Oops, I forgot to attach more details about the graphics card that I use. Here's the output of lspci -v:
02:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Turks
[Radeon HD 6670] (prog-if 00 [VGA controller])
Subsystem: ASUSTeK Computer Inc. Device 03ea
Flags: bus master, fast devsel, latency 0, IRQ 47
Memory at d0000000 (64-bit, prefetchable) [size=256M]
Memory at febe0000 (64-bit, non-prefetchable) [size=128K]
I/O ports at e800 [size=256]
Expansion ROM at febc0000 [disabled] [size=128K]
Capabilities: [50] Power Management version 3
Capabilities: [58] Express Legacy Endpoint, MSI 00
Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010
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Capabilities: [150] Advanced Error Reporting
Kernel driver in use: radeon
02:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI Turks/Whistler HDMI
Audio [Radeon HD 6000 Series]
Subsystem: ASUSTeK Computer Inc. Device aa90
Flags: bus master, fast devsel, latency 0, IRQ 48
Memory at febbc000 (64-bit, non-prefetchable) [size=16K]
Capabilities: [50] Power Management version 3
Capabilities: [58] Express Legacy Endpoint, MSI 00
Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010
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Capabilities: [150] Advanced Error Reporting
Kernel driver in use: snd_hda_intel
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