Commit 1f597c99 authored by Joe Rayhawk's avatar Joe Rayhawk
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moin2mdwn: convert page Optimus

parent 1df77a27
= Nvidia Optimus =
'Optimus technology' is a software [and possibly hardware] solution for automatically switching between an integrated graphics chip or IGP (such as on onboard intel chip) and a more powerful [nvidia] graphics chip. This technology is intended specifically for laptops. The precursor to this technology was 'switchable graphics,' in which the user could manually switch between the graphics card. It may require that the Nvidia GPU has the PCOPY engine.
There is currently no kernel-land support for optimus in linux (more below).
== Why is Optimus not currently supported under Linux? ==
First lets review:
The graphics system in a laptop has a gpu with some memory, in the case of an IGP, this memory may be a piece of system memory, but otherwise it is usually dedicated memory living on the GPU. This GPU connects to a laptop display, or output port.
There are two main problems to solve in order to support optimus under linux:
1) Currently we do not have a way to a priori know what outputs (displays) are connected to what GPU's.
2) The supposed optimus software, should perform the task of switching between the which of the two graphics processors drive your display. Ideally this would be done by directly flipping a hardware switch, called a mux ([[http://en.wikipedia.org/wiki/Multiplexer|multiplexer]]). However such a mux does not always exist!
If a hardware mux does not exist, there there is no physical way to perform this GPU switching. Thus Optimus is used to effectively "implement" a software mux. Specifically it ensures that relevant data is sent to and processed on the right GPU then the data needed for display is copied to the device that displays the image.
When it comes to how a specific machine is configured, there are a number of possibilities:
Again, if the hardware mux exists it would be used to select which GPU drives the internal panel, or the external monitor, or possibly both.
It is also possible, that a GPU is hardwired to the internal panel, so the other GPU cannot possibly drive the internal panel. The same goes for external monitor output.
In the worst case we have that the:the Intel GPU hardwired to the internal panel and the Nvidia GPU hardwired to the external output!
The best case scenario is a mux, which can select which GPU drivers control which outputs.
Basically, you can have ''any'' combination of these possibilities. There is no standard on how to wire things. There should be ways to detect the wirings and whether there is a mux and where, but the documentation is not available to the developers (maybe you can help us figure out how to do this, have any ideas? You can also 'petition' nvidia for releasing these specs: [[http://nvidia.custhelp.com/|nvidia customer help]] ? )
Detecting the wiring is not the only hard part. Currently we are missing the entire infrastructure for doing the image copying between graphics cards as described earlier. Everything from the kernel DRM drivers, X drivers and the X server itself need to be modified to support Optimus. Not even the proprietary drivers support it on Linux, save a few exceptions.
On the other hand, power saving is a key point of Optimus. Switching off a GPU is relatively easy. Everything else is hard.
== What works right now ==
If your laptop has a hardware mux, the kernel switcheroo driver may be able to set the wanted GPU at boot. There are also hacks based on the switcheroo, like asus-switcheroo, but they offer no extra value. If one of the hacks happens to work, and the switcheroo does not, the switcheroo has a bug. There might already be pending patches waiting to go towards the mainline kernel.
In all other cases, you are stuck with what happens to work by default. No switching, no framebuffer copying. Yet.
== PRIME ==
'PRIME GPU offloading' is an attempt to support muxless hybrid graphics in the Linux kernel: http://www.phoronix.com/scan.php?page=news_item&px=MTAyMDM
# Nvidia Optimus
'Optimus technology' is a software [and possibly hardware] solution for automatically switching between an integrated graphics chip or IGP (such as on onboard intel chip) and a more powerful [nvidia] graphics chip. This technology is intended specifically for laptops. The precursor to this technology was 'switchable graphics,' in which the user could manually switch between the graphics card. It may require that the Nvidia GPU has the PCOPY engine.
There is currently no kernel-land support for optimus in linux (more below).
## Why is Optimus not currently supported under Linux?
First lets review:
The graphics system in a laptop has a gpu with some memory, in the case of an IGP, this memory may be a piece of system memory, but otherwise it is usually dedicated memory living on the GPU. This GPU connects to a laptop display, or output port.
There are two main problems to solve in order to support optimus under linux:
1) Currently we do not have a way to a priori know what outputs (displays) are connected to what GPU's.
2) The supposed optimus software, should perform the task of switching between the which of the two graphics processors drive your display. Ideally this would be done by directly flipping a hardware switch, called a mux ([[multiplexer|http://en.wikipedia.org/wiki/Multiplexer]]). However such a mux does not always exist!
If a hardware mux does not exist, there there is no physical way to perform this GPU switching. Thus Optimus is used to effectively "implement" a software mux. Specifically it ensures that relevant data is sent to and processed on the right GPU then the data needed for display is copied to the device that displays the image.
When it comes to how a specific machine is configured, there are a number of possibilities:
Again, if the hardware mux exists it would be used to select which GPU drives the internal panel, or the external monitor, or possibly both.
It is also possible, that a GPU is hardwired to the internal panel, so the other GPU cannot possibly drive the internal panel. The same goes for external monitor output.
In the worst case we have that the:the Intel GPU hardwired to the internal panel and the Nvidia GPU hardwired to the external output!
The best case scenario is a mux, which can select which GPU drivers control which outputs.
Basically, you can have _any_ combination of these possibilities. There is no standard on how to wire things. There should be ways to detect the wirings and whether there is a mux and where, but the documentation is not available to the developers (maybe you can help us figure out how to do this, have any ideas? You can also 'petition' nvidia for releasing these specs: [[nvidia customer help|http://nvidia.custhelp.com/]] ? )
Detecting the wiring is not the only hard part. Currently we are missing the entire infrastructure for doing the image copying between graphics cards as described earlier. Everything from the kernel DRM drivers, X drivers and the X server itself need to be modified to support Optimus. Not even the proprietary drivers support it on Linux, save a few exceptions.
On the other hand, power saving is a key point of Optimus. Switching off a GPU is relatively easy. Everything else is hard.
## What works right now
If your laptop has a hardware mux, the kernel switcheroo driver may be able to set the wanted GPU at boot. There are also hacks based on the switcheroo, like asus-switcheroo, but they offer no extra value. If one of the hacks happens to work, and the switcheroo does not, the switcheroo has a bug. There might already be pending patches waiting to go towards the mainline kernel.
In all other cases, you are stuck with what happens to work by default. No switching, no framebuffer copying. Yet.
## PRIME
'PRIME GPU offloading' is an attempt to support muxless hybrid graphics in the Linux kernel: [[http://www.phoronix.com/scan.php?page=news_item&px=MTAyMDM|http://www.phoronix.com/scan.php?page=news_item&px=MTAyMDM]]
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