[linux-dvb] Re: Tuning problems with 2.5.47 driver

["Andrew de Quincey" <adq_dvb@lidskialf.net>]

> Hi, I'm using a WinTV Nova DVB-T card in the UK.
> 
> I've used the older 0.9.4 drivers, and the newer drivers from 
> the 2.5.47 kernel. I am having the same problem reported 
> earlier by others... Namely that the 2.5.47 drivers do not 
> tune in as well as the 0.9.4 ones.
> 
> Observing the output of dvbtune with 2.5.47, I see the driver 
> attempts to tune for a short time, but then returns 
> FE_TIMEDOUT to the application.. Which then gives up.... 
> However if you try it repeatedly, it eventually works fine.. 
> I'm using exactly the same parameters for tuning with both, 
> so I suspect this has to be a driver problem.
> 
> With the 0.9.4 drivers, tuning is pretty much instantaneous.
> 
> I am wondering if anyone else is looking into this behaviour? 
> If not, I shall try and fix it...

Hi, I've found exactly where the problem is in the DVB code now. Somehow
the kernel HZ timing also affected this as I reported in my other mail,
but I think this must be a side effect, perhaps something just being "on
the edge".

The issue is in dvb_frontend_thread(), specifically when it calls
dvb_frontend_recover(). The problem seems to be that on the initial
tuning, the card doesn't have long enough to tune itself, so the thread
loops round, and calls dvb_frontend_recover(). Which then starts off a
zigzag scan to compensate for LNB drift (which isn't actually
happening).

I tested this by commenting dvb_frontend_recover() out... Now my card
always tunes perfectly. However, this is obviously not a solution as it
disables LNB drift compensation.

A solution might be to differentiate between tuning
because-the-application-requested-it, and tuning-because-sync-was-lost
in dvb_frontend_thread(). I'm going to have a look into doing this
tomorrow. I see there is already something like this elsewhere in the
code (the first_trial parameter in dvb_frontend_set_parameters()).



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