I have a Dell E7240. I’m pretty happy with it - my main complaint is that it has a very shiny screen, and that seems to be because it’s the touchscreen variant. While I don’t care about that feature I do care about the fact it means I get FullHD in 12.5”
Anyway. I’ve had issues with using a dock and an external monitor with the laptop for some time, including getting so far as mentioning the problems on the appropriate bug tracker. I’ve also had discussions with a friend who has the same laptop with the same issues, and has some time trying to get it reliably work. However up until this week I haven’t had a desk I’m sitting at for any length of time to use the laptop, so it’s always been low priority for me. Today I sat down to try and figure out if there had been any improvement.
Firstly I knew the dock wasn’t at fault. A Dell E6330 works just fine with multiple monitors on the same dock. The E6330 is Ivybridge, while the E7240 is Haswell, so I thought potentially there might be an issue going on there. Further digging revealed another wrinkle I hadn’t previously been aware of; there is a DisplayPort Multi-Stream Transport (MST) hub in play, in particular a Synaptics VMM2320. Dell have a knowledge base article about Multiple external display issues when docked with a Latitude E7440/E7240 which suggests a BIOS update (I was already on A15) and a firmware update for the MST HUB. Sadly the firmware update is Windows only, so I had to do a suitable dance to be able to try and run it. I then discovered that the A05 update refused to work, complaining I had an invalid product ID. The A04 update did the same. The A01 update thankfully succeeded and told me it was upgrading from 2.00.002 to 2.15.000. After that had completed (and I’d power cycled to switch to the new firmware) I tried A05 again and this time it worked and upgraded me to 2.22.000.
Booting up Linux again I got further than before; it was definitely detecting that there was a monitor but it was very unhappy with lots of
[drm:intel_dp_start_link_train] *ERROR* too many full retries, give up errors being logged. This was with 4.2, and as I’d been meaning to try 4.3-rc2 I thought this was a good time to give it a try. Lo and behold, it worked! Even docking and undocking does what I’d expect, with the extra monitor appearing / disappearing as you’d expect.
Now, I’m not going to assume this means it’s all happy, as I’ve seen this sort-of work in the past, but the clue about MST, the upgrade of that firmware (and noticing that it made things better under Windows as well) and the fact that there have been improvements in the kernel’s MST support according to the post 4.2 log gives me some hope that things will be better from here on.