serial: imx: reduce RX interrupt frequency

Triggering RX interrupt for every byte defeats the purpose of aging
timer and leads to interrupt storm at high baud rates. The interrupt
storm can starve line discipline worker and prevent tty throttling,
rendering hardware/software flow control useless.

Increase receiver trigger level to 8 to increase the minimum period
between RX interrupts to 8 characters time. The tradeoff is increased
latency.

Aging timer resets with every received character. Worst case scenario
happens when RX data intercharacter delay is slightly less than the
aging timer timeout (8 characters time). The upper bound of the time
a character can wait in RxFIFO before interrupt is raised is:
  (RXTL - 1) * (8 character time timeout + received 1 character time)

Usually the data is received in frames, with low intercharacter delay.
In such case the latency increase is 8 characters time at the end of
the frame with probability (RXTL - 1) / RXTL.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Link: https://lore.kernel.org/r/20220117060417.624613-1-tomasz.mon@camlingroup.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -1258,7 +1258,7 @@ static void imx_uart_clear_rx_errors(struct imx_port *sport)
 }
 
 #define TXTL_DEFAULT 2 /* reset default */
-#define RXTL_DEFAULT 1 /* reset default */
+#define RXTL_DEFAULT 8 /* 8 characters or aging timer */
 #define TXTL_DMA 8 /* DMA burst setting */
 #define RXTL_DMA 9 /* DMA burst setting */