[Dell Venue] johnsag49:
I got the Dell Venue 8 7840 as an early Christmas gift (for myself, at Best Buy) and was saddened to learn getting root was (nearly) impossible. I followed various guides (many written this summer, before the latest batch of updates - as of December 2015) but found myself nowhere. Then, I found my answer.

Well, fellow root-wanter, I'm here to tell you it is possible, but it won't be easy.

How I Got Here

Right out of the box, I hastily updated to all the latest firmware updates from Dell. That was my biggest hurdle. So if you plan to do this and you haven't updated the device yet, DON'T UPDATE! Follow the several guides here to get root.

Too late? Already screwed the pooch? Read on.

Generally speaking, you need to do a few steps to get where you want to be. They are (roughly) as follows:

Unlock the Bootloader (developer settings change, adb commands)

Use the Intel Flash Tool to flash an earlier firmware, thus making your device able to boot a temporary ClockworkMod Recovery (CWM)

Use clockwork to wipe the system and cache, then reinstall the OS and SuperUser

Reboot and Enjoy!

Here's the long version of steps I took to make this possible:

Getting Started

As a prerequisite, please install at least the minimum adb/fastboot tools. For first timers, ADB is the Android Debug Bridge, which allows command-line access to system level tools when working with an Android device. It's the backbone of this entire process. Fastboot is a debug mode wherein you can run certain commands on the system, without the presence of an OS. As an Android user, and certainly as somebody who's found themselves on XDA, you should become familiar with fastboot and its abilities. You also can (and should) download the entire set of platform tools (part of the Software Development Kit) as a matter of course.

There are several other tools, firmware files, images, etc. that you will need. I've put all the files I used together in a single zip file, which you can download here:

Dell Venue 8 7840 Tools and Files (zip)

* (Thanks to the many contributors on this forum, like toxic_garden, enzymeyllw, mrgardiner and of course xBIGREDDx. Also Chainfire for SuperSU!)

Once you have all your files downloaded, you need to grab the Intel flash tool. My god is that tool a pain in the ballsack. It's also a necessary evil. Give Dell your firstborn, install the damn tool, and let's move on.

Seperately, you will need an SD card with the following files on it: update.zip and UPDATE-SuperSU-v2.46.zip

Ready to Go? Good.

First, enable Developer Options in Android. To do that, go into Settings >> About Tablet >> and tap the Build Number a bunch of times. After some vigorous tapping, a toast message (The little gray-oblong, semitransparent box near the bottom) will pop up and tell you "You're now a developer!". Congrats.

Go into Developer Options, look a few items down the list for "OEM Unlocking" and enable that sucker. If you were successful, it was the first step toward greatness.

If you have to back anything up, now's the time to do it. Because these next steps will wipe your device clean.

Install the Intel Drivers if you haven't already, and in Developer Options enable USB Debugging (it's not too far down below OEM Unlocking)

Some more drivers will install on your PC, and switch it over to Camera (PTP) mode when you see the selection option in the Notification tray. Eventually, if you played your cards right, you'll see an RSA Security request on your device, asking if you want to trust your PC and displaying a hex address. Click OK.

On your PC, find your platform-tools folder (in the Android SDK folder) or wherever you installed the minimal adb/fastboot tools.

In this folder, hold down Shift and right-click, then choose "Open Command Window Here" (alternately, go to the Start menu, type cmd and right-click "Run as Administrator" then navigate to your folder with adb.exe and fastboot.exe in it.)

Cool, take a breather. How are you feeling? Good? Good.

In the black window, type adb devices. You should see a string of numbers or numbers/letters and something like 'unauthorized' in the list. If it's blank, the ADB drivers are not installed properly. Go to your device manager, find anything with a question mark/yellow icon that looks like intel or android or moorefield, and install the drivers manually. You can search XDA if you get stuck on this step.

Back in the black window, type: adb reboot bootloader

Your device should immediately black out and, after a short while, go to the bootloader menu.

From here, go back to the black window and type: fastboot devices

You should see another set of numbers/letters and a word after. As long as you see something, you're golden. If it doesn't show up, you may need to double check those drivers. Another set of drivers will need to install for your device to be seen in this state. Just stick with it, you'll get there.

