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