is necessary to support both to ensure that all
  applications have a chance of working with the device.  Videobuf makes it
  easy to do that with the same code.  To implement read(), the driver need
  only make a call to one of:
  
      ssize_t videobuf_read_one(struct videobuf_queue *q,
  			      char __user *data, size_t count,
  			      loff_t *ppos, int nonblocking);
  
      ssize_t videobuf_read_stream(struct videobuf_queue *q,
  				 char __user *data, size_t count,
  				 loff_t *ppos, int vbihack, int nonblocking);
  
  Either one of these functions will read frame data into data, returning the
  amount actually read; the difference is that videobuf_read_one() will only
  read a single frame, while videobuf_read_stream() will read multiple frames
  if they are needed to satisfy the count requested by the application.  A
  typical driver read() implementation will start the capture engine, call
  one of the above functions, then stop the engine before returning (though a
  smarter implementation might leave the engine running for a little while in
  anticipation of another read() call happening in the near future).
  
  The poll() function can usually be implemented with a direct call to:
  
      unsigned int videobuf_poll_stream(struct file *file,
  				      struct videobuf_queue *q,
  				      poll_table *wait);
  
  Note that the actual wait queue eventually used will be the one associated
  with the first available buffer.
  
  When streaming I/O is done to kernel-space buffers, the driver must support
  the mmap() system call to enable user space to access the data.  In many
  V4L2 drivers, the often-complex mmap() implementation simplifies to a
  single call to:
  
      int videobuf_mmap_mapper(struct videobuf_queue *q,
  			     struct vm_area_struct *vma);
  
  Everything else is handled by the videobuf code.
  
  The release() function requires two separate videobuf calls:
  
      void videobuf_stop(struct videobuf_queue *q);
      int videobuf_mmap_free(struct videobuf_queue *q);
  
  The call to videobuf_stop() terminates any I/O in progress - though it is
  still up to the driver to stop the capture engine.  The call to
  videobuf_mmap_free() will ensure that all buffers have been unmapped; if
  so, they will all be passed to the buf_release() callback.  If buffers
  remain mapped, videobuf_mmap_free() returns an error code instead.  The
  purpose is clearly to cause the closing of the file descriptor to fail if
  buffers are still mapped, but every driver in the 2.6.32 kernel cheerfully
  ignores its return value.
  
  ioctl() operations
  
  The V4L2 API includes a very long list of driver callbacks to respond to
  the many ioctl() commands made available to user space.  A number of these
  - those associated with streaming I/O - turn almost directly into videobuf
  calls.  The relevant helper functions are:
  
      int videobuf_reqbufs(struct videobuf_queue *q,
  			 struct v4l2_requestbuffers *req);
      int videobuf_querybuf(struct videobuf_queue *q, struct v4l2_buffer *b);
      int videobuf_qbuf(struct videobuf_queue *q, struct v4l2_buffer *b);
      int videobuf_dqbuf(struct videobuf_queue *q, struct v4l2_buffer *b,
  		       int nonblocking);
      int videobuf_streamon(struct videobuf_queue *q);
      int videobuf_streamoff(struct videobuf_queue *q);
  
  So, for example, a VIDIOC_REQBUFS call turns into a call to the driver's
  vidioc_reqbufs() callback which, in turn, usually only needs to locate the
  proper struct videobuf_queue pointer and pass it to videobuf_reqbufs().
  These support functions can replace a great deal of buffer management
  boilerplate in a lot of V4L2 drivers.
  
  The vidioc_streamon() and vidioc_streamoff() functions will be a bit more
  complex, of course, since they will also need to deal with starting and
  stopping the capture engine.
  
  Buffer allocation
  
  Thus far, we have talked about buffers, but have not looked at how they are
  allocated.  The scatter/gather case is the most complex on this front.  For
  allocation, the driver can leave buffer allocation entirely up to the
  videobuf layer; in this case, buffers will be allocated as anonymous
  user-space pages and will be very scattered indeed.  If the application is
  using user-space buffers, no allocation is needed; the videobuf layer will
  take care of calling get_user_pages() and filling in the scatterlist array.
  
