Qualcomm Snapdragon 415/615/616 (MSM8929/MSM8939)

MSM8939 (or Snapdragon 615) is a Qualcomm SoC released in 2014, with great mainline support. There is a slightly higher clocked variant (CPU cores up to 1.7/1.2 GHz) available as Snapdragon 616. There is also a slightly lower clocked variant available as Snapdragon 415 (MSM8929). Efforts to coordinate mainlining of this SoC and porting of new devices can be discussed at #msm8916:postmarketos.org.

Mainlining
MSM8939 is a great platform to get started with mainlining! It can use the MSM8916 Mainlining guide that explains how to get started (as both SoC's are very similar). Most of the time you need to change  to. https://github.com/msm8916-mainline/linux has support for msm8939.

msm8916-mainline/linux is closer to upstream, better maintained and thoroughly reviewed. New and existing devices should be added there.

Some device can be found on the stale https://gitlab.com/msm8939-mainline/linux where you need to build  with. For initial device tree compare with other msm8939 devices, as compatible, include and regulators differ a bit. Most other stuff should just work™ and you can always compare to the other msm8939 devices

Tree
A shared kernel fork with some out-of-tree patches which were not upstreamed yet can be found at https://gitlab.com/msm8939-mainline/linux and https://github.com/msm8916-mainline/linux.