All good? Then type fastboot oem unlock

You'll get a warning, but just continue on.

Once unlocked, it's time to flash new firmware. Note: For me, this was absolutely the biggest PITA step. The tool was written with virtually no instructions. What follows is how I got it to work.

While you're still in the bootloader menu, you should see confirmation that your device is unlocked. YAY!

Use the menu to boot the device normally. Note: All your user data is gone, so your tablet will act like it's brand new again. Skip through as much BS as you can and get back to the home screen of your tab. Go to settings, and re-enable USB Debugging like last time.

While that's happening, go to your PC and fire up the Intel Phone Flash Tool. (Nice name, Intel.)

Load up the firmware folder "Dell_Venue_8_7840_Lollipop_5.0.2_qs_rd_opensource-blankphone-userdebug-1397", and specifically choose from the dropdown to flash the file: flash-DDO_BB.xml

If the tool thinks your device is not connected, that's okay. If the tool thinks your device is connected, that's OK too. Don't sweat it.

With the tool still running, as soon as its connected (allow Android Debugging, change to PTP or what have you) hit the Start to Flash button.

It will try about 100 times to find your device (you can see a little console log at the bottom of the tool) ... while it's searching, hold the power button on your device and POWER IT OFF. (Yes, off. Counter-intuitive, I know.)

As it powers off, it will enter some quasi-dream state that Intel dreamed up that will allow it to flash the firmware. Give it a bit of time, and eventually the firmware will flash successfully. HOORAY! You just paved the way for CWM.

Now go back to the folder of files and tools I told you to download, and find the folder "IntelAndroid-FBRL-05-16-2015-PTR" (alternately, search on XDA and find that tool yourself)

Load up the tool, hit accept, then choose option T4 (not T1-3, T4)

If you were successful, it will slowly begin copying over files and having its way with your tablet, and eventually some little white text will appear on your Tablet screen. Soon, ClockwordMod Recovery will start up.

From here, go to Mounts and Storage. Make sure /system is not mounted, and External SD is mounted. Wipe the /system partition, just to be sure. And again, make sure it's not mounted.

Go back to the main CWM screen, and choose "install zip from sd card", and then the option for the external sd.

Find and flash the update.zip file (this will give your cleared out tablet Android 5.1)

Now find and flash UPDATE-SuperSU-v2.46.zip

Go back to the main menu, and Wipe Cache Partition.

Back at the main menu, Reboot System Now.

If all went well and I didn't totally screw up this guide, your newly rooted tablet should boot up. It may appear to be stuck on the Dell loading screen a while, but it's not. Give it time, and Lollipop will come right up.

From here, download SuperUser and BusyBox from Google Play, if either of those things aren't present on your tablet. Now take advantage of Titanium Backup to remove all that unwanted crapware, and Greenify (at least) to keep those pesky apps at bay.

Enjoy! If this worked, please let me know. As I said, this is what worked for me - but everybody's situation is different, and I'm sorry if this did not work for you. Obligatory I am not responsible if your device bricks, dies or otherwise craps the bed. Use this guide at your own risk.

Some threads that helped me through, increased my sanity, and eased my upset stomach:

Toxic_Garden's Guide: http://xdaforums.com/showpost.php?p=64391058&postcount=16

Mr Gardiner's Guides: http://xdaforums.com/showpost.php?p=62833258&postcount=4, http://xdaforums.com/dell-venue/general/7840-stuck-bootloop-bricked-t3178937

Enzymeyllw's Guide: http://xdaforums.com/dell-venue/general/7840-succeed-to-unbrick-t3205024

Insight from xBIGREDDx: http://xdaforums.com/dell-venue/general/venue-8-7840-developer-files-posted-t3144232/page4

A side note: a few of those unbricking guides tried to have me use the adb shell to send the system image to the device, but I didn't find that necessary. Just flashing update.zip with CWM got me far enough along to get the tablet running. I'm not sure if that will come back to bite me in the ass, but hopefully not.