V4L/DVB (10570): v4l2-framework: documments videobuf usage on drivers

Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/video4linux/v4l2-framework.txt

@@ -47,7 +47,9 @@ All drivers have the following structure:
 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and
    /dev/vtxX) and keeping track of device-node specific data.
 
-4) Filehandle-specific structs containing per-filehandle data.
+4) Filehandle-specific structs containing per-filehandle data;
+
+5) video buffer handling.
 
 This is a rough schematic of how it all relates:
 
@@ -525,3 +527,91 @@ void *video_drvdata(struct file *file);
 You can go from a video_device struct to the v4l2_device struct using:
 
 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
+
+video buffer helper functions
+-----------------------------
+
+The v4l2 core API provides a standard method for dealing with video
+buffers. Those methods allow a driver to implement read(), mmap() and
+overlay() on a consistent way.
+
+There are currently methods for using video buffers on devices that
+supports DMA with scatter/gather method (videobuf-dma-sg), DMA with
+linear access (videobuf-dma-contig), and vmalloced buffers, mostly
+used on USB drivers (videobuf-vmalloc).
+
+Any driver using videobuf should provide operations (callbacks) for
+four handlers:
+
+ops->buf_setup   - calculates the size of the video buffers and avoid they
+		   to waste more than some maximum limit of RAM;
+ops->buf_prepare - fills the video buffer structs and calls
+		   videobuf_iolock() to alloc and prepare mmaped memory;
+ops->buf_queue   - advices the driver that another buffer were
+		   requested (by read() or by QBUF);
+ops->buf_release - frees any buffer that were allocated.
+
+In order to use it, the driver need to have a code (generally called at
+interrupt context) that will properly handle the buffer request lists,
+announcing that a new buffer were filled.
+
+The irq handling code should handle the videobuf task lists, in order
+to advice videobuf that a new frame were filled, in order to honor to a
+request. The code is generally like this one:
+        if (list_empty(&dma_q->active))
+		return;
+
+        buf = list_entry(dma_q->active.next, struct vbuffer, vb.queue);
+
+        if (!waitqueue_active(&buf->vb.done))
+		return;
+
+	/* Some logic to handle the buf may be needed here */
+
+        list_del(&buf->vb.queue);
+        do_gettimeofday(&buf->vb.ts);
+        wake_up(&buf->vb.done);
+
+Those are the videobuffer functions used on drivers, implemented on
+videobuf-core:
+
+- videobuf_queue_core_init()
+  Initializes the videobuf infrastructure. This function should be
+  called before any other videobuf function.
+
+- videobuf_iolock()
+  Prepares the videobuf memory for the proper method (read, mmap, overlay).
+
+- videobuf_queue_is_busy()
+  Checks if a videobuf is streaming.
+
+- videobuf_queue_cancel()
+  Stops video handling.
+
+- videobuf_mmap_free()
+  frees mmap buffers.
+
+- videobuf_stop()
+  Stops video handling, ends mmap and frees mmap and other buffers.
+
+- V4L2 api functions. Those functions correspond to VIDIOC_foo ioctls:
+   videobuf_reqbufs(), videobuf_querybuf(), videobuf_qbuf(),
+   videobuf_dqbuf(), videobuf_streamon(), videobuf_streamoff().
+
+- V4L1 api function (corresponds to VIDIOCMBUF ioctl):
+   videobuf_cgmbuf()
+      This function is used to provide backward compatibility with V4L1
+      API.
+
+- Some help functions for read()/poll() operations:
+   videobuf_read_stream()
+      For continuous stream read()
+   videobuf_read_one()
+      For snapshot read()
+   videobuf_poll_stream()
+      polling help function
+
+The better way to understand it is to take a look at vivi driver. One
+of the main reasons for vivi is to be a videobuf usage example. the
+vivi_thread_tick() does the task that the IRQ callback would do on PCI
+drivers (or the irq callback on USB).