How does Optimus really work?

Helena · Posted: Thu Jan 12, 2023 11:12 am Post subject: How does Optimus really work?

After a lot of experimentation and with a little help from my friends I finally managed to get X working on my Optimus-enabled laptop. The final step I needed was https://wiki.gentoo.org/wiki/NVIDIA/Optimus.

However, Gentoo is about understanding a lot of what's going on, and in this case I seem to be missing something with respect to what and how the screen is drawn. I'll explain.

When booting, the screen is driven by the BIOS. I use Grub to boot and only when the kernel is loaded into memory the next phase starts; the EFI frame buffer is now in control. I boot to a command prompt, so after logging in this goes on until I fire up X. Correct so far?

X reads the configuration file that I generated using

NeddySeagoon · Posted: Thu Jan 12, 2023 7:28 pm Post subject:

Helena,

Optimus can be though of as one and a half graphics systems.
The Intel part is complete. It can work alone.
The nVidia part has no connections to the display surface, so it can draw images but not move the images to the display. Attempting to use the nVidia GPU alone results in a blank screen.
That's a design feature, not a bug.

The images in the pixel buffer can be drawn by either GPU but you need to use Intel alone or both GPUs together to get an image.

When the nVidia GPU is doing the rendering, the Intel GPU must be controlled by the modesetting driver. Its only role is to read the images in the pixel buffer to the display surface.
x11-drivers/xf86-video-intel is not used.

This saves an expensive power hungry video multiplexer chip in your laptop. That's good for for space, battery life, thermal performance and your pocket but it does add some complexity.

Ran.Rutenberg · n00b Joined: 23 Mar 2007 Posts: 27

The new (modesetting) intel driver is part of the kernel. The kernel module is called i915 but x calls it intel.
The intel driver in x11-drivers/xf86-video-intel is an old driver that is not needed in newer systems.

At least in my laptop, it won't work without the intel kernel module (i915), but the nvidia modules are optional.

Helena · Posted: Sat Jan 14, 2023 9:41 am Post subject:

Ran,

Thx for replying.

Neddy,

Your comments are useful and add to my understanding. Still, the use of i915 vs. intel is very confusing for me.

Anyway, I've got the system running on the closed-source nvidia drivers. I chose this as using nouveau is being reported as suboptimal. However, the whole idea of hybrid graphics is being able to switch between the iGPU only and the dGPU (which then also requires the iGPU). Right now I do not know how to switch back and forth between the 2. Does it require a separate xorg.conf file? The one I use now is generated by the nvidia driver, as mentioned before:

NeddySeagoon · Posted: Sat Jan 14, 2023 12:13 pm Post subject:

Helena,

The hardware bit is easy for me, I'm a retired electronics engineer.

The software bit is a much harder. From the Optimus Wiki page, XRandR is used to choose the nVidia GPU for rendering at Xorg start time, so a restart of Xorg is required to switch GPUs

This bit of the auto generated xorg.conf always makes me smile

mrbassie · l33t Joined: 31 May 2013 Posts: 772 Location: over here