1.. SPDX-License-Identifier: GPL-2.0 2 3.. _vb_framework: 4 5Videobuf Framework 6================== 7 8Author: Jonathan Corbet <corbet@lwn.net> 9 10Current as of 2.6.33 11 12.. note:: 13 14 The videobuf framework was deprecated in favor of videobuf2. Shouldn't 15 be used on new drivers. 16 17Introduction 18------------ 19 20The videobuf layer functions as a sort of glue layer between a V4L2 driver 21and user space. It handles the allocation and management of buffers for 22the storage of video frames. There is a set of functions which can be used 23to implement many of the standard POSIX I/O system calls, including read(), 24poll(), and, happily, mmap(). Another set of functions can be used to 25implement the bulk of the V4L2 ioctl() calls related to streaming I/O, 26including buffer allocation, queueing and dequeueing, and streaming 27control. Using videobuf imposes a few design decisions on the driver 28author, but the payback comes in the form of reduced code in the driver and 29a consistent implementation of the V4L2 user-space API. 30 31Buffer types 32------------ 33 34Not all video devices use the same kind of buffers. In fact, there are (at 35least) three common variations: 36 37 - Buffers which are scattered in both the physical and (kernel) virtual 38 address spaces. (Almost) all user-space buffers are like this, but it 39 makes great sense to allocate kernel-space buffers this way as well when 40 it is possible. Unfortunately, it is not always possible; working with 41 this kind of buffer normally requires hardware which can do 42 scatter/gather DMA operations. 43 44 - Buffers which are physically scattered, but which are virtually 45 contiguous; buffers allocated with vmalloc(), in other words. These 46 buffers are just as hard to use for DMA operations, but they can be 47 useful in situations where DMA is not available but virtually-contiguous 48 buffers are convenient. 49 50 - Buffers which are physically contiguous. Allocation of this kind of 51 buffer can be unreliable on fragmented systems, but simpler DMA 52 controllers cannot deal with anything else. 53 54Videobuf can work with all three types of buffers, but the driver author 55must pick one at the outset and design the driver around that decision. 56 57[It's worth noting that there's a fourth kind of buffer: "overlay" buffers 58which are located within the system's video memory. The overlay 59functionality is considered to be deprecated for most use, but it still 60shows up occasionally in system-on-chip drivers where the performance 61benefits merit the use of this technique. Overlay buffers can be handled 62as a form of scattered buffer, but there are very few implementations in 63the kernel and a description of this technique is currently beyond the 64scope of this document.] 65 66Data structures, callbacks, and initialization 67---------------------------------------------- 68 69Depending on which type of buffers are being used, the driver should 70include one of the following files: 71 72.. code-block:: none 73 74 <media/videobuf-dma-sg.h> /* Physically scattered */ 75 <media/videobuf-vmalloc.h> /* vmalloc() buffers */ 76 <media/videobuf-dma-contig.h> /* Physically contiguous */ 77 78The driver's data structure describing a V4L2 device should include a 79struct videobuf_queue instance for the management of the buffer queue, 80along with a list_head for the queue of available buffers. There will also 81need to be an interrupt-safe spinlock which is used to protect (at least) 82the queue. 83 84The next step is to write four simple callbacks to help videobuf deal with 85the management of buffers: 86 87.. code-block:: none 88 89 struct videobuf_queue_ops { 90 int (*buf_setup)(struct videobuf_queue *q, 91 unsigned int *count, unsigned int *size); 92 int (*buf_prepare)(struct videobuf_queue *q, 93 struct videobuf_buffer *vb, 94 enum v4l2_field field); 95 void (*buf_queue)(struct videobuf_queue *q, 96 struct videobuf_buffer *vb); 97 void (*buf_release)(struct videobuf_queue *q, 98 struct videobuf_buffer *vb); 99 }; 100 101buf_setup() is called early in the I/O process, when streaming is being 102initiated; its purpose is to tell videobuf about the I/O stream. The count 103parameter will be a suggested number of buffers to use; the driver should 104check it for rationality and adjust it if need be. As a practical rule, a 105minimum of two buffers are needed for proper streaming, and there is 106usually a maximum (which cannot exceed 32) which makes sense for each 107device. The size parameter should be set to the expected (maximum) size 108for each frame of data. 109 110Each buffer (in the form of a struct videobuf_buffer pointer) will be 111passed to buf_prepare(), which should set the buffer's size, width, height, 112and field fields properly. If the buffer's state field is 113VIDEOBUF_NEEDS_INIT, the driver should pass it to: 114 115.. code-block:: none 116 117 int videobuf_iolock(struct videobuf_queue* q, struct videobuf_buffer *vb, 118 struct v4l2_framebuffer *fbuf); 119 120Among other things, this call will usually allocate memory for the buffer. 