Hi Wolfgang, it finally works :-). I had to make a small modification to the x.org source code. The file xc/programs/Xserver/hw/xfree86/i2c/xf86i2c.c contains the initialization of the i2c bus. In function xf86I2CBusInit, I added the two lines near the beginning:
if (b->I2CStart == NULL) b->I2CStart = I2CStart; which sets the start function to a default function. The problem was that mb86290_i2c.c does not provide a start function, whereas xf86I2CBusInit expects one under some circumstances. Best regards, Matthias