  If the driver needs to do its own memory allocation, it should be done in
  the vidioc_reqbufs() function, *after* calling videobuf_reqbufs().  The
  first step is a call to:
  
      struct videobuf_dmabuf *videobuf_to_dma(struct videobuf_buffer *buf);
  
  The returned videobuf_dmabuf structure (defined in
  <media/videobuf-dma-sg.h>) includes a couple of relevant fields:
  
      struct scatterlist  *sglist;
      int                 sglen;
  
  The driver must allocate an appropriately-sized scatterlist array and
  populate it with pointers to the pieces of the allocated buffer; sglen
  should be set to the length of the array.
  
  Drivers using the vmalloc() method need not (and cannot) concern themselves
  with buffer allocation at all; videobuf will handle those details.  The
  same is normally true of contiguous-DMA drivers as well; videobuf will
  allocate the buffers (with dma_alloc_coherent()) when it sees fit.  That
  means that these drivers may be trying to do high-order allocations at any
  time, an operation which is not always guaranteed to work.  Some drivers
  play tricks by allocating DMA space at system boot time; videobuf does not
  currently play well with those drivers.
  
  As of 2.6.31, contiguous-DMA drivers can work with a user-supplied buffer,
  as long as that buffer is physically contiguous.  Normal user-space
  allocations will not meet that criterion, but buffers obtained from other
  kernel drivers, or those contained within huge pages, will work with these
  drivers.
  
  Filling the buffers
  
  The final part of a videobuf implementation has no direct callback - it's
  the portion of the code which actually puts frame data into the buffers,
  usually in response to interrupts from the device.  For all types of
  drivers, this process works approximately as follows:
  
   - Obtain the next available buffer and make sure that somebody is actually
     waiting for it.
  
   - Get a pointer to the memory and put video data there.
  
   - Mark the buffer as done and wake up the process waiting for it.
  
  Step (1) above is done by looking at the driver-managed list_head structure
  - the one which is filled in the buf_queue() callback.  Because starting
  the engine and enqueueing buffers are done in separate steps, it's possible
  for the engine to be running without any buffers available - in the
  vmalloc() case especially.  So the driver should be prepared for the list
  to be empty.  It is equally possible that nobody is yet interested in the
  buffer; the driver should not remove it from the list or fill it until a
  process is waiting on it.  That test can be done by examining the buffer's
  done field (a wait_queue_head_t structure) with waitqueue_active().
  
  A buffer's state should be set to VIDEOBUF_ACTIVE before being mapped for
  DMA; that ensures that the videobuf layer will not try to do anything with
  it while the device is transferring data.
  
  For scatter/gather drivers, the needed memory pointers will be found in the
  scatterlist structure described above.  Drivers using the vmalloc() method
  can get a memory pointer with:
  
      void *videobuf_to_vmalloc(struct videobuf_buffer *buf);
  
  For contiguous DMA drivers, the function to use is:
  
      dma_addr_t videobuf_to_dma_contig(struct videobuf_buffer *buf);
  
  The contiguous DMA API goes out of its way to hide the kernel-space address
  of the DMA buffer from drivers.
  
  The final step is to set the size field of the relevant videobuf_buffer
  structure to the actual size of the captured image, set state to
  VIDEOBUF_DONE, then call wake_up() on the done queue.  At this point, the
  buffer is owned by the videobuf layer and the driver should not touch it
  again.
  
  Developers who are interested in more information can go into the relevant
  header files; there are a few low-level functions declared there which have
  not been talked about here.  Also worthwhile is the vivi driver
  (drivers/media/platform/vivi.c), which is maintained as an example of how V4L2
  drivers should be written.  Vivi only uses the vmalloc() API, but it's good
  enough to get started with.  Note also that all of these calls are exported
  GPL-only, so they will not be available to non-GPL kernel modules.