Force UART to use serial 8250 driver

In order to simulate transceiver control signal of RS485 on TK1, I would like to use 8250 driver on UART4 without DMA. As Tegra HS serial driver requires DMA, it is not accurate to toggle transceiver control signal when transmitting with it(6ms latency is safe for transceiver control signal toggle, but it is longer than requirement). Device tree tegra124-apalis-eval.dts is modified as following.

	/* Apalis UART4 */
	serial@70006300 {
		compatible = "nvidia,tegra114-uart", "nvidia,tegra20-uart";
		status = "okay";


	uartd: serial@70006300 {
		compatible = "nvidia,tegra114-uart", "nvidia,tegra20-uart";
		reg = <0x0 0x70006300 0x0 0x40>;
		reg-shift = <2>;
		interrupts = <0 90 0x04>;
		/*  nvidia,dma-request-selector = <&apbdma 19>;
		nvidia,memory-clients = <14>;
		status = "disabled";
		dmas = <&apbdma 19>, <&apbdma 19>;
		dma-names = "rx", "tx"; 
		dma-names = "", ""; */

But UART4 is not accessible.

ubuntu@tegra-ubuntu:/proc/tty$ sudo cat driver/serial
serinfo:1.0 driver revision:
0: uart:Tegra mmio:0x70006000 irq:68 tx:47475 rx:362 RTS|DTR|DSR|CD|RI
1: uart:unknown port:00000000 irq:0
2: uart:unknown port:00000000 irq:0
3: uart:unknown port:00000000 irq:0
4: uart:unknown port:00000000 irq:0
5: uart:unknown port:00000000 irq:0
6: uart:unknown port:00000000 irq:0
7: uart:unknown port:00000000 irq:0
8: uart:unknown port:00000000 irq:0
ubuntu@tegra-ubuntu:/proc/tty$ sudo cat driver/serial-hs-tegra
serinfo:1.0 driver revision:
1: uart:SERIAL_TEGRA mmio:0x70006040 irq:69 tx:0 rx:0 CTS
2: uart:SERIAL_TEGRA mmio:0x70006200 irq:78 tx:0 rx:0 CTS

Where should be modified to use 8250 driver on uart4?

It seems not easy to apply serial 8250 driver on UART. I tried to initial it by apalis_tk1_uart_init,but failed. Does serial-hs-tegra chose PIO or DMA mode according to data length when transmitting ? Here is the driver code. If TEGRA_UART_MIN_DMA is a large value e.g 1024, then PIO mode will be used by most case ? Because just deleting 'dma-names = “rx”, “tx”; ’ in device tree will rise error when using uart .

Using our mainline based machine may allow for much easier such integration.