Re: Writing first v4l2 driver

[Mauro Carvalho Chehab <mchehab@infradead.org>]

On Thu, 05 Jun 2008 10:26:17 -0400
Jon Dufresne <jon.dufresne@infinitevideocorporation.com> wrote:

> Hi,
> 
> I'm in the process of writing my first v4l2 linux driver. I have written
> drivers in the past but this is my first time with a video device. I
> have read as much documentation as I could get my hands on.
> 
> My device is a pci capture device and I am trying to use streaming
> mmaped buffers. Right now I am trying to integrate the video-buf helper
> functions to do the actual frame grabbing. I am quite confused as how
> this all fits together. 

Does your device support DMA scatter/gather mode (e.g. instead of a continuous
DMA memory, you would be using a range of non-continuous memory areas)? If not,
then you'll need to write a different video-buf driver.
> 
> Is there a good guide on using video-buf for video dma transfer? I did
> quite a few google searches but I didn't find anything.
> 
> What is the simplest example of a capture device using video-buf for a
> streaming device in the source tree to use as an example? I've looked at
> a few, but I want to look at the simplest one.

The simplest driver is vivi.c. Yet, it uses videobuf-vmalloc, which is a little
bit different. However, since this is a fake device, it helps to understand how
videobuf works. If you get into vivi.c previous versions, at mercurial repo,
you'll find even a vivi example using the video-buf-dma-sg. Most of stuff
will be similar. 

The other two examples for PCI devices are saa7134-video and cx88-video. bttv
also uses it, but bttv driver is more complex to understand, since its code is
bigger.

Cheers,
Mauro

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