[lm-sensors] [PATCH] I2C: i2c_bit_add_bus should initialize SDA and SCL lines

Mark M. Hoffman mhoffman at lightlink.com
Mon Jun 12 14:36:26 CEST 2006

Hi Krzysztof:

> "Mark M. Hoffman" <mhoffman at lightlink.com> writes:
> > SCL and SDA must be pulled high by hardware.  If a driver inits to
> > setting them low, that's a bug in the driver.

* Krzysztof Halasa <khc at pm.waw.pl> [2006-06-11 00:27:14 +0200]:
> Thanks for your response.
> The question is rather who inits the lines: a) the hw driver,
> b) the I2C algorithm driver.
> With a) every hw driver has to know how to init them (duplicated
> code but there might be positive side).
> With b) I2C algorithm driver inits the lines and hw driver
> doesn't worry about but it might have some limitations such
> as unknown SCL state.
> I understand the current case is a) - right?

I think it should be, yes.

> The other question is _how_ to init the lines. There are 4 possible
> hardware initial conditions:
> a)  0   0 (outputs zeroed by default)
> b)  0   1 (uncommon but may be left in this state by previous operations)
> c)  1   0 (ditto)
> d)  1   1 (I/O lines configured as input by default)
> The internal state of devices connected to the bus is potentially
> unknown. Some implementations just start with STOP to eliminate
> this problem, I don't know what Linux driver is supposed to do.

If you can read the line state, then yes... I would detect which of
the above four states you're in, and generate a STOP condition for
a, b, and c:

a) setscl(1), setsda(1)
b) setsda(0), setscl(1), setsda(1)
c) setsda(1)

If you can't read the line state... well, that's not actually a proper
I2C bus anyway.  At that point I suggest 'whatever works'.

But even if you need to generate a 'null transfer' (prohibited by spec)
to get reliable operation... that doesn't belong in the algorithm driver.

> (Other implementation I know are rather specialized and thus they
> know their hardware init state, Linux I2C algorithm handles many
> devices with potentially different initial state of hardware lines).
> To summarize questions:
> - is it the hw driver who has to init the bus
> - how to init the bus (depending on init state)


Mark M. Hoffman
mhoffman at lightlink.com

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