i2c ultra fast mode

One of the things that slows down the I2C bus is arbitration and allowing for multiple masters.  In Ultra Fast mode, they eliminate the need for these features allowing the bus to switch faster.   Another higher speed alternative to I2C is SPI, where speed is increased by making each line unidirectional, but they need to use a third line to accomplish this.

I am not worried about multi master issue in UFm. Let it be single master.

But, why is it unidirectional. What is the limiting factor?

Limiting here is the bus driver architecture. Clause 3.2 of the i2c specification states that UFM i2c is a push-pull bus.

The push-pull is required for fast bus switching as opposed to open drain (bidirectional).

Push-pull drivers have very low On resistance needed for momentary (fast) charging/discharging the bus capacity.

okay.....