Discrete graphics always DynOff

On my Ubuntu 18.04, I solved it this way. There is no need to edit /etc/default/grub file.

1. Check that the integrated GPU is used by default by running glxheads

   $ glxheads                                                              
      ...                                                                     
      GL_VERSION:  3.0 Mesa 18.0.5                                            
      GL_VENDOR:   Intel Open Source Technology Center                        
      GL_RENDERER: Mesa DRI Intel(R) Sandybridge Mobile
   

2. Check that the discrete GPU is visible but DynOff

   $ sudo cat /sys/kernel/debug/vgaswitcheroo/switch                       
     0:DIS: :DynOff:0000:01:00.0                                             
     1:IGD:+:Pwr:0000:00:02.0     
   

3. Enable the discrete GPU for the glxheads and run it again. You can see that the renderer has changed.

   $ DRI_PRIME=1 glxheads                                                  
     GL_VERSION:  3.0 Mesa 18.0.5                                            
     GL_VENDOR:   X.Org                                                      
     GL_RENDERER: AMD TURKS (DRM 2.50.0 / 4.15.0-32-generic, LLVM 6.0.0)
   

4. Additionally, while running DRI_PRIME=1 glxheads, in the different window check that now the discrete GPU is DynPwr instead of DynOff

   $ sudo cat /sys/kernel/debug/vgaswitcheroo/switch                         
     0:DIS: :DynPwr:0000:01:00.0                                             
     1:IGD:+:Pwr:0000:00:02.0
   

5. So you need to run each application with DRI_PRIME=1 in order to use discrete GPU. Also, you can set this environmental variable permanently https://unix.stackexchange.com/questions/117467/how-to-permanently-set-environmental-variables

1. Set kernel parameter:

   sudo nano /etc/default/grub
   

2. Find GRUB_CMDLINE_LINUX_DEFAULT, append radeon.runpm=0 like this

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.runpm=0"
   

3. Update config

   sudo update-grub
   

4. To /etc/rc.local add

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

5. Restart machine

   sudo shutdown -r now
   

6. PROFIT!

   sudo cat /sys/kernel/debug/vgaswitcheroo/switch 
   
   0:IGD:+:Pwr:0000:00:02.0
   
   1:DIS: :Off:0000:01:00.0
   

My set to ATI discrete hybrid works.

# lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Thames [Radeon HD 7500M/7600M Series]

gedit /etc/default/grub 
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.runpm=0"
GRUB_CMDLINE_LINUX_DEFAULT="radeon.runpm=0"
GRUB_CMDLINE_LINUX=""

cat /sys/kernel/debug/vgaswitcheroo/switch 
0:DIS: :Pwr:0000:01:00.0
1:IGD:+:Pwr:0000:00:02.0

Add DRI_PRIME=1 string in files env to work with ATI graphics card in apps.

gedit /etc/bash.bashrc 
DRI_PRIME=1
gedit /etc/environment 
DRI_PRIME=1
gedit /etc/profile
DRI_PRIME=1

At the end it's used with all apps by default.

# glxheads 
glxheads: exercise multiple GLX connections (any key = exit)
Usage:
  glxheads xdisplayname ...
Example:
  glxheads :0 mars:0 venus:1
Name: :0.0
  Display:     0x55a68ea5b670
  Window:      0x4200002
  Context:     0x55a68ea6cd80
  GL_VERSION:  3.0 Mesa 17.2.2
  GL_VENDOR:   X.Org
  GL_RENDERER: AMD TURKS (DRM 2.50.0 / 4.13.0-25-generic, LLVM 5.0.0)