121Finally, the buf_prepare() function should set the buffer's state to 122VIDEOBUF_PREPARED. 123 124When a buffer is queued for I/O, it is passed to buf_queue(), which should 125put it onto the driver's list of available buffers and set its state to 126VIDEOBUF_QUEUED. Note that this function is called with the queue spinlock 127held; if it tries to acquire it as well things will come to a screeching 128halt. Yes, this is the voice of experience. Note also that videobuf may 129wait on the first buffer in the queue; placing other buffers in front of it 130could again gum up the works. So use list_add_tail() to enqueue buffers. 131 132Finally, buf_release() is called when a buffer is no longer intended to be 133used. The driver should ensure that there is no I/O active on the buffer, 134then pass it to the appropriate free routine(s): 135 136.. code-block:: none 137 138 /* Scatter/gather drivers */ 139 int videobuf_dma_unmap(struct videobuf_queue *q, 140 struct videobuf_dmabuf *dma); 141 int videobuf_dma_free(struct videobuf_dmabuf *dma); 142 143 /* vmalloc drivers */ 144 void videobuf_vmalloc_free (struct videobuf_buffer *buf); 145 146 /* Contiguous drivers */ 147 void videobuf_dma_contig_free(struct videobuf_queue *q, 148 struct videobuf_buffer *buf); 149 150One way to ensure that a buffer is no longer under I/O is to pass it to: 151 152.. code-block:: none 153 154 int videobuf_waiton(struct videobuf_buffer *vb, int non_blocking, int intr); 155 156Here, vb is the buffer, non_blocking indicates whether non-blocking I/O 157should be used (it should be zero in the buf_release() case), and intr 158controls whether an interruptible wait is used. 159 160File operations 161--------------- 162 163At this point, much of the work is done; much of the rest is slipping 164videobuf calls into the implementation of the other driver callbacks. The 165first step is in the open() function, which must initialize the 166videobuf queue. The function to use depends on the type of buffer used: 167 168.. code-block:: none 169 170 void videobuf_queue_sg_init(struct videobuf_queue *q, 171 struct videobuf_queue_ops *ops, 172 struct device *dev, 173 spinlock_t *irqlock, 174 enum v4l2_buf_type type, 175 enum v4l2_field field, 176 unsigned int msize, 177 void *priv); 178 179 void videobuf_queue_vmalloc_init(struct videobuf_queue *q, 180 struct videobuf_queue_ops *ops, 181 struct device *dev, 182 spinlock_t *irqlock, 183 enum v4l2_buf_type type, 184 enum v4l2_field field, 185 unsigned int msize, 186 void *priv); 187 188 void videobuf_queue_dma_contig_init(struct videobuf_queue *q, 189 struct videobuf_queue_ops *ops, 190 struct device *dev, 191 spinlock_t *irqlock, 192 enum v4l2_buf_type type, 193 enum v4l2_field field, 194 unsigned int msize, 195 void *priv); 196 197In each case, the parameters are the same: q is the queue structure for the 198device, ops is the set of callbacks as described above, dev is the device 199structure for this video device, irqlock is an interrupt-safe spinlock to 200protect access to the data structures, type is the buffer type used by the 201device (cameras will use V4L2_BUF_TYPE_VIDEO_CAPTURE, for example), field 202describes which field is being captured (often V4L2_FIELD_NONE for 203progressive devices), msize is the size of any containing structure used 204around struct videobuf_buffer, and priv is a private data pointer which 205shows up in the priv_data field of struct videobuf_queue. Note that these 206are void functions which, evidently, are immune to failure. 