What is the purpose of i2c-bcm2708?

[MartyMacGyver]

Note: This may have been asked before. I dutifully searched for i2c-bcm2708 first: "No posts were found because the word i2c-bcm2708 is not contained in any post." Eh?

In all my dabblings with the RPi and RPi2, I've been careful to ensure i2c-bcm2708 is not blacklisted and i2c-dev is in /etc/modules, but I've never added i2c-bcm2708 to that file and yet I2C has always seemed to work fine.

In troubleshooting a potentially related issue I stumbled on the recommendations to have both i2c-bcm2708 and i2c-dev in /etc/modules... the addition of i2c-bcm2708 had no effect on the problem (it may be entirely unrelated to anything I2C) but it made me wonder: What is i2c-bcm2708 supposed to be adding to my RPi's functionality that i2c-dev didn't already (somehow, apparently)?

[joan]

I have no idea. My guess is that i2c-dev creates the /dev/i2c-0 and /dev/i2c-1 devices and i2c-bcm2708 is the actual driver which talks to the hardware I2C peripheral.

Now that device tree is in use you should have i2c-dev in /etc/modules but i2c-bcm2708 should NOT be in /etc/modules (although whether it does any harm or not is an unanswered question, it doesn't seem to do any harm currently).

The device tree entry dtparam=i2c1=on in /boot/config.txt should take care of loading i2c-bcm2708.

[MartyMacGyver]

I was already using dtparam=i2c_arm=on so yes, if that's loading i2c-bcm2708 already then the extra i2c-bcm2708 module reference is redundant (also explains why it had no apparent effect). Unless others disagree I think this answers my question - thank you!

[DougieLawson]

Using any one of dtparam=i2c-rtc, dtparam=i2c=on, dtparam=i2c_arm=on or dtparam=i2c_vc=on will load i2c-bcm2708

[MartyMacGyver]

I guess the fact that device tree things have worked for me so far has led to them flying under my radar... the RTC part is interesting as well (looks like it's supposed to be an overlay (dtoverlay=i2c-rtc,ds3231) rather than a dtparam, replacing the module invocation rtc-ds1307 (which I previously used)). This may or may not improve things for me, but it's clear I needed to update my settings either way to be current. Thanks again!

As an aid to those with similar issues, further reading:

viewtopic.php?t=97314

https://www.raspberrypi.org/documentati ... ce-tree.md

[DougieLawson]

The ds1307 stuff is orders of magnitude easier than the old way (echo strange arcane command to something in /sysfs). You add the config.txt stuff and i2c address 0x68 is activated and captured by the kernel and /dev/rtc0 appears by magic.