I2C

Inter-Integrated Circuit (I²C) is a bidirectional, two-wire serial bus for short-distance communication between integrated circuits, where devices communicate using assigned bus addresses.

Overview

Key Characteristics

Bidirectional 2-wire serial bus (SDA + SCL)
7-bit or 10-bit target addressing
Multi-controller mode support

Operating Modes

Mode	Speed
Standard mode	100 kbps
Fast mode	400 kbps
Fast mode plus	1 Mbps (maximum supported)
High-speed mode	3.4 MHz (not supported)

Communication Sequence

Packet structure: Start → Address frame (7/10-bit) → R/W bit → ACK/NACK → Data frames (8-bit) → Stop Signal rules:

SCL HIGH: SDA must remain stable
SCL LOW: SDA may change
Exception: START and STOP sequences may change SDA when SCL is HIGH

Transfer Modes by Subsystem

Subsystem	Transfer Mode	Bus Speeds
Linux	FIFO, CPU DMA, GSI	100 kHz, 400 kHz, 1000 kHz
Boot	FIFO	100 kHz, 400 kHz, 1000 kHz
aDSP / Qualcomm TEE / SDC	FIFO	Standard

Interface Components

Device Tree Sources

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

APIs

Subsystem	Header
Linux	`include/linux/i2c.h`, `include/linux/i2c-dev.h`
Boot	`boot_images/boot/QcomPkg/Include/i2c_api.h`
aDSP	`adsp_proc/core/api/buses/i2c_api.h`
Qualcomm TEE	`trustzone_images/core/buses/api/i2c/qupv3/i2c_api.h`

Software Device Tree Configuration

Linux Device Tree Example

i2c21: i2c@b80000 {
   compatible = "qcom,geni-i2c";
   reg = <0x0 0xb80000 0x0 0x4000>;
   #address-cells = <1>;
   #size-cells = <0>;
   interrupts = <GIC_SPI 831 IRQ_TYPE_LEVEL_HIGH>;
   clocks = <&gcc GCC_QUPV3_WRAP3_S0_CLK>;
   clock-names = "se";
   pinctrl-0 = <&qup_i2c21_default>;
   pinctrl-names = "default";
   power-domains = <&rpmhpd SA8295P_CX>;
   dmas = <&gpi_dma3 0 0 QCOM_GPI_I2C>,
          <&gpi_dma3 1 0 QCOM_GPI_I2C>;
   dma-names = "tx", "rx";
   status = "disabled";
};

QUPAC Access Control

Ensure the correct protocol is set in QUPAC_Access.c:

{ QUPV3_0_SE0, QUPV3_PROTOCOL_I2C, QUPV3_MODE_FIFO, AC_HLOS, TRUE, TRUE, FALSE },
{ QUPV3_1_SE5, QUPV3_PROTOCOL_I2C, QUPV3_MODE_GSI,  AC_HLOS, FALSE, TRUE, FALSE },

Configuration Steps

Enable kernel configurations

Edit kernel_platform/kernel/arch/arm64/configs/qcom_defconfig:

CONFIG_QCOM_GENI_SE=y
CONFIG_I2C_QCOM_GENI=m
CONFIG_I2C_CHARDEV=m
CONFIG_QCOM_GPI_DMA=m

Enable I2C node in device tree

+&i2c1 {
+    status = "okay";
+};

Compile and flash

Compile the kernel and device tree changes, then load images to the device.

Verification

Check Device Nodes

ls /dev/i2c*
# Expected: /dev/i2c-0  /dev/i2c-1  /dev/i2c-16

Use i2c-tools

sudo apt update && sudo apt install i2c-tools
i2cdetect -y -r 0
i2cdump -r 0-0xff 0 0x2b b
i2cget -y 0 0x2b 0x04
i2cset -y 0 0x2b 0x04 0xFF

Debugging

Enable Debug Logs

mount -t debugfs none /sys/kernel/debug
echo -n "file i2c-qcom-geni.c +p" > /sys/kernel/debug/dynamic_debug/control
echo -n "file i2c-core-base.c +p" > /sys/kernel/debug/dynamic_debug/control
echo -n "file gpi.c +p" > /sys/kernel/debug/dynamic_debug/control
dmesg | grep i2c

Troubleshooting

I2C Device Not Detected

cat /sys/firmware/devicetree/base/soc/i2c@984000/status
lsmod | grep i2c_qcom_geni
modprobe i2c_qcom_geni
cat /sys/kernel/debug/clk/gcc_qupv3_wrap0_s1_clk/clk_enable_count

I2C Transfer Timeout

Causes: No pull-up resistors, slave not responding, wrong clock frequency, bus stuck low.Add 4.7 kΩ pull-ups on SDA/SCL; adjust clock-frequency in device tree.

Invalid Protocol Error

Symptom: geni_i2c a94000.i2c: Invalid proto 1Verify QUPAC_Access.c sets QUPV3_PROTOCOL_I2C, AC_HLOS, bAllowFifo=TRUE, bLoad=TRUE.

DMA Transfer Failures

Verify CONFIG_QCOM_GPI_DMA=m; check dmas and dma-names in device tree.To test without DMA:

&i2c1 {
    /delete-property/ dmas;
    /delete-property/ dma-names;
};

Quick Diagnostic Commands

ls -l /dev/i2c*
i2cdetect -y -r 0
dmesg | grep -i i2c
cat /sys/kernel/debug/clk/clk_summary | grep -i qup
lsmod | grep i2c

IQ-8275 EVK

Overview

Key Characteristics

Operating Modes

Communication Sequence

Transfer Modes by Subsystem

Interface Components

Device Tree Sources

APIs

Software Device Tree Configuration

Linux Device Tree Example

QUPAC Access Control

Configuration Steps

Verification

Check Device Nodes

Use i2c-tools

Debugging

Enable Debug Logs

Troubleshooting

Quick Diagnostic Commands

Resources

​Overview

​Key Characteristics

​Operating Modes

​Communication Sequence

​Transfer Modes by Subsystem

​Interface Components

​Device Tree Sources

​APIs

​Software Device Tree Configuration

​Linux Device Tree Example

​QUPAC Access Control

​Configuration Steps

​Verification

​Check Device Nodes

​Use i2c-tools

​Debugging

​Enable Debug Logs

​Troubleshooting

​Quick Diagnostic Commands

​Resources

Overview

Key Characteristics

Operating Modes

Communication Sequence

Transfer Modes by Subsystem

Interface Components

Device Tree Sources

APIs

Software Device Tree Configuration

Linux Device Tree Example

QUPAC Access Control

Configuration Steps

Verification

Check Device Nodes

Use i2c-tools

Debugging

Enable Debug Logs

Troubleshooting

Quick Diagnostic Commands

Resources