207 208V4L2 capture drivers can be written to support either of two APIs: the 209read() system call and the rather more complicated streaming mechanism. As 210a general rule, it is necessary to support both to ensure that all 211applications have a chance of working with the device. Videobuf makes it 212easy to do that with the same code. To implement read(), the driver need 213only make a call to one of: 214 215.. code-block:: none 216 217 ssize_t videobuf_read_one(struct videobuf_queue *q, 218 char __user *data, size_t count, 219 loff_t *ppos, int nonblocking); 220 221 ssize_t videobuf_read_stream(struct videobuf_queue *q, 222 char __user *data, size_t count, 223 loff_t *ppos, int vbihack, int nonblocking); 224 225Either one of these functions will read frame data into data, returning the 226amount actually read; the difference is that videobuf_read_one() will only 227read a single frame, while videobuf_read_stream() will read multiple frames 228if they are needed to satisfy the count requested by the application. A 229typical driver read() implementation will start the capture engine, call 230one of the above functions, then stop the engine before returning (though a 231smarter implementation might leave the engine running for a little while in 232anticipation of another read() call happening in the near future). 233 234The poll() function can usually be implemented with a direct call to: 235 236.. code-block:: none 237 238 unsigned int videobuf_poll_stream(struct file *file, 239 struct videobuf_queue *q, 240 poll_table *wait); 241 242Note that the actual wait queue eventually used will be the one associated 243with the first available buffer. 244 245When streaming I/O is done to kernel-space buffers, the driver must support 246the mmap() system call to enable user space to access the data. In many 247V4L2 drivers, the often-complex mmap() implementation simplifies to a 248single call to: 249 250.. code-block:: none 251 252 int videobuf_mmap_mapper(struct videobuf_queue *q, 253 struct vm_area_struct *vma); 254 255Everything else is handled by the videobuf code. 256 257The release() function requires two separate videobuf calls: 258 259.. code-block:: none 260 261 void videobuf_stop(struct videobuf_queue *q); 262 int videobuf_mmap_free(struct videobuf_queue *q); 263 264The call to videobuf_stop() terminates any I/O in progress - though it is 265still up to the driver to stop the capture engine. The call to 266videobuf_mmap_free() will ensure that all buffers have been unmapped; if 267so, they will all be passed to the buf_release() callback. If buffers 268remain mapped, videobuf_mmap_free() returns an error code instead. The 269purpose is clearly to cause the closing of the file descriptor to fail if 270buffers are still mapped, but every driver in the 2.6.32 kernel cheerfully 271ignores its return value. 272 273ioctl() operations 274------------------ 275 276The V4L2 API includes a very long list of driver callbacks to respond to 277the many ioctl() commands made available to user space. A number of these 278- those associated with streaming I/O - turn almost directly into videobuf 279calls. The relevant helper functions are: 280 281.. code-block:: none 282 283 int videobuf_reqbufs(struct videobuf_queue *q, 284 struct v4l2_requestbuffers *req); 285 int videobuf_querybuf(struct videobuf_queue *q, struct v4l2_buffer *b); 286 int videobuf_qbuf(struct videobuf_queue *q, struct v4l2_buffer *b); 287 int videobuf_dqbuf(struct videobuf_queue *q, struct v4l2_buffer *b, 288 int nonblocking); 289 int videobuf_streamon(struct videobuf_queue *q); 290 int videobuf_streamoff(struct videobuf_queue *q); 291 292So, for example, a VIDIOC_REQBUFS call turns into a call to the driver's 293vidioc_reqbufs() callback which, in turn, usually only needs to locate the 294proper struct videobuf_queue pointer and pass it to videobuf_reqbufs(). 295These support functions can replace a great deal of buffer management 296boilerplate in a lot of V4L2 drivers. 297 298The vidioc_streamon() and vidioc_streamoff() functions will be a bit more 299complex, of course, since they will also need to deal with starting and 300stopping the capture engine. 301 302Buffer allocation 303----------------- 304 305Thus far, we have talked about buffers, but have not looked at how they are 306allocated. The scatter/gather case is the most complex on this front. For 307allocation, the driver can leave buffer allocation entirely up to the 308videobuf layer; in this case, buffers will be allocated as anonymous 309user-space pages and will be very scattered indeed. If the application is 310using user-space buffers, no allocation is needed; the videobuf layer will 311take care of calling get_user_pages() and filling in the scatterlist array. 