Thank you Robin 2 for your great tutorial about serial in. But my problem is about Tx !
I use a serial port run at 115200 to communicate with a display module. The communication use command like long text strings. At times when there is more data to send, some text will be missed in the display module. So I think increase Tx buffer may correct this problem. I can't afford increase all 4 Tx buffer because there is less than 30% memory left.
For the suggestion of use a Due, sadly I don't know how to use it, and I think it's learning curve is too steep for me !