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If you have checked everything else, then you most likely are not using the required 1.8volt interface level to the serial port.
Note:
The same serial port is used, in succession:
SoC built-in ROM code ;
U-Boot ;
Linux kernel ;
So there is plenty to be seen.
Unless you have fried the serial port with the wrong voltage or made a wiring error.
Those are the only choices at this point.
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