IQ-615 - Qualcomm Dragonwing Documentation

Debug the DisplayPort

The mini DisplayPort connector maps to DP-1. Run connector=DP-1 before the following checks. This selects the mini DisplayPort connector.

Connect to the target through SSH and run target commands from the root shell. For instructions, see Sign in using SSH.

Board connector	Linux DRM connector
Mini DisplayPort	DP-1

Figure: IQ-615 DisplayPort connection

The connector mapping above is platform-specific and determines the following used connector value. The seven troubleshooting steps are otherwise the same on every DisplayPort page.

1. Identify the connector

Run the following command:

modetest -M msm -c

This lists the DRM connectors, their IDs, and their connection states.

2. Check connector status and modes

Run the following command:

cat "/sys/class/drm/card0-${connector}/status"

This reports whether the selected connector is connected. The expected result is connected.

Reference output

connected

Run the following command:

cat "/sys/class/drm/card0-${connector}/modes"

This lists the modes reported by the connected display.A connected output normally reports at least one mode. If it is disconnected, check the cable, monitor power, and selected monitor input. If card0 is absent, use the DRM card reported by modetest.

3. Check the active DRM state

If debugfs is not mounted, run the following command:

mount -t debugfs none /sys/kernel/debug

This makes DRM state and interface debug data available.Run the following command:

modetest -M msm -c -p

This shows the selected connector’s encoder, CRTC, mode, and plane.

Reference output

CRTCs:
id      fb      pos     size
75      78      (0,0)   (1920x1080)
  #0 1920x1080 60.00

Run the following command:

cat /sys/kernel/debug/dri/0/state

This shows the kernel DRM state for each connector, encoder, CRTC, and plane.For the selected connector, confirm one active display pipeline:

status: connected
An encoder assigned to that connector
A nonzero CRTC ID from that encoder
active=yes for that CRTC
A plane whose crtc field matches that CRTC ID

A visible output requires every item.

Restore Weston after testing a display mode.To test a different mode, follow Run DRM modetest, then Relaunch Weston before returning to Weston tests.

4. Check interface signals

Run the following command:

cat "/sys/kernel/debug/dri/0/${connector}/dp_debug"

This shows DisplayPort link rate, lane count, and panel timing when the driver provides dp_debug.

Reference output

name = msm_dp
drm_dp_link
        rate = 270000
        num_lanes = 4
dp_panel_info:
        active = 1920x1080
        refresh rate = 60

Run the following command:

cat /sys/kernel/debug/clk/*dp*link_clk/clk_rate

This prints the DisplayPort link-clock rates in Hz.

Reference output

disp_cc_mdss_dp_link_clk: 270000000

Run the following command:

cat /sys/kernel/debug/clk/*dp*pixel*clk/clk_rate

This prints the DisplayPort pixel-clock rates in Hz.

Reference output

disp_cc_mdss_dp_pixel_clk: 74250000

A working link has nonzero rate and lane values and reports at least one mode. Compare the pixel clock with the active CRTC mode, which modetest reports in kHz.

5. Check current-boot kernel messages and live display events

Run the following command:

journalctl -k -b --no-pager -o short-monotonic | grep -Ei 'msm[_-]dp|displayport|edp|dp_aux|link.*train|hpd|typec|ucsi|pmic_glink|dpu|drm'

This shows DisplayPort and DRM messages from the current boot only. If it returns no lines, capture live data while reproducing the issue.

Reference output

[   12.512759] [drm] Initialized msm 1.13.0 for ae01000.display-controller

To capture display events while reproducing the issue, run the following command:

journalctl -f

This follows new Weston and kernel messages. Run it without -k so the output includes compositor connector changes and output enablement. Reproduce the issue, then press Ctrl+C.Look for connector-state changes, EDID or mode detection, output enablement, and driver errors. Compare the relevant messages with the connector status and active DRM state.AUX or DPCD timeouts point to the cable or port, and failed link training points to link quality.

6. Interpret the result

Result	Likely area	Next action
`disconnected`	Cable or monitor input	Reseat the cable and select the correct monitor input.
Connected with no modes or AUX/DPCD timeouts	Cable, port, or EDID	Check the cable and monitor, then reconnect while following the live journal.
Link-training failure	Cable or signal quality	Try a known-good cable and a lower-bandwidth mode.
Modes present, but the connector, encoder, CRTC, and plane do not form one active pipeline	DRM or compositor	Check the active DRM state and Weston status.
No new display message and no connector-status change after reconnecting	HPD or Type-C Alt Mode	Check the port, Type-C orientation, and cable.

Verify the Weston service before checking the output layout.Run the following command:

systemctl is-active weston

The expected result is active. If the service is not active, Relaunch Weston before continuing.

7. Check Weston and the output layout

Run the following command:

journalctl -b -u weston -n 40 --no-pager

This shows the latest Weston messages from the current boot.Run the following command if Weston is active but the display is blank:

find /run/user/<weston-uid> -maxdepth 1 -type s -name 'wayland-*' -printf '%f\n'

This lists the available Wayland socket names.Run the following command after replacing the placeholders:

runuser -u weston -- env XDG_RUNTIME_DIR=/run/user/<weston-uid> WAYLAND_DISPLAY=<socket-name> wayland-info

Compare the Weston output with DRM state.This prints Weston output information. Find the selected display output and compare it with the DRM connector and CRTC state. If Weston is inactive, Relaunch Weston before continuing.

​Debug the DisplayPort

​1. Identify the connector

​2. Check connector status and modes

​3. Check the active DRM state

​4. Check interface signals

​5. Check current-boot kernel messages and live display events

​6. Interpret the result

​7. Check Weston and the output layout

Debug the DisplayPort

1. Identify the connector

2. Check connector status and modes

3. Check the active DRM state

4. Check interface signals

5. Check current-boot kernel messages and live display events

6. Interpret the result

7. Check Weston and the output layout