AW: BSD video capture emulation question

John-Mark Gurney gurney_j at
Mon Jul 14 09:56:40 PDT 2003

Andrew Gordon wrote this message on Mon, Jul 14, 2003 at 16:29 +0100:
> On Sun, 13 Jul 2003, John-Mark Gurney wrote:
> >
> > If the mbuf code seperated it out, this MIGHT be a possibility, I'm
> > pretty sure we don't want to run into some of the problem with resource
> > contention on mbufs..  Also, the buffer space may need more fine
> > management...  The idea of a sink sending a packet that a source fills
> > also is kinda wierd (necessary for some dma operations)..
> There seems to be some lack of clarity in these discussions about what
> level of API you are trying to create.  There's at least two
> possibilities:
> a) The low-level API for shifting bulk data and timing information
>    between hardware devices and/or processing modules.  Here the
>    device drivers and encoders/decoders are the providers and consumers
>    of the API, and we're inevitably talking about a kernel interface.
> b) A higher level API to control the 'plumbing'.  Here, user-interface
>    programs are the consumer of the API, with the details of the
>    bulk transfer mechanisms being hidden below the API.


> Then there's your stated aim that things like USB videocams shouldn't have
> to be implemented with all the logic in the kernel (an aim I agree with
> BTW).  So, you end up with several different APIs for the core data
> transfer, with scope for a unifying higher-layer API on top.  But it's a
> lot of work....

Thank you for clarifing this for everyone.  Yes, I plan to try to address
both a and b of above.  Right now my priority is a since that will get
most hardware available, but b is necessary in order to properly and fully
support userland devices.  B is also necessary because I plan to implement
a in such a way that programmers will kill me if b doesn't come around.

The reason I say this is that the hardware interface will probably have
five different fd's for a card like an MJPEG card.  Remebering what
ioctl's for what fd is to be simplifed by b.  I have recently changed
the verbage of:
to make this more clear.

  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."

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