Linux on the Mac — state of the union [LWN.net]

Linux on the Mac — state of the union

Posted Dec 11, 2016 1:56 UTC (Sun) by aaahaaap (guest, #112900)
Parent article: Linux on the Mac — state of the union

> Apple is the only remaining vendor to build a multiplexer into hybrid graphics laptops, which have both high-end and low-power GPUs. The multiplexer allows the panel to be switched between the GPUs and the unused GPU to be powered off. All other manufacturers use a "muxless" solution, whereby the discrete GPU is headless and copies rendered data over PCIe into the integrated GPU's framebuffer. "Muxed" solutions, such as the one used by Apple, offer superior power saving and latency, but are more difficult to implement.

Where does the XPS15 (9550) stand in this regard? Since it's discrete GPU can apparently be fully disabled, even when using HDMI (see https://bbs.archlinux.org/viewtopic.php?pid=1675168#p1675168), how could Apple's solution be more efficient?

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Linux on the Mac — state of the union

Posted Dec 11, 2016 2:13 UTC (Sun) by raven667 (subscriber, #5198) [Link] (5 responses)

Without a mux then only one of the GPUs is be connected to the outputs and that GPU has to always be on, it can't be turned off even if it is the "inactive" one, IIUC.

Linux on the Mac — state of the union

Posted Dec 11, 2016 16:48 UTC (Sun) by l1k (subscriber, #112260) [Link] (4 responses)

Yes, with "muxless" hybrid graphics, the integrated GPU is always on whereas "muxed" solutions need only one of the two GPUs to be on at any given moment. On top of that, muxless requires the frames to be copied over PCIe, so you have higher latency and need to spend additional CPU cycles for fencing to avoid tearing. With Thunderbolt eGPUs latency increases further.

Linux on the Mac — state of the union

Posted Dec 11, 2016 21:22 UTC (Sun) by aaahaaap (guest, #112900) [Link] (3 responses)

I think I understand, thanks for the info!

In my case I'm looking for a decent laptop purely for development, but almost all of them, except for a custom config of the 2015 base model macbook pro 15", have discrete GPUs.
If I understand it correctly in cases where the integrated GPU controls the display outputs I should be able to fully disable the discrete GPU and not incur it's the power usage penalty.

Is there an easy way to figure out to which GPU the display outputs are connected? Maybe the info can be extracted from xrandr and the Xorg + kernel log?

Linux on the Mac — state of the union

Posted Dec 12, 2016 12:42 UTC (Mon) by l1k (subscriber, #112260) [Link] (1 responses)

almost all of them, except for a custom config of the 2015 base model macbook pro 15", have discrete GPUs

The MacBookPro11,4 is the 2015 version without discrete GPU, the MacBookPro11,5 is the one with discrete GPU.

If I understand it correctly in cases where the integrated GPU controls the display outputs I should be able to fully disable the discrete GPU and not incur it's the power usage penalty.

Correct, but that's the case for both muxed and muxless systems. One thing to keep in mind though is that the external DP/HDMI ports can only be driven by the discrete GPU on 2011+ MacBook Pros.

Is there an easy way to figure out to which GPU the display outputs are connected? Maybe the info can be extracted from xrandr and the Xorg + kernel log?

This is e.g. logged in dmesg: "vgaarb: overriding boot device: PCI:0000:01:00.0" (that's the discrete GPU, the integrated is usually 0000:00:02.0). By default the firmware switches to the discrete GPU on boot, but this can be influenced with an EFI variable using tools like gpu-switch (linked in the article) or gfxCardStatus (on macOS). The MacBookPro5 (2008/09, dual Nvidia) is the exception to that rule, it defaults to the integrated GPU.

Linux on the Mac — state of the union

Posted Dec 12, 2016 19:39 UTC (Mon) by aaahaaap (guest, #112900) [Link]

Awesome! Thanks a lot for your help/the info!

Linux on the Mac — state of the union

Posted Dec 17, 2016 12:24 UTC (Sat) by jkt_ (guest, #90352) [Link]

> In my case I'm looking for a decent laptop purely for development, but almost all of them, except for a custom config of the 2015 base model macbook pro 15", have discrete GPUs.

I can recommend Thinkpad T460s -- it works just fine under recent Linux. I have colleagues that are happy with their Dell XPSs without a discrete GPU -- perhaps you're talking about Macs?