312 313If the driver needs to do its own memory allocation, it should be done in 314the vidioc_reqbufs() function, *after* calling videobuf_reqbufs(). The 315first step is a call to: 316 317.. code-block:: none 318 319 struct videobuf_dmabuf *videobuf_to_dma(struct videobuf_buffer *buf); 320 321The returned videobuf_dmabuf structure (defined in 322<media/videobuf-dma-sg.h>) includes a couple of relevant fields: 323 324.. code-block:: none 325 326 struct scatterlist *sglist; 327 int sglen; 328 329The driver must allocate an appropriately-sized scatterlist array and 330populate it with pointers to the pieces of the allocated buffer; sglen 331should be set to the length of the array. 332 333Drivers using the vmalloc() method need not (and cannot) concern themselves 334with buffer allocation at all; videobuf will handle those details. The 335same is normally true of contiguous-DMA drivers as well; videobuf will 336allocate the buffers (with dma_alloc_coherent()) when it sees fit. That 337means that these drivers may be trying to do high-order allocations at any 338time, an operation which is not always guaranteed to work. Some drivers 339play tricks by allocating DMA space at system boot time; videobuf does not 340currently play well with those drivers. 341 342As of 2.6.31, contiguous-DMA drivers can work with a user-supplied buffer, 343as long as that buffer is physically contiguous. Normal user-space 344allocations will not meet that criterion, but buffers obtained from other 345kernel drivers, or those contained within huge pages, will work with these 346drivers. 347 348Filling the buffers 349------------------- 350 351The final part of a videobuf implementation has no direct callback - it's 352the portion of the code which actually puts frame data into the buffers, 353usually in response to interrupts from the device. For all types of 354drivers, this process works approximately as follows: 355 356 - Obtain the next available buffer and make sure that somebody is actually 357 waiting for it. 358 359 - Get a pointer to the memory and put video data there. 360 361 - Mark the buffer as done and wake up the process waiting for it. 362 363Step (1) above is done by looking at the driver-managed list_head structure 364- the one which is filled in the buf_queue() callback. Because starting 365the engine and enqueueing buffers are done in separate steps, it's possible 366for the engine to be running without any buffers available - in the 367vmalloc() case especially. So the driver should be prepared for the list 368to be empty. It is equally possible that nobody is yet interested in the 369buffer; the driver should not remove it from the list or fill it until a 370process is waiting on it. That test can be done by examining the buffer's 371done field (a wait_queue_head_t structure) with waitqueue_active(). 372 373A buffer's state should be set to VIDEOBUF_ACTIVE before being mapped for 374DMA; that ensures that the videobuf layer will not try to do anything with 375it while the device is transferring data. 376 377For scatter/gather drivers, the needed memory pointers will be found in the 378scatterlist structure described above. Drivers using the vmalloc() method 379can get a memory pointer with: 380 381.. code-block:: none 382 383 void *videobuf_to_vmalloc(struct videobuf_buffer *buf); 384 385For contiguous DMA drivers, the function to use is: 386 387.. code-block:: none 388 389 dma_addr_t videobuf_to_dma_contig(struct videobuf_buffer *buf); 390 391The contiguous DMA API goes out of its way to hide the kernel-space address 392of the DMA buffer from drivers. 393 394The final step is to set the size field of the relevant videobuf_buffer 395structure to the actual size of the captured image, set state to 396VIDEOBUF_DONE, then call wake_up() on the done queue. At this point, the 397buffer is owned by the videobuf layer and the driver should not touch it 398again. 399 400Developers who are interested in more information can go into the relevant 401header files; there are a few low-level functions declared there which have 402not been talked about here. Also worthwhile is the vivi driver 403(drivers/media/platform/vivi.c), which is maintained as an example of how V4L2 404drivers should be written. Vivi only uses the vmalloc() API, but it's good 405enough to get started with. Note also that all of these calls are exported 406GPL-only, so they will not be available to non-GPL kernel modules. 407