PCIe - Qualcomm Dragonwing Documentation

PCIe (Peripheral Component Interconnect Express) uses a bidirectional connection to send and receive information simultaneously. The path between devices is called a Link, made up of one or more transmit/receive pairs called Lanes.

Hardware Specifications

IQ-9075 PCIe Controllers

Parameter	PCIe RC1	PCIe RC0
Speed	Gen4 2-lane (8 GT/s)	Gen4 4-lane (8 GT/s)
Configuration Space	0x40100000 (1 MB)	0x60100000 (1 MB)
I/O Space	0x40200000 (1 MB)	0x60200000 (1 MB)
BAR Space	0x40300000 (509 MB)	0x60300000 (509 MB)
Power Rails	vreg_l5a (VDD_A_PCIE_0_CORE), vreg_l1c (VDD_A_PCIE_0_PLL_1P2)	vreg_l5a (VDD_A_PCIE_1_CORE), vreg_l1c (VDD_A_PCIE_1_PLL_1P2)
Interrupts	MSI and PCI legacy	MSI and PCI legacy
Power Management	ASPM (L1/L1ss, L0s)	ASPM (L1/L1ss, L0s)

EVK Hardware Configuration

PCIe0: Gen4 2-lane — M.2 E key (Wi-Fi) or expansion (switch-selectable via SW1-1)
PCIe1: Gen4 4-lane — PCIe x4 slot or expansion (switch-selectable via SW1-2)
JPCIE: PCIe Express v4 receptacle connector (PCIe1)

Architecture

Layer	Key Features
Physical	Link training, 8b/10b encoding, serial conversion
Data Link	DLLP assembly, LCRC generation, ACK/NAK replay
Transaction	TLP assembly, ECRC, flow control, QoS (TC→VC mapping)

PCIe Connection Types

Type	Description
Root Complex (RC)	Connects CPU to PCIe topology
Switch	Routes packets between more than 2 ports
Bridge	Connects different buses
Endpoint (EP)	Leaf device with only an upstream port

Software Driver Configuration

Kernel Configuration

CONFIG_HOTPLUG_PCI=y
CONFIG_PCI_HOST_GENERIC=y
CONFIG_PCIE_DW_PLAT_HOST=y
CONFIG_PCIE_QCOM=y
CONFIG_PCI_ENDPOINT=y
CONFIG_PCIEASPM=y
CONFIG_PCIEASPM_POWER_SUPERSAVE=y
CONFIG_BLK_DEV_NVME=y
CONFIG_NVME_CORE=y

Device Tree Sources

Platform	File
Dragonwing IQ-9075	`arch/arm64/boot/dts/qcom/sa8775p.dtsi`

Key Driver Files

Component	Path
PCIe Driver	`drivers/pci/controller/dwc/pcie-qcom.c`
DesignWare Host	`drivers/pci/controller/dwc/pcie-designware-host.c`
PHY Driver	`drivers/phy/qualcomm/phy-qcom-qmp-pcie.c`

Verification

List PCIe Devices

lspci

Example output:

00:00.0 PCI bridge: Qualcomm Device 010b
01:00.0 PCI bridge: Toshiba Corporation Device 0623
05:00.0 Ethernet controller: Toshiba Corporation Device 0220

Check Link Status

lspci -vv | grep -i "lnk"
cat /sys/bus/pci/devices/*/current_link_speed
cat /sys/bus/pci/devices/*/current_link_width

USB over PCIe

After connecting a USB-over-PCIe card:

lspci
# Expected: USB controller: NEC Corporation uPD720200 USB 3.0 Host Controller

lsusb
# Shows connected USB devices

NVMe over PCIe

lspci | grep -i nvme
ls /dev/nvme*
nvme list

Debugging

Enable Debug Logs

sudo su
echo 8 > /proc/sys/kernel/printk
echo "file pcie-qcom.c +p" > /sys/kernel/debug/dynamic_debug/control
echo "file pcie-designware-host.c +p" > /sys/kernel/debug/dynamic_debug/control

dmesg | grep -i pcie

Troubleshooting

Link Training Fails

Possible causes: PHY not initialized, clock/power issues, incorrect device tree.

dmesg | grep -i phy | grep -i pcie
cat /sys/kernel/debug/regulator/regulator_summary | grep -i pcie

Check PHY power rails and clock configuration
Verify device tree settings

Device Not Enumerated

Possible causes: Link not established, endpoint not powered, switch misconfigured.

lspci -tv
dmesg | grep -i pcie

Verify DIP switch positions (SW1-1, SW1-2)
Check endpoint power supply

MSI Not Working

Enable MSI in device tree
Check interrupt controller configuration
Verify pci_enable_msi() returns success in driver

Performance Issues

lspci -vv | grep -i "lnk"
# Check Speed and Width fields

Verify link is running at expected Gen4 speed
Adjust ASPM settings if too aggressive

Quick Reference

Essential Commands

lspci                                              # List all PCIe devices
lspci -vv                                          # Detailed device info
lspci -tv                                          # Tree view
lspci -nn                                          # With numeric IDs
dmesg | grep -i pcie                               # Kernel messages
cat /sys/bus/pci/devices/*/current_link_speed      # Link speeds
cat /sys/bus/pci/devices/*/current_link_width      # Link widths

​Hardware Specifications

​IQ-9075 PCIe Controllers

​EVK Hardware Configuration

​Architecture

​PCIe Connection Types

​Software Driver Configuration

​Kernel Configuration

​Device Tree Sources

​Key Driver Files

​Verification

​List PCIe Devices

​Check Link Status

​USB over PCIe

​NVMe over PCIe

​Debugging

​Enable Debug Logs

​Troubleshooting

​Quick Reference

​Essential Commands

​Resources