[linux-dvb] Re: CX88 i2c issue w/ DVB tuners

[Holger Waechtler <holger@qanu.de>]

Andrew de Quincey wrote:
> On Friday 10 Sep 2004 11:02, John Dalgliesh wrote:
>
> > On Fri, 10 Sep 2004, Andrew de Quincey wrote:
> >
> > > On Thursday 09 Sep 2004 23:41, Gerd Knorr wrote:
> > >
> > > > > Just hope there isn't a card out there with both DVB AND analogue
> > > > > tuners on it....
> > > > class isn't a value but a bitmask, so you can actually say the card
> > > > can do both analog and digital tv ...
> > > Ah right cool. Eurgh, nasty problem waiting down the line when those
> > > cards Holger mentioned appear.
> > I don't think I understand the condition you are worried about. Do you
> > mean that the card has to have two separate (i2c-addressable) tuners, one
> > for analogue and one for digital? Or would a card that has one tuner that
> > does both like <http://www.dvico.com/products_mul_hd3.html> be a worry?
> >
> > Just wondering :)
>
> If the card is marked as both analogue and digital, and happens to use one of 
> the i2c addresses which several devices use, you could end up with the wrong 
> i2c device associated with the wrong device - e.g. the tda9987 (analogue 
> tuner) driver was being used incorrectly as the driver for the cx22702 
> (digtial demodulator) because they both commonly reside at the same i2c 
> address. And theres no way to detect the tda9887 more specifically.
>
> Gah, if only Philips had mandated that all i2c devices must have a unique 32 
> bit identifier!
The i2c bus is well-designed for this particular purpose, it is the API 
that does not matches the reality.

i2c devices are usually dumb tiny special chips that do a very special 
job, which heavily depend on the hardware around them and how everything 
is wired together. Not really the environment where autoprobing is a 
good idea.

And there is no urge doing anything so complicated at all: all 
infrastructure we need are two simple functions for every card's 
i2c/SPI/8bit-parallel/whatever uC bus:

   int card->uC_write(u8 addr, const u8 *buf, int len);
   int card->uC_read(u8 addr, u8 *buf, int len);

This is enough to implement the access functions for a particular device:

   #define DEVICE_ADDR XXX

   static
   int device_writereg (u8 reg, u8 val)
   {
       u8 buf [] = { reg, val };
       return card->uC_write(DEVICE_ADDR, buf, sizeof(buf));
   }

   static
   int device_readreg (u8 reg, u8 *val)
   {
       u8 buf [] = { reg };
       int ret;
       if ((ret = card->uC_write(DEVICE_ADDR, buf, sizeof(buf))))
           return ret;
       return card->uC_read(DEVICE_ADDR, val, 1);
   }

These function pointers are passed to the frontend_spec struct (e.g. 
mt312_cardspec) and from there together with the card-specific constants 
and initialisation tables to the "frontend library" functions.

that's all we need...

Holger