Options For Driver i2c-dev

Option: --use-file-io

Driver i2c-dev provides two interfaces for applications. The first uses ioctl(). The second, simpler, interface hides the ioctl() calls behind a traditional file io based interface, using functions write() and read().

Normally, ddcutil uses the ioctl() interface. Unfortunately, when the ioctl() interface is used with older versions of the Nvidia proprietary video driver, the Nvidia driver rejects calls from i2c-dev. When this error is detected, ddcutil switches to using the file io interface.

This option forces use of the file io interface, avoiding the need for recovery.

For details about how this problem was diagnosed, see ddcutil issue #341.

Option: --force-slave-address

When the i2c-dev file io interface is used, occasionally communication cannot be established because driver i2c-dev has marked the I2C bus as being in use by another program (error EBUSY). This option allows ddcutil to take control of the bus, potentially impacting the other program. (Internally, ddcutil uses ioctl(I2C_SLAVE_FORCE) instead of ioctl(I2C_SLAVE).)

This option has no effect in the normal case where the i2c-dev ioctl() interface is used.

Option: --edid-read-size 128|256

Sometimes a display's EDID cannot be read on I2C slave address x50. As a result, ddcutil does not report the existence of a display connected to the /dev/i2c device. ddcutil attempts to work around this by trying to read the EDID multiple times with both 128 and 256 block sizes. Depending on the monitor, usually one or the other will work, but sometimes neither do. Option --edid-read-size, introduced in release 1.0.1, can force the block size to 128 or 256.

Option: --i2c-source-address <hex address>

Recent LG monitors allow for splitting the display screen between multiple input sources. Instead of the (relatively) simple VCP feature x60 (Input Source), they use an undocumented protocol, to control how the screen is split and which input source is shown where. Thanks to a community effort, particularly by user Max Thomas (Github handle shinyquagmire23), some of this protocol has been reverse engineered. It entails using an alternative I2C source address, x50 instead of x51, in the I2C packet. Option --i2c-source-addr provides a way to use this alternative address in DDC communication. For further details, see this wiki page. The extended discussion summarized on the wiki page occurred on ddcutil issue 100: LG 29UM69G fails switching input.