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-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/video/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.