Revision 10 as of 2011-02-25 00:24:47

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Hybrid Graphics

Introduction

Some laptops come with two graphics cards: one for use in applications that require a lot of computing power such as games, called the discrete GPU, and one that is less powerful, but conserves energy, called the integrated GPU. The integrated GPU is often embedded in the CPU, hence the name. This concept is called Hybrid Graphics.

How to switch GPU

The kernel shipped in Ubuntu 10.10 supports hybrid graphics by means of the vga_switcheroo flag. See vga_switcheroo below.

You can also compile a kernel module that allows you to directly make ACPI calls. See the Linux Hybrid Graphics Blog.

Enabling vga_switcheroo

vga_switcheroo is the kernel mechanism that allows you to switch between GPUs.

To check whether your kernel is compiled with the proper option you can exam the file config-2.6.nn-mm-generic in the /boot directory: 

grep -i switcheroo /boot/config-2.6.*

The vga_switcheroo mechanism will only be active when the kernel is booted with either the "modeset=1" kernel option, and/or the "nomodeset" option being absent.

To test if vga_switcheroo is enabled, look for the switch file: 

ls -l /sys/kernel/debug/vgaswitcheroo/switch

Using vga_switcheroo

Once you've ensured that vga_switcheroo is available, you can use these options to switch between GPUs. 

echo ON > /sys/kernel/debug/vgaswitcheroo/switch

Turns on the GPU that is disconnected (not currently driving outputs), but does not switch outputs. 

echo IGD > /sys/kernel/debug/vgaswitcheroo/switch

Connects integrated graphics with outputs. 

echo DIS > /sys/kernel/debug/vgaswitcheroo/switch

Connects discrete graphics with outputs. 

echo OFF > /sys/kernel/debug/vgaswitcheroo/switch

Turns off the graphics card that is currently disconnected.

There are also a couple of options that are useful from inside an X-Windows session: 

echo DIGD > /sys/kernel/debug/vgaswitcheroo/switch

Queues a switch to integrated graphics to occur when the X server is next restarted. 

echo DDIS > /sys/kernel/debug/vgaswitcheroo/switch

Queues a switch to discrete graphics to occur when the X server is next restarted.

Script for use inside an X session

Roberto Martinez has created a script which creates a graphical interface to switch between the graphics cards. He has released it over here. But you have to replace all the killall -u "$USER"" commands in his script with gnome-session-save --logout

To use this script, non-root users need permissions to write to /sys/kernel/debug/vgaswitcheroo/switch. If you run the script with sudo, you will just freeze the computer, since the script will then not end the current gnome session correctly.

To have permanent write permissions to the switch file, add the following line to /etc/init.d/rc.local: 

chown root:plugdev /sys/kernel/debug/vgaswitcheroo/switch

right before the "exit 0" line. This gives permission to switch GPUs to anyone in the plugdev group, which is usually all regular users.

You can also assign this script a global hotkey using the keyboard shortcuts menu (System > Preferences > Keyboard Shortcuts). You'll need to rename the script from Roberto Martinez to something without a .sh extension and place it into /usr/bin (as root) and make sure it is executable. Test it by running "switch_between_cards" without quotes from the terminal.

You may assign it any key combo you like so long as it's not already defined (See Keyboard Shortcuts). Super + G or Ctrl + Alt + G work well.

Script for use during bootup

You may want to switch to a given graphics adapter during boot, for example to enable output for the GPU that Linux uses to query for passwords to decrypt the root volume (and other boot-time messages).

Or you may want to disable one of the adapters during boot to conserve battery power.

There are currently no kernel options to control the vga_switcheroo mechanism, but here is an initramfs script that accomplishes the same. Put it in a file named /etc/initramfs-tools/scripts/local-top/hybrid_boot_options: 

#
# Standard initramfs preamble
#
prereqs()
{
:
}

case $1 in
prereqs)
        prereqs
        exit 0
        ;;
esac

# source for log_*_msg() functions, see LP: #272301
. /scripts/functions

#
# Helper functions
#
message()
{
        if [ -x /bin/plymouth ] && plymouth --ping; then
                plymouth message --text="$@"
        elif [ -p /dev/.initramfs/usplash_outfifo ] && [ -x /sbin/usplash_write ]; then
                usplash_write "TEXT-URGENT $@"
        else
                echo "$@" >&2
        fi
        return 0
}

run_switcheroo()
{
        local hybridopts
        hybridopts="$1"

        if [ -z "$hybridopts" ]; then
                return 1
        fi

        local IFS=" ,"
        for x in $hybridopts; do
                message "Switching hybrid graphics to $x"
                echo $x > /sys/kernel/debug/vgaswitcheroo/switch
        done
        return 0
}

#
# Begin real processing
#

# Do we have any kernel boot arguments?
for opt in $(cat /proc/cmdline); do
        case $opt in
        hybridopts=*)
                run_switcheroo "${opt#hybridopts=}"
                ;;
        esac
done

exit 0

Set it executable, and create new initramfs files with initramfs-tools: 

chmod +x /etc/initramfs-tools/scripts/local-top/hybrid_boot_options
initramfs-tools -c -k all

Now, you can add a command line to your kernel boot options in /etc/default/grub: 

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash modeset=1 hybridopts=ON,IGD,OFF"

The above example enables both GPUs, switches to the integrated GPU, and finally turns off the discrete GPU.

Finally, the options needs to be actually applied to GRUB (since the above is just a template), which is done with grub-update: 

grub-update

And we're done! You can also experiment with options during boot-time by pressing "e" in GRUB and adding/removing commands (separated by comma) from the "hybridopts=" option.

Various small fixes

Fix fans running at full speed

Roberto Martinez explains in his blog post, that he ran into a problem where his graphics card's fan would run at full speed, if he didn't have both graphics cards turned ON before shutting down his computer. He provides a script to fix it. All it does is run 

echo ON > /sys/kernel/debug/vgaswitcheroo/switch

before shutdown. See the blog post for more details.

Fix brightness

The brightness of the laptop display upon boot may be turned very low. If you have a nearly black screen during boot, you may be lucky and just need to press the correct key to increase brightness.

Default GPU

The GPU that is enabled by the BIOS during boot may be dependent on whether the laptop is plugged into AC power or not.

Console FB

You can select which device the linux kernel uses for the boot-time console using the kernel command line option fbcon, such as "fbcon=map:1" for /dev/fb1.