Using the Linux kernel 4.19-rt provided by the TISDK 6.03, I was able to use a GPS device connected to the uart4 of the AM5749 device (custom board).
As a basic test I'm using gpscat, gpsmon or gpsctl to open the /dev/gnss0 interface.
Note: The UART4 doesn't use RTS/CTS signals, Only Tx and Rx are defined in the pinmux project (SW flow control).
Now I'm using a kernel v5.10.87-rt59 from the ti-linux branch  but I noticed a kernel crash while opening /dev/gnss0:
__irq_svc from mem_serial_in+0x4/0x1c mem_serial_in from omap8250_set_mctrl+0x38/0xa0 omap8250_set_mctrl from uart_update_mctrl+0x4c/0x58 uart_update_mctrl from uart_dtr_rts+0x60/0xa8 uart_dtr_rts from tty_port_block_til_ready+0xd0/0x2a8 tty_port_block_til_ready from uart_open+0x14/0x1c uart_open from ttyport_open+0x64/0x148 ttyport_open from serdev_device_open+0x28/0xb0 serdev_device_open from gnss_serial_open+0x14/0x98 gnss_serial_open from gnss_open+0x68/0xb4 gnss_open from chrdev_open+0xa8/0x1a0 chrdev_open from do_dentry_open+0x110/0x3ec do_dentry_open from path_openat+0xb64/0xd6c path_openat from do_filp_open+0x80/0xf4 do_filp_open from do_sys_openat2+0x304/0x3d8 do_sys_openat2 from do_sys_open+0x7c/0xcc do_sys_open from ret_fast_syscall+0x0/0x4c
It's not 100% reproducible but if you try 10 time the same command, the kernel will crash.
It seems that the driver fail to read the UART_LCR register from omap8250_set_mctrl():
"lcr = serial_in(up, UART_LCR);"
PC is at mem_serial_in+0x2c/0x30
LR is at omap8250_set_mctrl+0x48/0xb0
The problem only occurs with a -rt kernel, I tried with several kernel version: 5.10, 5.15 and 5.17.
There is not crash with a standard kernel.
Testing again today with the 4.19-rt kernel, I didn't reproduce the issue.