Re: [RFC PATCH 3/5] i2c: core: add function to request an alias

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On Wed, Jan 08, 2020 at 02:27:08PM +0100, Wolfram Sang wrote:
> 
> > > It would be nice, but I'm not sure this is really doable. Say the DT for
> > > board X lists all the used slave addresses. Then the kernel would assume
> > > all the other addresses are available. But then somebody includes the DT
> > > of board X in the DT for product Z, based on board X + add-on board Y.
> > > Add-on board Y has 2 I2C chips, but only one is described in DT. Now the
> > > kernel still thinks it knows all the used address, but this is wrong.
> > 
> > That's the fault of the system integrator though. We can't prevent
> > people from making incorrect DT, and we shouldn't go to great length to
> > still support them.
> 
> Currently, there is no paradigm that all I2C busses must be fully
> described. Enforcing it now all of a sudden is not too user-friendly,
> or?

We're only enforcing it for systems that want to make use of this new
API, so it's not breaking backward compatibility.

> Especially since calling read_byte once is not necessarily "great
> length" in my book. If you have 8 cameras on a 400kHz bus, the 8 * 18
> bits should take 360us if I am not mistaken?

That's assuming the first scanned address is free. There could also be
I2C-controller I2C muxes or gates in front of the bus. Things can
quickly get more expensive.

-- 
Regards,

Laurent Pinchart



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