Public bug reported: # Bug report: External monitor fails EDID detection through Dell WD25Z dock (separate from known UCSI duplicate-altmode bug)
## Summary One external monitor connected via DisplayPort to a Dell Pro Dock WD25Z is never assigned a valid EDID by the kernel, and is limited to a small set of generic fallback resolutions (max 1024x768) instead of its native 1920x1080. A second external monitor connected via DisplayPort to the same dock, and the same monitor/cable/port under Windows 11, work correctly at 1920x1080. This report also documents a related, already- fixed UCSI firmware-duplicate-altmode bug encountered on the same hardware, which is **not** the cause of the EDID issue but was investigated in the same session and may be relevant context. ## System information - **Laptop:** Dell Latitude 7420 (BIOS 1.51.0 and 1.52.0 both tested, no change) - **OS:** Ubuntu 26.04.1 LTS - **Kernels tested:** - `7.0.0-31-generic` (default Ubuntu kernel) - `7.1.13-070113-generic` (mainline build from kernel.ubuntu.com, includes upstream commit `67c92c6419ea6dbc5b1f3e9691aecea956e3e81c`) - **Dock:** Dell Pro Dock WD25Z (USB4/DisplayPort Alt Mode; does not negotiate as a Thunderbolt device — confirmed absent from `boltctl` output and from Windows' Thunderbolt Control Center on both ports) - **Affected monitor:** connected via DisplayPort on the dock - **Working monitor for comparison:** LG Electronics 22" (model reports as "LG FULL HD"), connected via a second DisplayPort output on the same dock ## Steps to reproduce 1. Connect the Dell WD25Z dock to the Latitude 7420 via USB-C. 2. Connect two external monitors to the dock's two DisplayPort outputs (one HDMI + one DP was also tested with the same result on the DP-connected monitor). 3. Boot Ubuntu. ## Expected behaviour Both external monitors are detected with correct EDID (vendor/product/serial) and offer their native 1920x1080 resolution, as happens under Windows 11 on the same hardware. ## Actual behaviour - One monitor (LG) is detected correctly every time. - The other monitor is detected as "Unknown Display" (GNOME/Mutter) with `Vendor: unknown`, `Product: unknown`, `Serial: unknown`, and is limited to a short list of fallback modes (`[email protected]`, `[email protected]`, `[email protected]`, `[email protected]`, `[email protected]`) with no 1920x1080 option. - `/sys/class/drm/cardX-DP-Y/edid` for the affected connector reads back **0 bytes**, while the working monitor's EDID reads back correctly at the same time. - The DRM connector number assigned to the affected monitor changes on every boot and every resume from suspend (observed as `DP-5`, `DP-6`, `DP-9`, `DP-10` across different sessions for what is physically the same port), consistent with the dock's internal MST/DP hub re-enumerating non-deterministically. - DisplayPort Alt Mode itself negotiates successfully on the USB-C link for this monitor: `/sys/class/typec/portN/portN-partner/portN-partner.X/active` = `yes` with the DisplayPort mode active. The failure appears to be downstream, in the actual DP-AUX/EDID read over the dock's internal hub for this specific output. ## Separate, already-resolved issue encountered during investigation While investigating, the following kernel WARNING was found on kernel `7.0.0-31-generic` when hot-plugging the dock: ``` ucsi_acpi USBC000:00: con2: Firmware bug: duplicate partner altmode SVID 0x413c (VDO 0x00000001 vs 0x00000002) at offset 2, ignoring. Please update your system firmware. ------------[ cut here ]------------ WARNING: drivers/usb/typec/class.c:311 at typec_altmode_update_active+0x101/0x110 [typec] ... Call Trace: ucsi_altmode_update_active+0x116/0x1b0 [typec_ucsi] ucsi_check_altmodes+0x92/0xd0 [typec_ucsi] ucsi_poll_worker+0x41/0xe0 [typec_ucsi] ``` This matches the upstream fix "usb: typec: ucsi: Detect and skip duplicate altmodes from buggy firmware" (commit `67c92c6419ea6dbc5b1f3e9691aecea956e3e81c`, landed for v7.1.6/v7.2). After building and booting mainline kernel `7.1.13`, this WARNING/call trace no longer occurs. However: - `/sys/class/typec/port1/port1.2/` and `port1.3/` (port side) and `port1-partner.1/` and `.2/` (partner side) still both advertise SVID `413c` with distinct VDOs (`0x00000001` and `0x00000002`) — the duplication is still present in the dock's firmware, it is simply no longer causing a kernel warning/crash, which appears to be intentional given the port itself also advertises the SVID twice (a case the current fix logic treats as tolerable rather than as the "buggy firmware" case it filters). - **This fix did not resolve the EDID/resolution issue**, which persisted identically on kernel 7.1.13. The two issues are independent. I am not certain whether the EDID read failure is best classified as a kernel/DRM driver issue, a udev/enumeration-ordering issue, or purely a dock firmware issue outside the kernel's control — filing here in case it's the former, and in parallel reporting to Dell in case it's a dock firmware issue. ## Workaround currently in use Forcing a synthetic EDID via the `drm.edid_firmware` kernel parameter, mapped to the range of connector names observed across boots, restores 1920x1080 output but does not restore correct monitor identification and is not a real fix for the underlying EDID read failure or the connector renumbering. ## Additional information available on request - Full `dmesg` output from both kernel versions - `xrandr --verbose` output - `gdctl show --modes` output - `/sys/class/typec/` and `/sys/class/drm/` sysfs dumps ## Report generated by Claude AI ** Affects: linux (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2167904 Title: xternal monitor fails EDID detection through Dell WD25Z dock To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2167904/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
