The new Kernel DMA Protection mode does require support from hardware/firmware, OS, and drivers.  I'm not sure which Windows 10 release added that.  But it's a useful mode because it means that when you use a peripheral that supports it, the peripheral is automatically allowed without any need to approve it.  The reason approval was historically required is because Thunderbolt allows direct access to system memory, which meant that it was possible for a malicious device to be plugged into a Thunderbolt 3 port and dump out the entire contents of the system memory, which might include passwords, encryption keys, etc.  That's why the default security mode is to require approval for devices before the system will use them and why you really shouldn't set "No security" on your Thunderbolt 3 controller.  But now there's Kernel DMA Protection, which allows the system to tell a peripheral that it only has direct access to a specific portion of system memory, which is why if the peripheral supports being constrained that way, approval isn't required.  For peripherals that don't support that, which would include all Thunderbolt 2 devices, the legacy security mode is used instead.

So if you have Kernel DMA Protection enabled, you might have to be on a pretty recent Windows 10 release.  1803 might be too old; I just don't remember anymore.

As to your question, what specific Thunderbolt 2 device are you trying to connect and what specific adapter are you using?

So the device is a certified tb3 to tb2 adapter from star tech and controversially an apple Thunderbolt Display, which connected to my other Xps machine, latitude 7390, boot camp MacBook and Lenovo Thinkpad.

Do we know how to turn off DMA Kernel Protection?

@RMB86  it's amazing to me how many people seem to cling to these old Apple Thunderbolt Displays....  Anyhow, I've seen a few threads indicating that they only work through Apple's official adapter and specifically not StarTech's (as indicated in this thread), but I remember threads where people found that they don't work at all on certain systems.  This thread mentions the need to enable a particular BIOS option to make it work on an XPS 15, but I've also seen other threads where people said, "I found that thread, but my system doesn't have that BIOS option and my Apple Thunderbolt Display doesn't work."  And to my knowledge they never found a fix.  If you put "Apple Thunderbolt Display site:dell.com" into Google, you'll find all sorts of reading about this.

As for Kernel DMA Protection, you should be able to disable it in the BIOS if you really want to, but I wouldn't recommend it.  It's only actually used if the device you're connecting affirmatively says it supports that technology, otherwise the system falls back to the legacy security level options -- and if the device DOES support Kernel DMA Protection, you want to use it.

Hi Appreciate the sign post to the other articles however the majority of these are resolved in one way or another and whilst I get the finer points on "why would anyone still use an apple thunderbolt display", I am where I am and I am hoping for consistency amongst Dell hardware especially when TB is a backwards compatible platform when executed correctly

Lets go through the steps and please add any as appropriate:

1. Adapters - I have a StarTech and a Apple TB3 to TB2 adapter both of which work on the other machines I have at home with the display
2. Windows - 1809 is the corporate version of windows I am running with however I have managed to put 1909 on the machine as well. Windows has also been wiped.
3. TB Firmware - The firmware version I am running is v44 and was upto date as it came out of the box, I cannot downgrade to v40. The thunderbolt generation is 15D2.
4. Bios - bios is upgraded to current version
5. Controller Driver - 1.41.648.0 up to date
6. Bios Settings - Set to either user or no security no response
7. Unavailable Settings - Thunderbolt Auto switch is not available in the BIOS setup but is described here 

I really could do with bringing the Thunderbolt Display into use as per the other threads and would welcome any additional steps which I have missed from the many threads on this matter.

@DELL-Cares 

Hello @RMB86,

Did you ever get your Apple Thunderbolt Display to connect to your 7400? I'm in the same situation. Still can't connect after trying the Apple adapter and the steps mentioned in the other threads above.

Thanks,

Jim

I'm having the exact same issue, works on Latitude 7480 2017, doesn't work on Latitude 7400 2020. Disabling Thunderbolt Security or SMM, doesn't help. Any clues? 

-Ronald