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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.

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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

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Transfer Modes by Subsystem

SubsystemTransfer ModeBaud RatesNotes
LinuxFIFO, CPU DMA300 bps – 4 MbpsDMA ideal for Bluetooth modules
Boot (UEFI)FIFO onlyUp to 1152005–8 bits per character
aDSPFIFO only115200, 230400, 460800, 921600, 1M, 3M, 6M5–8 bits per character

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Interface Components

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Device Tree Sources

PlatformFile
Dragonwing IQ-9075arch/arm64/boot/dts/qcom/lemans.dtsi

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APIs

SubsystemHeader
Linuxinclude/linux/tty.h
BootQcomPkg/Include/HSUart.h
aDSPadsp_proc/core/api/buses/uart.h

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Software Configuration

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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 SC7280_CX>;
    interconnects = <&clk_virt MASTER_QUP_CORE_0 0 &clk_virt SLAVE_QUP_CORE_0 0>,
                    <&gem_noc MASTER_APPSS_PROC 0 &cnoc2 SLAVE_QUP_0 0>;
    interconnect-names = "qup-core", "qup-config";
    status = "disabled";
};
GPIO pinctrl: 
qup_uart7_cts: qup-uart7-cts-state { pins = "gpio28"; function = "qup07"; };
qup_uart7_rts: qup-uart7-rts-state { pins = "gpio29"; function = "qup07"; };
qup_uart7_tx:  qup-uart7-tx-state  { pins = "gpio30"; function = "qup07"; };
qup_uart7_rx:  qup-uart7-rx-state  { pins = "gpio31"; function = "qup07"; };
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 }

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Kernel Configuration

Edit kernel_platform/kernel/arch/arm64/configs/qcom_defconfig: 
CONFIG_QCOM_GENI_SE=y
CONFIG_SERIAL_QCOM_GENI=y
Enable the UART node in the device tree: 
+serial1 = &uart7;

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

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Verification

1

Verify UART device registration

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

dmesg | grep ttyH
# Expected: 99c000.serial: ttyHS1 at MMIO 0x99c000 ...
2

Configure UART settings

stty -F /dev/ttyHS1 115200
3

Run loopback test

Open two SSH terminals.Terminal 1 (write):
echo "Hello UART" > /dev/ttyHS1
Terminal 2 (read):
cat /dev/ttyHS1
Expected output: Hello UART

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Debugging

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Enable Debug Logs

mount -t debugfs none /sys/kernel/debug

echo -n "file qcom_geni_serial.c +p" > /sys/kernel/debug/dynamic_debug/control
echo -n "file qcom-geni-se.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

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Troubleshooting

lsmod | grep qcom_geni
cat /proc/device-tree/soc@0/geniqup@*/serial*/status
ls -la /dev/tty*
  • Verify device tree status is "ok"
  • Confirm CONFIG_SERIAL_QCOM_GENI=y in kernel config
  • Check GPIO pin configurations and QUPAC access control settings
# Check current UART config
stty -F /dev/ttyHS1 -a

# Set baud rate
stty -F /dev/ttyHS1 115200

# Enable/disable hardware flow control
stty -F /dev/ttyHS1 crtscts    # enable
stty -F /dev/ttyHS1 -crtscts   # disable
  • Verify baud rate matches on both ends
  • Check flow control configuration (CTS/RTS)
  • Test with loopback mode first
ls -la /dev/ttyHS*
usermod -a -G dialout $USER
  • Switch from FIFO to DMA mode for high-speed transfers
  • Check interrupt statistics: cat /proc/interrupts | grep uart
  • Review power management: cat /sys/kernel/debug/pm_genpd/pm_genpd_summary

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Debugging Checklist

  • Kernel config includes CONFIG_SERIAL_QCOM_GENI=y
  • Device tree status set to "ok" for UART node
  • GPIO pins properly configured and mapped
  • QUPAC access control properly configured
  • Baud rate matches on both sides
  • Flow control settings are consistent
  • Device file permissions are correct

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Resources