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

[Johannes Stezenbach <js@convergence.de>]

On Tue, Apr 13, 2004 at 11:42:07AM +0100, Rob.McConnell@Zarlink.Com wrote:
> Hi Johannes,
> 
> Thanks for getting back to me on this ; )
> 
> > - one section filter per fd
> > - multiple section filters for the same PID (on different fds)
> > - section data is copied from hw DMA buffer of the PID filter
> >   to the output buffer of the corresponding section filter fd
> 
> This certainly is more s/w + CPU overhead for h/w that has a single buffer
> for all section filters associated with a common PID to drop into.
> 
> > Note: The (non-av7110) hw I know puts tags into the section ringbuffer
> > of the PID filter, so software can determine which section filter
> > matched.
> 
> The h/w that I am familiar with also performs tagging.  Is there any h/w
> out there that actually has multiple section filters attached to a single
> PID with a single h/w output buffer *WITHOUT* tagging?  If this were the
> case, then it would make the task of demuxing the h/w circular buffer much
> more CPU intensive/a nightmare.  I wonder if there is more h/w out there
> that matches what I am used to rather than the AV7110?  If this were the
> case, wouldn't it make more sense to support the majority of h/w rather
> than legacy h/w such as the AV7110?

If you have tags mixed with section data in your hw ringbuffer, then
the driver will have to copy section data anyway to an output
buffer to remove the tags. Or, if you want to pass the tags to
userspace, you would have to convert the tags to a hw independent
format so userspace software would be portable.

And since we have the requirement of supporting multiple section filters
for the same PID on different fds, we need to do the copy anyway.
(Assuming hw does not support multiple "feeds" for the same PID.)

> Incidentally, it looks as though I will have to diverge from the V4 API to
> support customers who insist on having a single fd to read back all the
> section tables associated with a single PID :(

That does not help to convince me.
One can use poll/select to process multiple section filters in
userspace. So I don't see a real need for multiple section filters
per fd, it is just a "nice to have" thing. Am I wrong?


Johannes


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