[Intel-gfx] [PATCH v5 00/12] Enable GPU switching on pre-retina?MacBook Pro

[Lukas Wunner]

Hi Bastien,

On Fri, Mar 04, 2016 at 04:12:27PM +0000, Bastien Nocera wrote:
> Lukas Wunner <lukas <at> wunner.de> writes:
> > Enable GPU switching on the pre-retina MacBook Pro (2008 - 2013), v5.
> 
> I've tested your patchset on a MacBookPro8,1, with an integrated Intel and
> discrete AMD/ATI GPUs.

Hm, it must be either an 8,2 or 8,3. The 8,1 was a 13" machine and only
had an integrated GPU.


> I've used the COPR repository here to cut down on my compilation time:
> https://copr.fedorainfracloud.org/coprs/firstyear/kernel-mbp/
> 
> I'm not certain how to test out your changes, or what the consequences should
> be on a stock Fedora 23/GNOME 3.18 installation. After booting (note that I
> did not change any command-line options in grub), a gnome-shell/gdm X11
> session comes up (I disabled Wayland, to rule out behavioural changes), I'd
> log in to GNOME and gnome-shell (which starts another X11 session on
> another VT).

Switching and power control currently requires manual intervention
by echoing commands to /sys/kernel/debug/vgaswitcheroo/switch
as documented here:
https://01.org/linuxgraphics/gfx-docs/drm/modes_of_use.html

As you've correctly observed, the machine is initially switched to
the discrete GPU and both GPUs are turned on. By echoing "IGD" to
the sysfs file, you'll switch to the integrated GPU and turn off
the discrete GPU.

It's possible to let the EFI firmware switch to the integrated GPU
on boot by using this tool: https://github.com/0xbb/gpu-switch
However still both GPUs will be powered up, so you have to issue
the "OFF" command to sysfs to power the discrete GPU down. Also,
once you boot into OS X, the setting made by the gpu-switch tool
will be overwritten and the machine will be switched to the discrete
GPU again the next time you boot Linux.

Note that switching is only possible from the text console, with
X11/Wayland shut down. Obviously this is not great in terms of UX.
A few years ago there was a GSoC proposal to get hot GPU switching
to work on Linux (akin to what OS X does) but nothing ever came of it:
http://www.phoronix.com/scan.php?page=news_item&px=OTIyMQ
https://lists.x.org/archives/xorg/2011-March/052522.html

Unfortunately this seems to be a low priority item for kernel graphics
developers since nowadays most dual GPU notebooks no longer have a mux
and cannot switch. The MacBook Pro seems to be the last one supporting
this but I've witnessed a bit of an anti-Apple sentiment among kernel
graphics developers since everything is non-standard there. Which is
unfortunate because these machines have a large market share and Apple
software quality is deteriorating rapidly so a lot of Mac users are
ripe for converting to Linux.

Anyway, one short-term improvement will be to add runtime pm support
(called "Driver power control" in the vga_switcheroo documentation
linked above). That way it'll no longer be necessary to power the
discrete GPU up and down manually, this will happen automatically
as needed (when switching or using render offloading with DRI PRIME).
I have patches to enable this for radeon but they're completely untested:

http://wunner.de/mbp_switcheroo_v5-4.5.tar.gz       => gpu switching for 4.5
http://wunner.de/mbp_switcheroo_v5-4.5-runpm.tar.gz => runtime pm for radeon

I have an Nvidia based machine and runtime pm doesn't work there yet
because of bugs in nouveau that I haven't had the time to look into.


> I did not use any external screens to test this.

Since your machine has Thunderbolt, the external port is no longer
switchable between GPUs, it can only be driven by the discrete GPU.
So you need to power it up manually for this to work. You don't need
to switch to it, but it's probably recommendable to save energy.
(Otherwise both GPUs are on with the integrated GPU driving the panel
and the discrete GPU driving the DP port.)

The runpm tarball linked above contains a patch to automatically
wake the discrete GPU on hotplug.

I've heard that the AMD GPU is picky about external monitors and
doesn't recognize them unless they're plugged in at exactly the
right moment, so you may need to retry a couple of times until it
works.

Best regards,

Lukas