[Galaxy S 4 Developer Discussion [Developers-Only]] Jon8RFC:
I own a JFLTEVZW i545, but I understand that this affects other variants as well.

I'm not a developer, but in the last 4 weeks or so, I've been trying to learn more about android, linux, and kernels. Hopefully what I've come up with can be attempted by someone with a more advanced skill set, because although I've had what appears to be success in attempted fixes, I really don't know if I'm implementing the changes appropriately because I don't see the appropriate fix. I'm also wasting many, MANY hours (200-300?) learning, tinkering, and waiting on compiling because I'm not skilled enough to make a quick change that I want, or to implement that change easily, without a complete recompile (which costs me 2 hours each time). Someone who is more knowledgeable with kernels and building/compiling from source could probably do everything I'm doing in 1%, or less, of the time that I'm doing it.

Core issue:

The magnetometer's X and Z axes are off 180degrees. This has been a consistent issue since early/mid 2014 builds in both CM11 and CM12, as well as CM12.1 (as of a few days ago when I last tested). This causes problems with navigation and multiple user apps.

Ways to experience the issue:

If you aren't familiar with this bug, or if you're of the opinion that the compass doesn't have any problems, fire up google sky and you'll see that things are wonky when the phone flips around crazily and none of the constellations, planets, or moon are where they should be via the augmented reality. This app is

NOT

incompatible--the data it's being fed is erroneous.

Alternatively, you can use Physics Toolbox Sensor Suite to view the true raw data (other sensor apps are either adulterated or show false or useless data). With this, the sections I've found most worth looking at are the Linear Accelerometer, Magnetometer, and Orientation, and you can compare the data to an OEM phone if you have another one handy.

For the magnetometer, I've found that the absolute best way to calibrate it is by performing a figure-8 movement in three-dimensional space, rather than two-dimensional as shown in some apps and videos, or by the method mentioned in GPS Status & Toolbox. See this video for an example. I perform a larger figure-8 and do it multiple times--once can work, but a few times really settles it down.

What aren't contributing factors:

Calibration

Hardware malfunction (many others have confirmed)

App malfunction

Magnetometer driver source code (note: the code itself in the files I've looked at are the same as Samsung's source, but the way in which it's implemented may not be)

Please keep in mind that because I'm very new to this, I don't have instant intuitive feedback to know how to confirm these things in the contributing factors or possible solutions. I really need to pass the reigns on this one to someone much more advanced than myself, who will see this post and churn out the fix in a half hour.

Possible contributing factors (could be more than this):

Driver implementation. The source in /kernel/samsung/jf/drivers/sensors/ is new, and OEM, but I don't see it compiling.

Driver implementation. I'm having difficulty knowing which source files from Samsung need to be dropped in to try and compile a kernel without CM modifications which pertain to the sensors. I also have significant difficulty knowing if it succeeded, correctly, rather than something in CM taking over and undoing my changes. This is the case for one particular thing, so I have no idea how to confirm the things that I can't readily see.

Sensor(s) orientation configuration(s).

Possible solutions:

Should /kernel/samsung/jf/drivers/sensors/ actually be compiling? I don't think it is (because I don't see the folder), or even know if it's necessary for our phones, but Samsung has it in their source and I cannot successfully compile Samsung source to try and compare. I also don't know if it gets merged in with other files somewhere else.

Dropping in all OEM necessary files and compiling, without CM interrupting. I don't understand linux and the filesystem enough to know what happens when, and I've resorted to using shred/srm to try and truly delete files, but I still struggle with understanding what's going on. I also don't know what encompasses a swap like this. I don't know if replacing sensorhub is all that needs to be done or if there are 3 other files in completely different directories that are critical, and must overwrite the CM modifications for things to compile appropriately.

Setting sensor orientation correctly with CONFIG_SENSORS_SSP_ACCELEROMETER_POSITION=0, CONFIG_SENSORS_SSP_GYROSCOPE_POSITION=0, CONFIG_SENSORS_SSP_MAGNETOMETER_POSITION=0 being different values than zero.

I've tried the first two and had intermittent success. Sometimes things compile, sometimes they don't. But I also don't know if what I'm changing even matters. I've been checking file hashes to see when things change, but it's becoming tedious and someone who knows all of the linux commands and knows how the source gets compiled would know without having to check.

My favorite possible solution is the third. This is in the .config file which is made by the make menuconfig process, which I believe is influenced by various defcofig files. I've tried changing the .config directly, but CM undoes that. I've tried adding those lines to the defconfig files, but CM either undoes or ignores that. I've tried compiling a kernel outside of compiling CM as a whole and am hitting roadblocks with my lack of experience and knowledge. I've successfully compiled kernels, but I don't even know if my changes are sticking. I've taken what I though may have been an appropriately compiled kernel (Image and zImage) by modifying the .config and then manually doing a make zImage, but even dropping those in to compile with CM, chmod 555, chown/chgrp root and CM somehow manages to overwrite the renamed zImage-->kernel file, but it would actually leave them alone when I did all that nonsense to the Image and zImage in their normal output spot, /arch/arm/boot/kernel/ I believe.

The third possible solution sets how the sensors are physically placed within the phone. If the readings are off by right-angles, it seems that a coding change for one or more of these would be appropriate:

CONFIG_SENSORS_SSP_ACCELEROMETER_POSITION=0

CONFIG_SENSORS_SSP_GYROSCOPE_POSITION=0

CONFIG_SENSORS_SSP_MAGNETOMETER_POSITION=0

I've had no success in making this happen, but as I said, someone who is a genuine programmer would be able to make these things happen and compile and test quickly...rather than me spend an entire day trying 20 different ways to see if I can get something to stick, and then not even being able to confirm if what I changed, actually made its way into the final compiled files.

Hopefully someone is willing to take a stab at this, because I'm apparently the equivalent of an elderly person having their first encounter with a computer when it comes to this stuff. It seems so simple, but I'm not the one to make it happen, and I feel like this may be the route to take. Thanks, y'all!

EDIT: I've tried other methods to make this work that I didn't list, I just can't remember everything and my mind is breaking down after going at this for about 13 hours straight today.