For the long exposure mode, long exposure operation can be performed by lengthening the frame period.
When the sensor is operating in slave mode, this is done by lengthening the input vertical sync signal (XVS) pulse interval.
When the sensor is operating in master mode, it is done by designating a larger register VMAX [19:0] value compared to normal operation. When the integration time is extended by increasing the number of lines, the rear Vblanking increases by an equivalent amount.
We do some test and found that, in fact in long exposure mode, the VMAX can be set to 0xFFFFF, and the long exposure time is Texp = (hts/pixclk)(VMAX-1) = (2200/148500000)(0XFFFFF-1） = 15.53s
For some speical environment, need long expposure mode, maybe we can try using
libcamera-still -o long_exposure.jpg --shutter 15530000
You can also modify the value after the shutter parameter to change the exposure time
libcamera-still -t 1000 --shutter 1000000 -o 1000000_picture.jpg
Exposure time corresponding to different values (reference)