Fix for blank screen or black display after resume with open source radeon drivers in Xorg

The bug was reported in detail here. The fix comes in two ways:

For just echo mem > /sys/power/state to work, use the 

1. kernel boot parameters nomodeset acpi_sleep=s3_bios,s3_mode

-> You have to test which of the s3_bios or s3_mode settings work for you. You may also use number 1, 2, and 3 in /proc/sys/kernel/acpi_video_flags instead to change the setting at run time. 

2. For pm-suspend to work (e.g. *Ubuntu) you need to create a file

/etc/pm/config.d/radeon  which contains  QUIRK_S3_BIOS="true"  QUIRK_S3_MODE="true"

3. To have working mode setting, load the radeon module with modeset=1, by creating a file 
    

"/etc/modprobe.d/radeon.conf"
    
with
    
"options radeon modeset=1"
    

    
Good luck!