[Nouveau] [PATCH v2 8/9] acpi: Add support for Apple Gmux _DMS

Lukas Wunner lukas at wunner.de
Sat May 30 11:23:46 PDT 2015

Hi Pierre,

On Thu, May 28, 2015 at 10:52:56AM +0200, Pierre Moreau wrote:
> How is it supposed to work, by simply echo'ing ON or OFF to
> vga_switcheroo/switch? Then I probably forgot some stuff as
> it doesn't want to work on my laptop.

What exactly doesn't work and which version of the MacBook Pro did
you try this on? The expected behaviour when switching GPUs on the
MBP is that the GPU that was inactive on bootup will only show a
black screen because it was unable to read the EDID (and DPCD on
retina MBPs) on bootup. In other words, it's broken.

There have been attempts to solve this by multiple folks, the last
one was submitted to dri-devel in April by yours truly:

That initial version of my patch set turned out to only work on
pre-retina MBPs, and only if the active GPU on bootup is the
discrete one. I'm currently working on a v2 to solve all that.

We need to get switching working before we can enable runtime PM on
MBPs, otherwise the user will be greeted by a black screen when the
Nvidia GPU goes to sleep and hands over to the integrated GPU.
So your patch is a bit premature I'm afraid. :-(

As to the patch itself, you're checking for existence of the gmux
but we can't make use of it unless its driver is loaded. Hence it
may be easier to simply query for the existence of the driver,
using something along the lines of find_module("apple-gmux").
I imagine this is much less code than checking for the DMI IDs.

Also, Dave Airlie and Matthew Garrett already wrote a patch set to
enable runtime PM for nouveau on MBPs which is more generic: It works
with any vga_switcheroo handler, not just gmux:


The fourth one in that list is by Dave Airlie and all the others are
by Matthew Garrett. But, as I said, we can't apply these either until
switching works. I'm working on it. ;-)

As to your questions:

> How is it supposed to work, by simply echo'ing ON or OFF to
> vga_switcheroo/switch?

Echoing ON/OFF to the "switch" file will power the inactive device
up or down. If the currently active gpu is the discrete one (which
is the default when booting a MacBook Pro), this will power up/down
the integrated gpu. You can use the tool "gpu-switch" to force the
MBP to the integrated GPU on next bootup, then you can echo ON/OFF
to change the power state of the Nvidia GPU:

> If I understand it correctly, if the laptop is an Optimus one or
> has a mux, we are not "allowed" to opt-out of DynPwr/DynOff by
> powering down the card?

If we power the GPU down manually by echoing OFF to the "switch"
file, the runtime PM code would probably get confused, I imagine
that's the reason why ON/OFF is ignored if client->driver_power_control
is set to true.

Speaking of which, the external ports of the MBP are soldered to
the discrete GPU and can't be switched. Only the panel can be
switched. So if there are DP or HDMI connectors with status
connected, we can't suspend. That's one of the MBP's numerous
peculiarities that I haven't verified yet they're handled properly
by the code.

Best regards,


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