[linux-dvb] Re: cinergyT2: which kernel/usb module to use?

[Holger Waechtler <holger@qanu.de>]

Johannes Stezenbach wrote:

> Holger Waechtler wrote:
>  
> > Johannes Stezenbach wrote:
> >    
> > > Holger Waechtler wrote:
> > >      
> > > > the FE_GET_EVENT ioctl was simply not implemented (I already rendered it 
> > > > obsolete in my mind, since GET_STATUS and GET_FRONTEND provide the same 
> > > > functionality...).
> > > > please try again,
> > > >        
> > > Nah, I won't try because to me the currect code looks
> > > suspiciously like an endless recursion.
> > >
> > > It would also not work the way you tried to implement it,
> > > as VDR does the following before tuning:
> > >
> > > 	while (GetFrontendEvent(event))
> > > 		; // discard stale events
> > >
> > > Which would run into an endless loop if you create
> > > frontend events "on demand".
> > >      
> > The above code is in any case wrong, you assume that the hardware 
> > delivers less events than you process, this is surely in most cases 
> > right but definitely not safe.
> >    
> This code is correct. It's the same code that Convergence has
> been using for ages. I also had it in an early szap version.
> What it does is to flush the event queue before tuning. Sure,
> we could've added FE_FLUSH_EVENTS but what the hell...
>  
The entire idea of caching pasttime events in a ringbuffer was bogus, do 
you know any use for this? Your code contains a race condition that can 
show up at particular combinations of CPU speed and status update rates. 
Maybe only a theoretical one but in any case an example for bad programming.


> > The correct implementation is this:
> >
> >       while (!exit) {
> >               select(fd, ...);
> >               ioctl(fd, FE_READ_STATUS, &status);
> >               /* process/display status */
> >       };
> >    
> Select what? If the driver sees an event so it can wake up poll(),
> it surely can generate a frontend event, no?
> Certainly it is valid to call FE_READ_STATUS instead of looking
> at the event retrieved with FE_GET_EVENT, but the API guarantees
> that FE_GET_EVENT will also give you the tuning status.
>  
a pasttime event, yes. But who cares about the lockbit status some while 
ago?

Holger