> Hi All,
>
> Nathan Lutchansky wrote:
>>
>> At the moment, anyway. It will become more and more necessary as the
>> number of different I2C sensor chips on the market increases. V4L/DVB
>> has
>> already hit this problem in a big way.
>>
>>
>
>
> DVB doesn't probe for i2c devices. The DVB adapter, ie the card knows
> what devices that are present and works under that assumption.
>
> Earlier DVB had the problem with probing. ie a different device is found
> at the expected location of another device(In the case of probing) and
> this device doesn't like to be fiddled around with. And "lo" we have a
> perfect freeze.
>
> After many such cases, DVB no longer probes for i2c devices (we now
> longer have anymore i2c issues)
> V4L on the other hand probes for all devices. This is IMHO wrong, due to
>
> (1) When the probe list grows long, it takes longer time for the probe
> to succeed (V4L guys themselves would agree to the fact that they have
> seen probes in the order of ~30 minutes ! Yuck)
>
> (2) Probing wrong devices
>
> So in any case probing can never be right, unless the card information
> is passed to the i2c core. But in that sense it is no longer a probe. It
> is indeed just an attach method. In that case, it makes no sense to make
> i2c core go around in circles.
>
> The proper way to handle this, is that only the right device is
> attached. Rather than making i2c to do this, subsystems should be
> handling this.
OK, I'll repeat this one more time: for many of the older v4l cards we
have no idea on what addresses the i2c devices are. It was never
documented. And also some devices can move around depending on the card
revision (esp. true for the analog tuner).
There are also cards where the only way to determine which model it is is
by probing addresses to see if a chip is present as there is no other way
to detecting this. And I'm sure I've missed lots of other horrible
situations where probing is the only way to do discover what card you are
talking to.
And the whole idea of these patches is to at least allow v4l drivers to be
more precise in their probing (or use the precise i2c address if known) to
make the startup faster and to prevent misdetections, which are becoming a
major problem. BTW, I've never heard of 30 minute probes, compared to
things like firmware uploads over i2c the probe time is negligible.
Regards,
Hans
