[Galaxy S6 General] RaymanFX:
So, kindly allow me to chime in here.

I have received numerous requests from people about my work on the Galaxy Note 4 (Exynos 5433) and I they asked me if I was willing to contribute to Galaxy S6 AOSP development.

Now, let me try to explain why the situation is a little different with this device.

For the Exynos 5433 SoC, I was able to create device specific open-source drivers for most of its hardware, including power management, audio and lights (for the buttons and the notificiation LED). I also had to spend more than three weeks reverse engineering the RIL stack with the help of other developers to research what was needed to be able to use open-source radio interface layer libraries for the ST-Ericsson M7450 modem.

Now, the Exynos specific drivers (for GPU acceleration, HW video encoding/decoding, color conversion and more) were derived from the Exynos 5420 source code released by Insignal for their Exynos based development board.

I had hopes I would be able to hack into these sources and make them compatible to the Exynos 5433 SoC, and as it turns out, I was right.

However, both the Exynos 5420 and 5433 SoCs were only ever supported on arm32, not 64 bits platforms (although Exynos 5433 chips are 64 bit capable), so this is a huge difference from the Exynos 7420 chip. Since I haven't worked on an Exynos 7420 device, I cannot tell how much of an issue this is when attempting to create functional 7420 drivers.

On the other hand, there are different platforms like Nvidia Tegra, which run without any functional open-source drivers at all, they can only use prebuilt binaries, which works surprisingly well in most cases, but is a real pain for developers (because it involves lots of reverse engineering, disassembling and hexediting of blobs).

That said, I already did all these things for my Exynos 5433 device and I'm beginning to know my way around the Samsung blobs.

Considering all these points, I think an AOSP/CM port for the Galaxy S6 family is possible either by creating open-source drivers (which might take a VERY long time though) or trying to make the prebuilt blobs play nice. The only developer who put some efforts towards this is Brandon McAnsh (as someone pointed out some pages back). He already contacted me on numerous issues he was running into, but unfortunately his biggest blocker was the time as he does not own a S6 himself, but had to lend one from a coworker/relative (I don't remember the exact situation anymore).

On a more general topic, I feel like I need to state what I said to the Note 4 community when they were raising funds for a device: You need to keep in mind that there's always the possibility of the developer (including myself) not being able to achieve a functional AOSP port. Although he/she might be skilled, talented and already have experience and expertise with a similar chipset (i.e. Exynos 5433), Samsung hardware development is a box full of wonders, you never know what you get, sometimes it's a sweet donut and in other cases it'll be a rotten bagel (I like bagels, no hate plox).

Most of the reputable developers I know will/would do their best if a device is given to them by the community, but we do this in our free time, we're not getting paid to work on this at all, we're only given the utility to be able to work at all. This is a huge difference to keep in mind.

Overall, the Exynos 5433 project took me approximately half a year to figure out the most important issues.

I would probably be willing to invest time in Exynos 7420 research, but there's absolutely no guarantee that something useful (say: something that you guys can flash on your devices) will emerge from that.