How could we reconfige the ardunio's uart?

With test, we find that our PYNQ ardunio’s uart could only has 16 bytes,cause the FIFO configuration.
So we search the code in pynq’s dictionary.
And we find a variable XUL_FIFO_SIZE is set to 16.

But the problem is ,the file comment tell us it cann’t be changed, It is a pity. I think it is a important thing.
Could anyone tell me how to configurate it or is it really can’t be changed?"

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what is the function of FIFORESET ? IS it a way for us to change it ? How could we use this function in pynq environment?
THank you ,I have try Pynq for a long time. Really learn a lot of things.