serial: imx: Raise TX trigger level to 8

At the default TX trigger level of 2 in non-DMA mode (meaning that an
interrupt is generated when less than 2 characters are left in the
FIFO), we have observed frequent buffer underruns at 115200 Baud on an
i.MX8M Nano. This can cause communication issues if the receiving side
expects a continuous transfer.

Increasing the level to 8 makes the UART trigger an interrupt earlier,
giving the kernel enough time to refill the FIFO, at the cost of
triggering one interrupt per ~24 instead of ~30 bytes of transmitted
data (as the i.MX UART has a 32 byte FIFO).

Signed-off-by: Michael Krummsdorf <michael.krummsdorf@tq-group.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20240508133744.35858-1-matthias.schiffer@ew.tq-group.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: tq-schifferm

drivers/tty/serial/imx.c

@@ -1305,7 +1305,7 @@ static void imx_uart_clear_rx_errors(struct imx_port *sport)
 
 }
 
-#define TXTL_DEFAULT 2 /* reset default */
+#define TXTL_DEFAULT 8
 #define RXTL_DEFAULT 8 /* 8 characters or aging timer */
 #define TXTL_DMA 8 /* DMA burst setting */
 #define RXTL_DMA 9 /* DMA burst setting */