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Diffstat (limited to 'Documentation/video4linux')
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diff --git a/Documentation/video4linux/videobuf b/Documentation/video4linux/videobuf deleted file mode 100644 index 3ffe9e960b6f..000000000000 --- a/Documentation/video4linux/videobuf +++ /dev/null @@ -1,355 +0,0 @@ -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. |