The driver tried to use Linux' native software I2C bus master
(i2c-algo-bits) for exporting the I2C interface that talks to the SFP
cage(s) towards userspace. As-is, however, the physical SCL/SDA pins
were not moving at all, staying at logical 1 all the time.

The main culprit was the I2CPARAMS register where igb was not setting
the I2CBB_EN bit. That meant that all the careful signal bit-banging was
actually not being propagated to the chip pads (I verified this with a
scope).

The bit-banging was not correct either, because I2C is supposed to be an
open-collector bus, and the code was driving both lines via a totem
pole. The code was also trying to do operations which did not make any
sense with the i2c-algo-bits, namely manipulating both SDA and SCL from
igb_set_i2c_data (which is only supposed to set SDA). I'm not sure if
that was meant as an optimization, or was just flat out wrong, but given
that the i2c-algo-bits is set up to work with a totally dumb GPIO-ish
implementation underneath, there's no need for this code to be smart.

The open-drain vs. totem-pole is fixed by the usual trick where the
logical zero is implemented via regular output mode and outputting a
logical 0, and the logical high is implemented via the IO pad configured
as an input (thus floating), and letting the mandatory pull-up resistors
do the rest. Anything else is actually wrong on I2C where all devices
are supposed to have open-drain connection to the bus.

The missing I2CBB_EN is set (along with a safe initial value of the
GPIOs) just before registering this software I2C bus.

The chip datasheet mentions HW-implemented I2C transactions (SFP EEPROM
reads and writes) as well, but I'm not touching these for simplicity.

Tested on a LR-Link LRES2203PF-2SFP (which is an almost-miniPCIe form
factor card, a cable, and a module with two SFP cages). There was one
casualty, an old broken SFP we had laying around, which was used to
solder some thin wires as a DIY I2C breakout. Thanks for your service.
With this patch in place, I can `i2cdump -y 3 0x51 c` and read back data
which make sense. Yay.
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
See-also: https://www.spinics.net/lists/netdev/msg490554.htmlReviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>