[Upgrading, Modifying and Unlocking] Astralix:
This is difficult to answer, especially es we have to got into a certain depth of hardware knowledge. I hope the following longer text brings some light into this.

You cannot full-check this. You might disassemble your tablet, strip all chips from the PCB, xray your PCB and recreate schematics from the xrays. Then you try to get the complete developer manual from RockChip (which you won't get as they never reply to private emails) and check the kernel by comparision of all the previosly collected information...

But reality is much easier:

The critical things that really could break something or harm someone are the DC/DC converters and charge controllers. But these are only read by the software and sometimes could be switched by it, but they have hard wired cut off points. So blowing up your LiIon by using the wrong software cannot be done. I think there exist safety rules that say that you have to have a hardware cut off of this charging as LiIon and LiPoly bury the risk of fire or tend to explode.

As we found out while collecting RK2918 based tablet information, there are only a few designs, a few peripheral chips that are used. There are only a few companies producing these tablets, putting mostly the same set of chips behind different LCDs and that all into differnt plastic cases.

Technically there are some little chances that hardware may be damaged.

That is why kernel developers often write these sentences like 'Use at your own risk'.

The CPUs or SOCs in these tablets have GPIOs, pins that are I/O lines and can be configured to be input, output and different flavours of these two modes. So if a kernel assigns a GPIO to be an output but the PCB connects it to an output of a peripheral, things could go wrong. But normally you do not drive push-pull, i.e. the pin does not set itself to 3.3V or 0V actively. Instead it leaves the pin open if a 1 is written and connects it to 0V (GND) if a 0 is written to it. The high level is established by a resistor between 3.3V and the line or pin.

So there is no short circuit in case of two outputs connected to the same line.

A second security topic might be that often series resistors are places near the SOC into the PCB trace. This is for EMC purpose but also limits the maximum current to a value that might prevent the GPIO from being blewn up in case of a shortage.

Now, we do not know all and everything and we do not know if there wasn't a chinese guy that saved 0.02ct of production cost by designing out 10 resistors and using some GPIOs in push-pull mode... That is the risk you take if you use something else than your hardware manufacturer deliveres exactly four your device.

I for myself adopted and recompiled some of the kernels OMA uses for these tablets. I have Odys Xpress and Loox tablets and I disassembled them and put several traces to my scopes. I haven't found anything that I really had to take care of when trying to figure out GPIO connections.

That the backlight behaves different between two kernels is ok:

LEDs are controlled by current. But current to brightness is not linear. So if your kernel is not designed to the right PWM values, it will not behave linear or result in brighter or darker display. The kernel could be designed for a cheap current consuming LED or a bigger LCD that uses multiple LEDs that is already really dark at 20%PWM.

If you flash this kernel into a tablet where a more efficient LEDs or less LEDs are used, it needs a PWM of 10% or less to get it as dark as in the original tablet.

But full brightness most times means a PWM of 100%. If your manufacturer later found out that 100% PWM reduces runtime too much, he might have locked it down to less than 100% by software. But that should not imply that he designed the complete hardware to be capable of only that reduced current. If he did, consider throwing the tablet at him.

I consider the risks at a very low level. But as a kernel developer I must say that I am not responsible for anything broken and you take the risk yourself.

I hope I could describe the things and answerd your question, even you have to decide if you take that tiny risk or not.

Cheers,

Astralix