Some context before I ask this question:
I thought I understood how Optimus works, until I encountered this https://bugs.launchpad.net/ubuntu/+source/nvidia-prime/+bug/1765363 issue, with the prime tool on Ubuntu. Note that my question is not specific to Ubuntu.
I was under the impression that on an Optimus laptop, if the proprietary drivers are present, then the system will try to use the dGPU with the proprietary drivers. This will only result in a usable system though, if the display manager scripts described here ( https://wiki.archlinux.org/index.php/NVIDIA_Optimus#Display_Managers ) are in place for your chosen display manager.
I was also under the impression that if the proprietary drivers are not present, then the system will try to use Intel + Nouveau, and power saving is implemented into Nouveau, so in this case, the Nvidia card will be powered off, unless you use DRI_PRIME=1 to run some command.
If the proprietary drivers are not present, or they are blacklisted, and on top of that, nouveau is also blacklisted, then I assumed previously that the Nvidia GPU will not even power on at all.
On distros, in which there was no downstream implementation of any prime functionality (say, Arch Linux), what I did was that I had a script, which
when passed the "on" flag to, it copied the DM scripts to the proper location, and unblacklisted Nvidia;
when passed the "off" flag to, it deleted the DM scripts and blacklisted Nvidia.
I thought this script was working properly, but I never ran any serious tests on battery usage, cause I very rarely ran my laptop off battery, I just wanted to have power saving options in case I needed it.
In light of the first link (to launchpad) in this post, I feel my understanding is shaken. So I have a couple of questions.
If I understand correctly, the current (18.04) implementation of PRIME in Ubuntu only doesn't work if one uses a newer Nvidia GPU, in which Nouveau's power saving doesn't work (for example, in my Geforce GTX 950m, it should work), right?
If both the proprietary drivers and Nouveau are blacklisted, then does the dGPU still power on during boot?
If so, assuming a generic system (as in, with no in-built Optimus manager, for example Arch Linux), what needs to be done to have a reliable Optimus setup which actually saves me power in Intel mode? Is having bbswitch installed, and using it to turn off the GPU when Intel mode is active is the only way?