Troubleshooting/Docked-mode.md

@@ -191,6 +191,37 @@ Dell uses a 1-wire protocol to talk on the power adapters connection. By doing s
 
 To get a Dell usb-c dock working you'll need a compatible Dell power adapter to connect the dock to.
 
+# Training error
+
+In some cases you might find output like the following in your `sudo dmesg`:
+
+```
+[ 79.802137] [drm] DP revision: 0x11
+[ 79.805665] [drm] DP rate: 270000 Mbps
+[ 79.809489] [drm] DP number of lanes: 2
+[ 79.813390] [drm] DP capabilities: 0x1
+[ 79.817217] [drm] Mode: 1920x1080p148500
+...
+[ 81.184792] cdns-mhdp-imx 32c00000.hdmi: [drm:cdns_mhdp_train_link [cdns_mhdp_drmcore]] *ERROR* training failed: -110
+[ 81.195552] cdns-mhdp-imx 32c00000.hdmi: [drm:cdns_mhdp_train_link [cdns_mhdp_drmcore]] *ERROR* Failed to start training -110
+[ 81.206966] cdns-mhdp-imx 32c00000.hdmi: [drm:cdns_dp_mode_set [cdns_mhdp_drmcore]] *ERROR* Failed link train -110
+```
+
+That means: PD contract was successfully negotiated, DP alt-mode was negotiated, the imx8mq's DP core is up and running, DP Aux communication works (via the SBU lanes on the USB-C plug, so muxing works as well) but the calibration of the actual data lanes (which are muxed on the TX/RX pairs of the USB-C plug) failed. 
+
+This can be a cable issue, a problem with the attached monitor/dock or though unlikely a hw problem with the L5. You can check the TX/RX line pairs by testing some USB-3 devices as they use those as well.
+
+If it is not a hardware problem it could also be an outdated u-boot software as the the DP firmware contained therein is responsible for the link training and early firmwares where pretty bad at that.
+
+You can check your u-boot version like this:
+
+```
+purism@pureos:~$ cat /proc/cmdline 
+u_boot_version=2022.10-rc1-gc8c06b98b5 console=ttymxc1,115200 quiet fsck.repair=yes security=apparmor splash plymouth.ignore-serial-consoles vt.global_cursor_default=0
+```
+
+If you're not getting output containing the `u_boot_version=` part then your u-boot is likely outdated and you should contact support about whether to update your u-boot installation and how.
+
 # If all else fails, get help
 
 To track down the reason why a setup might not work as expected gather information about your setup and possible errors as described in this article.