How to use vesa graphic driver

If command xrandr output tells something like xrandr: Failed to get size of gamma for output default , the graphic driver may not work properly. Command inxi -G can tell something like

Graphics: Card: Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx PCIE VGA Display Adapter
Display Server: X.Org 1.16.4 drivers: fbdev (unloaded: modesetting,vesa)
Resolution: 800×600@0.00hz
GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.5, 128 bits)
GLX Version: 3.0

… so we can see that system uses fbdev driver

https://en.wikipedia.org/wiki/Linux_framebuffer

The Linux framebuffer (fbdev) is a graphic hardware-independent abstraction layer to show graphics on a computer monitor, typically on the console.[1] The word framebuffer means a part of video memory containing a current video frame, and the Linux framebuffer means “access method to the framebuffer under the Linux kernel”, without relying on system-specific libraries such as SVGALib or another user space software.

Vesa-driver may be better than fbdev-driver, but it requires a /etc/X11/xorg.conf file so that system uses a vesa-driver. Fortunately it is quite easy to make a xorg.conf file, which uses vesa-driver.

1. Open Terminal or command line and run

sudo Xorg :1 -configure

That command makes a $USER/xorg.conf.new file which probably uses “unworkable” driver. To make a workable xorg.conf you need to find a line Section “Device” and on the bottton of section lines like

Identifier “Card0”
Driver “sisimedia”
BusID “PCI:1:0:0”
EndSection

Replace Driver (“sisimedia“) using “vesa” and save the file as xorg.conf

2. Now when configured xorg.conf file is made, it is needed to copy to /etc/X11

sudo cp xorg.conf /etc/X11/xorg.conf

3. Reboot and see a difference. Command inxi -G tells what driver is on. An example from FUJITSU SIEMENS Amilo A1645 laptop.

Graphics: Card: Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx PCIE VGA Display Adapter
Display Server: X.Org 1.16.4 driver: vesa
Resolution: 1024×768@61.00hz
GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.5, 128 bits)
GLX Version: 3.0

Compairing vesa and fbdev-resolutions in FUJITSU SIEMENS Amilo A1645

vesa = 1024x768
fbdev = 800x600

Read also SiS 661/741/760 PCI/AGP driver installation

An excellent xorg.conf in LMDE 2 in Amilo A 1645 xorg.conf