Qualcomm mainline porting

Devices with Qualcomm SoCs are generally good candidates for porting mainline Linux, considering the great support those SoCs have. It usually doesn't take a lot of work to get a device running mainline with a decent amount of working features, provided the SoC is already supported.

Prerequisites

 * Downstream kernel source code: This is the main source of reference when porting the mainline kernel.
 * Board schematics: An additional source of reference that can be really helpful considering the usually poor quality of downstream code including the device trees. Will allow you to confirm the wiring of all components, and will also show any hardware that is misrepresented in the downstream device tree, such as fixed regulators which are often modeled as enable pins.
 * Having the mainline source code of a similar device can be very helpful: https://wiki.postmarketos.org/wiki/Mainlining#Supported_SoCs

First steps
You must find a method for getting feedback from the device when booting the mainline kernel. This will be necessary for debugging.

UART
Perhaps the most useful method, since it allows for getting early kernel messages, and even bootloader logs which can help when the device is not finding or refusing your DTB. You will have to find the location of the UART port on your device, see (video tutorial | Identifying serial ports) and Serial debugging

Simple Framebuffer
This should be the easiest method to get working, provided you have a video-mode panel and your bootloader leaves a framebuffer configured for continuous splash. It will allow Linux to print its logs to the screen, and it will also allow you to actually make use of the screen before writing a panel driver, although to a limited extent.

RAM Console / ramoops
This method is more of a last resort, considering it is usually unreliable and in many instances quite difficult to get working.

Relevant chat log
Thanks to opendata and bshah for the following log. Let's make this more wiki friendly once we try it out.
 * Copy fairphone2 DTs and change device info
 * (alas, if you are using Nexus 4, apq8064 based device would be good start i.e Nexus 7 2013 (yes, it's most complete device now))
 * Not sure about 8064, I would do the same thing but base off yuga
 * Look for a file named  in devices android kernel tree
 * Look for  then look at the voltages and make sure your DTS is the same, if not change it.
 * Do the same operation for the other regulators:
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet
 * Then look in your devices android DTSi and look for any regulator configs if you find any change the corresponding regulator in the dts
 * This should be enough to boot with USB telnet

SoC-specific guides
There are detailed guides for some SoCs: The mainlining procedures are quite similar, so you should be able to follow them to some extent even if you have a different SoC.
 * MSM8916 Mainlining
 * MSM8996 Mainlining
 * SDM845 Mainlining