AM623: UART DMA issues

Part Number: AM623

Tool/software:

UART DMA feature is very important for industry application, some application used up all UART port, even at high baud rate. 

As long as application loading increase, need DMA to offload, otherwise will drop data arbitrary/occasionally. 

As there are several UART DMA issue, some thread was messed up with other issue, for better tracking, start a new thread specifically for UART DMA issue.

#1. This thread reported UART transmit abnormal long with DMA, avoided by hack code to not use DMA for UART TX. It can be test purpose, not a solution for UART DMA for SDK10.x

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1472428/am625-uart-with-dma-takes-longer-in-continues-sending/

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 0b9c47172bc8..0952f08622c9 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -1042,6 +1042,10 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
        unsigned int    skip_byte = 0;
        int ret;
 
+       /* HACK: do not use DMA for TX */
+       ret = -EINVAL;
+       goto err;
+
        if (dma->tx_running)
                return 0;
        if (uart_tx_stopped(&p->port) || uart_circ_empty(xmit)) {

#2. Another use case need SDK9.2 support UART DMA.