[linux-dvb] Re: V4 API Implementation Q's

[Johannes Stezenbach <js@convergence.de>]

On Tue, Apr 13, 2004 at 12:15:56PM +0100, Rob.McConnell@Zarlink.Com wrote:
> Hi Johannes,
> 
> > However, the V3 API allows one to allocate+set a filter now, and
> > start the filter later. I don't know how useful this is. I left
> > this out in V4 because the software I know does not actually make
> > use of it, it always sets filters with DMX_IMMEDIATE_START.
> 
> > It sounds useful to make the distinction between filter allocation
> > and filter setting in the API. But how would this actually improve
> > applications?
> 
> OK, personally I don't know how useful this is either, but there is a lot
> of STB s/w out there at the moment that makes the distinction between:
> 
> 1. allocating a filter/feed.
> 2. setting up the filter/feed spec.
> 3. "hitting" the go button.
> 
> This API isn't Zarlink specific but if you ever deal with a few well known
> UK STB design houses you will see that this is how they operate -
> strangely. ;^)  So, really I/we could do with finding out if there is a
> real technical reason for performing these steps independently (rather than
> in one go). So if there is anyone out there with a plausible reason, they
> please feel free to contribute.

Well, I think if you write a simple zapping application you would
statically allocate filters for audio, video, pcr, and allocate
and set filters for PAT, PMT, EIT-p/f. Then for zapping you just
set some PIDs and stop/start the filters.

Linux, however, is a multitasking system, and allocation needs to
be more dynamic. Especially with MHP which can take over control
of zapping completely. Linux DVB software should release unneeded
resources, and must be prepared that setting a filter can fail
anytime.


Johannes


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