Em Dom, 2007-10-07 às 14:03 -0700, Trent Piepho escreveu:> On Sun, 7 Oct 2007, Mauro Carvalho Chehab wrote:> > I took a look at cx23885 code. It seems that there's a serious error on> > the way you're using cx23885_buffer there:> >> > cx23885-dvb.c: return cx23885_buf_prepare(q, port, (struct> > cx23885_buffer*)vb, field);> > cx23885-dvb.c: cx23885_buf_queue(port, (struct cx23885_buffer*)vb);> > cx23885-dvb.c: cx23885_free_buffer(q, (struct cx23885_buffer*)vb);> >> > It seems that you are forcing videobuf_buffer to be cx23885_buffer. This> > is not right!> >> > This is what is defined on cx23885.h:> >> > struct cx23885_buffer {> > /* common v4l buffer stuff -- must be first */> > struct videobuf_buffer vb;> > I'm not sure that it is competely wrong. Say one has a cx23885_buffer that> contains a videobuf_buffer. Now suppose you have a pointer to the> videobuf_buffer, and you want to get a pointer to the cx23885_buffer that> contains it. What you should write is:> > struct videobuf_buffer *vb = ...;> struct cx23885_buffer *buf = container_of(vb, struct cx23885_buffer, vb);> > But since vb is the first field of the cx23885_buffer struct, the container_of> will turn into just '(struct cx23885_buffer *)(vb)>> This code in videobuf-dma-sg.c looks odd to me:> > /* Allocated area consists on 3 parts:> struct video_buffer> struct <driver>_buffer (cx88_buffer, saa7134_buf, ...)> struct videobuf_pci_sg_memory> > static void *__videobuf_alloc(size_t size)> {> struct videbuf_pci_sg_memory *mem;> struct videobuf_buffer *vb;> > vb = kzalloc(size+sizeof(*mem),GFP_KERNEL);> > mem = vb->priv = ((char *)vb)+size;> > What is 'size', is that the size of the driver buffer? Shouldn't you> be> allocating size + sizeof(*vb) + sizeof(*mem)?> > Is there documentation for videobuf anywhere? It doesn't look like> any of> the videobuf functions have descriptions of that they do or what the> parameters are.
There aren't any videobuf doc yet. I intend to write one, when I havesome time. IMO, this is the most complex part of V4L core.
For now, let me give a quick explanation of the basics of videobuf.
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Videobuf uses a memory struct that looks what c++ compilers do when youuse inheritances.
It is something like:
class videobuf_core { public: // some data and code};
class videobuf_dma_sg: public videobuf_core { private: // some data and code}
class foo_buffer: public videobuf_dma_sg { public: // some data}
The "constructor" for any class derivated from videobuf_dma_sg is:void *videobuf_pci_alloc (size_t size);
Where size is the size of foo_buffer.
The other videobuf_dma methods are the external functions defined onvideobuf-dma-sg.h. All of them start with videobuf_dma_foo.
A similar inehitance concept happens with videobuf_queue. You have anabstract videobuf_queue class, defined on videobuf-core, where almostall methods are virtual, and, currently, two derivated class, thatimplements their methods: a DMA Scatter/Gather one (videobuf-dma-sg) anda vmalloc one (videobuf-vmalloc).
To use the full functionality of videobuf, you will need thevideobuf_queue, that is responsible for controlling the state machine ofthe videobuffers. The videobuf_queue constructor will allocate avideobuf_buffer inherited class. Also, when you need mmapped buffers,videobuf core will allocate one additional videobuf_buffer inheritedclass by each queue (generally, a driver allocates, by default, 8 queuesfor receiving data - this avoids data loss, when the machine is withhigh workloads).
It is also possible just to use a videobuf_buffer derivated class. ALSAsaa7134 and cx88 drivers implement this way. They have their own statemachine.
-- Cheers,Mauro
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