igb: unbreak I2C bit-banging on i350
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.html Reviewed-by: