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diff --git a/Documentation/video4linux/videobuf b/Documentation/video4linux/videobuf new file mode 100644 index 0000000..17a1f9a --- /dev/null +++ b/Documentation/video4linux/videobuf @@ -0,0 +1,360 @@ +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); + int videobuf_cgmbuf(struct videobuf_queue *q, struct video_mbuf *mbuf, + int count); + +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. videobuf_cgmbuf(), called from the driver's +vidiocgmbuf() function, only exists if the V4L1 compatibility module has +been selected with CONFIG_VIDEO_V4L1_COMPAT, so its use must be surrounded +with #ifdef directives. + +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. |
