I can’t get my monitor display brightness to stay up/adjust

This is an informational post for those of you who are using Linux on a laptop and are having a hard time adjusting the display brightness. This is problem that can cause your laptop to become more or less useless, assuming you want to see the screen but the brightness is wrong (usually too low, i.e., not maximum or near maximum) so I don’t want to call it a low level problem, but it is low level in that it affects only a few people. Complaints about this problem are scattered thinly across the internet, and every person with this problem seems to have two things: 1) A different computer or system from every other person with this problem and 2) only lame answers that generally don’t work.
Well, I have a generalizable solution that should work on a wide range of configurations. This might even work on Windows computers, but who cares about them. Anyway, here it is:

How to adjust the brightness of the monitor of your laptop running Linux:

This assumes that the two usual methods are not working. Those methods are:
1) find an adjustment slider bar or something in the system configuration thingie and turn it up or down; or
2) press the blue fn key (lower left part of your keyboard somewhere) along with the appropriate brightness adjustment key (there is a pair of function keys next to each other that do this) to invoke an on-screen doohickie virtual slider bar and make it go up and down.
If those two obvious answers are not working for you, there is a third temporary method that works intermittantly on some computers but not the real answer:
3) Unplug the power supply to your computer, if it is plugged in, and see what happens. Not the power supply INSIDE your computer, silly, just the batter charger wire coming from the brick.
OK, so none of those things work which is why you are here. So, now try this method which should work. There are four steps:
i) Accept, and be happy with, the fact that your computer will have one brightness setting and you can’t change it.
ii) Go to the system settings (#1 above), and especially your Power Settings, and change everything around so that the display brightness slider switch is where you want it (or would want it if it worked), ie., maximum, AND (this is important) disable/turn off any settings that promise to change the display settings, such as “dim screen after 20 minutes of inactivity” or “dim screen when using battery” etc. In other words, get the settings like you want them, but referring back to item i above, that will not include allowing your system to change the display brightness.
iii) Turn off your computer, turn it back on again (or “restart” if you like) and hit F2 (or another key as needed) to invoke the BIOS screen.
iv) Now that you are at the BIOS screen, you’ll notice that there is no adjustment there for this problem, but that does not matter. The thing is, the function key buttons (see #2 above) should work while you have the BIOS screen up! So, adjust the brightness to where you want it, and hit F10 or whatever to exit out of BIOS and let your computer boot.
Then, it should stay fixed. If it breaks later, repeat.