UART - Qualcomm Dragonwing Documentation

UART (Universal Asynchronous Receiver-Transmitter) transmits data asynchronously using start and stop bits instead of a clock signal. The receiving UART reads bits at a specific frequency known as the baud rate.

Key Parameters

Baud rate — Communication speed (bits per second)
Start/Stop bits — Frame delimiters
Parity bit — Optional error checking
Data bits — 5–9 bits per frame
Flow control — Hardware handshaking via CTS/RTS

Transfer Modes by Subsystem

Subsystem	Transfer Mode	Baud Rates	Notes
Linux	FIFO, CPU DMA	300 bps – 4 Mbps	DMA ideal for Bluetooth modules
Boot (UEFI)	FIFO only	Up to 115200	5–8 bits per character
aDSP	FIFO only	115200, 230400, 460800, 921600, 1M, 3M, 6M	5–8 bits per character

Interface Components

Device Tree Sources

Platform	File
Dragonwing IQ-8275	`arch/arm64/boot/dts/qcom/sa8295p.dtsi`

APIs

Subsystem	Header
Linux	`include/linux/tty.h`
Boot	`QcomPkg/Include/HSUart.h`
aDSP	`adsp_proc/core/api/buses/uart.h`

Software Configuration

Linux Device Tree Example

4-wire UART (with flow control):

uart7: serial@99c000 {
    compatible = "qcom,geni-uart";
    reg = <0 0x0099c000 0 0x4000>;
    clocks = <&gcc GCC_QUPV3_WRAP0_S7_CLK>;
    clock-names = "se";
    pinctrl-names = "default";
    pinctrl-0 = <&qup_uart7_cts>, <&qup_uart7_rts>,
                <&qup_uart7_tx>, <&qup_uart7_rx>;
    interrupts = <GIC_SPI 608 IRQ_TYPE_LEVEL_HIGH>;
    power-domains = <&rpmhpd SA8295P_CX>;
    interconnect-names = "qup-core", "qup-config";
    status = "disabled";
};

QUPAC access control:

// 4-wire HS UART
{ QUPV3_0_SE7, QUPV3_PROTOCOL_UART_4W, QUPV3_MODE_FIFO, AC_HLOS, TRUE, TRUE, FALSE }

// 2-wire UART (no flow control)
{ QUPV3_0_SE5, QUPV3_PROTOCOL_UART_2W, QUPV3_MODE_FIFO, AC_HLOS, TRUE, FALSE, FALSE }

Kernel Configuration

CONFIG_QCOM_GENI_SE=y
CONFIG_SERIAL_QCOM_GENI=y

Enable the UART node in the device tree:

+serial1 = &uart7;

+&uart7 {
+    status = "ok";
+};

Verification

Verify UART device registration

ls /dev/ttyHS*
# Expected: /dev/ttyHS1

dmesg | grep ttyH

Configure UART settings

stty -F /dev/ttyHS1 115200

Run loopback test

Open two SSH terminals.Terminal 1 (write):

echo "Hello UART" > /dev/ttyHS1

Terminal 2 (read):

cat /dev/ttyHS1

Debugging

mount -t debugfs none /sys/kernel/debug
echo -n "file qcom_geni_serial.c +p" > /sys/kernel/debug/dynamic_debug/control
echo -n "file serial_core.c +p" > /sys/kernel/debug/dynamic_debug/control
dmesg | grep -i uart
dmesg | grep ttyHS

Troubleshooting

UART Device Not Detected

lsmod | grep qcom_geni
cat /proc/device-tree/soc/serial@*/status
ls -la /dev/tty*

Verify device tree status is "ok"
Confirm CONFIG_SERIAL_QCOM_GENI=y

Data Transmission Failures / Garbled Data

stty -F /dev/ttyHS1 -a
stty -F /dev/ttyHS1 115200
stty -F /dev/ttyHS1 crtscts    # enable flow control
stty -F /dev/ttyHS1 -crtscts   # disable flow control

Permission Denied

ls -la /dev/ttyHS*
usermod -a -G dialout $USER

​Key Parameters

​Transfer Modes by Subsystem

​Interface Components

​Device Tree Sources

​APIs

​Software Configuration

​Linux Device Tree Example

​Kernel Configuration

​Verification

​Debugging

​Troubleshooting

​Resources