Re: [livid-gatos] [RFC] alternative kernel multimedia API

On Thu, 25 Oct 2001, Nate Dannenberg wrote:

> On Thu, 25 Oct 2001 [e-mail] wrote:
> 
> >
> >  After looking at and working with Xv, v4l and v4l2 I became somewhat
> > dissatisfied with the current state of affairs. I have attached a
> > description of the API that would make (at least) me much happier.
> >
> >  I would very much appreciate comments from interested people..
> 
> I'm not a coder for Linux or X, but I figured this would be a good read
> anyway, as it might provide some ideas for a similar (albeit less complex)
> issue on another platform I use >:-)
> 
> Anyways since this is an RFC, here's the only a couple of comments I can
> think of (the RFC otherwise looks clear to me):
> 
> 
> Commands/queries should also include some kind of (arbitrary, decided by the
> calling program) command serial number, so that an out-of-sequence reply
> (those prepended with a colon as you describe) can be matched to a previous
> command/query.  This could allow several commands to be sent and handled by
> multiple processes/threads/whatever.

out of sequence replies are meant to be volunteered by the driver, not
requested by the application (thought the application may want to turn
on/off their generation)  I thought about putting a restriction that
prohibits the user application from doing this: write one query, write
another query, read one reply, read another reply and instead require it
to always: write query, read all replies, repeat if necessary. This will
conserve the space used by buffers for the control device.

                        Vladimir Dergachev

> 
>  05,HUE=7\n
>  07,some unrelated command
> +05\n				# The HUE command was successful
> :07,reply to unrelated command
> :05,HUE=6\n			# Driver reported the HUE parameter as
> 				# different from that most recently set.
> 
> The program wanted to set HUE to 7, command successful, value later returned
> was 6 (maybe the device only allows even values), while at the same time some
> other command was sent and processed.
> 
> Also, it might be a nice idea to return the range of one or more parameters if
> a command given is out of range.  Suppose HUE is only valid for values 0-255:
> 
>  06,HUE=300\n
> -06,HUE=INVALID,0,255\n
> 
> ..Would tell the calling program that a value given for HUE is only valid for
> a range of 0 to 255.  This would be useful for a program that wants to attempt
> to guess the range a device accepts for a given parameter.
> 
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