By default, the MacBook Pro 8,2 boots Trisquel 6.0 64-bit in BIOS emulation mode. In this mode, the Intel integrated graphics is disabled, and the AMD Radeon card provides screen output. However, since the Radeon chip does not have free firmware available, the X server falls back to a generic driver which cannot display the MacBook Pro's native resolution of 1440x900 (up to 1152x864 only) with no 3D acceleration.

A better alternative is to boot the computer in EFI mode and use the Intel integrated graphics (which supports 3D acceleration) instead. For the Intel integrated graphics to work, a newer linux-libre kernel (tested with 3.11.1) was compiled and installed using the default options. Then, the options

i915.lvds_channel_mode=2

and

i915.lvds_use_ssc=0

were appended to the variable

GRUB_CMDLINE_LINUX_DEFAULT

in

/etc/default/grub

To turn off the Radeon chip and switch to Intel graphics at boot time, the following code was added in the menuentry of each kernel in grub.cfg right before the line beginning with linux ...:

# Switch to intel integrated graphics
outb 0x728 1
outb 0x710 2
outb 0x740 2
# Power off the ATI graphics
outb 0x750 0

Finally, a new grub.cfg was generated with the command

sudo update-grub

00:00.0 Host bridge: Intel Corporation 2nd Generation Core Processor Family DRAM Controller (rev 09)
00:01.0 PCI bridge: Intel Corporation Xeon E3-1200/2nd Generation Core Processor Family PCI Express Root Port (rev 09)
00:01.1 PCI bridge: Intel Corporation Xeon E3-1200/2nd Generation Core Processor Family PCI Express Root Port (rev 09)
00:16.0 Communication controller: Intel Corporation 6 Series/C200 Series Chipset Family MEI Controller #1 (rev 04)
00:1a.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Universal Host Controller #5 (rev 05)
00:1a.7 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #2 (rev 05)
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 05)
00:1c.0 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 1 (rev b5)
00:1c.1 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 2 (rev b5)
00:1c.2 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root Port 3 (rev b5)
00:1d.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Universal Host Controller #1 (rev 05)
00:1d.7 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #1 (rev 05)
00:1f.0 ISA bridge: Intel Corporation HM65 Express Chipset Family LPC Controller (rev 05)
00:1f.2 IDE interface: Intel Corporation 6 Series/C200 Series Chipset Family 4 port SATA IDE Controller (rev 05)
00:1f.3 SMBus: Intel Corporation 6 Series/C200 Series Chipset Family SMBus Controller (rev 05)
01:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Whistler [AMD Radeon HD 6600M Series]
01:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI Turks/Whistler HDMI Audio [Radeon HD 6000 Series]
02:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM57765 Gigabit Ethernet PCIe (rev 10)
02:00.1 SD Host controller: Broadcom Corporation NetXtreme BCM57765 Memory Card Reader (rev 10)
03:00.0 Network controller: Broadcom Corporation BCM4331 802.11a/b/g/n (rev 02)
04:00.0 FireWire (IEEE 1394): LSI Corporation FW643 [TrueFire] PCIe 1394b Controller (rev 08)

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 002: ID 0424:2513 Standard Microsystems Corp. 2.0 Hub
Bus 001 Device 003: ID 05ac:8509 Apple, Inc. FaceTime HD Camera
Bus 002 Device 002: ID 0424:2513 Standard Microsystems Corp. 2.0 Hub
Bus 001 Device 004: ID 0a5c:4500 Broadcom Corp. BCM2046B1 USB 2.0 Hub (part of BCM2046 Bluetooth)
Bus 001 Device 005: ID 05ac:0252 Apple, Inc. Internal Keyboard/Trackpad (ANSI)
Bus 002 Device 003: ID 05ac:8242 Apple, Inc. Built-in IR Receiver
Bus 001 Device 008: ID 05ac:821a Apple, Inc. Bluetooth Host Controller