[linux-dvb] Re: OOPS with budget card + cam (in dvb_ca_en50221_io_write)

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

> >>Bah! OK, I'll have a look into it later. Thanks for the log.
> >
> >Hi, this is coming from the middle of the msleep() call in
> >dvb_ca_en50221_io_write() - nothing to do with me (heheheh! :)
> >
> >You're using kernel 2.6.8.1. I had to downgrade to 2.6.7 because of
> > massive problems with that kernel (specifically NFS, but this looks like
> > the same kind of error). Do you have the preemptible kernel stuff turned
> > on? That was what caused it for me.
>
> In which context is this code running? Should it get delayed and get
> processed in a work queue?

I assume you're talking about the msleep(). 

The problem with CAMs is that you can't rely on their IRQ support - the 
standard was originally defined without them, so all your code has to be able 
to cope with them not being there.

Therefore, to write data, you have to have a loop which continually asks the 
CAM if its ready for data yet.. if not => sleep+keep looping.

If I moved the stuff in io_write() into a work queue, I would still have to 
have a thread somewhere doing the above loop. It was simpler just to leave it 
in the io_write() itself - easier to handle user aborting the syscall, or the 
CAM being yanked during a write.

It doesn't break the semantics of write() as it is - if you're in blocking 
mode, it will block until the write is complete, if you're in non-blocking, 
it will not proceed unless it can write the data.