Q:

Can't change screen resolution after upgrade to 16.04

I have a cheap 27" ebay monitor with unknown brand (says imon). It worked fine on 14.04, but after upgrading to 16.04 it's stuck to 800x600. Since I have an AMD graphic card, it could be the driver change to radeon ? It could also be the monitor EDID being misinterpreted. 
I went for what appeared to be the simplest fix: add a screen resolution.
$xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 800 x 600, current 800 x 600, maximum 800 x 600
default connected primary 800x600+0+0 0mm x 0mm
   800x600       75.00*

$xrandr --props | edid-decode
xrandr: Failed to get size of gamma for output default
Extracted contents:
header:          53 63 72 65 65 6e 20 30
serial number:   3a 20 6d 69 6e 69 6d 75 6d 20
version:         38 30
basic params:    30 20 78 20 36
chroma info:     30 30 2c 20 63 75 72 72 65 6e
established:     74 20 38
standard:        30 30 20 78 20 36 30 30 2c 20 6d 61 78 69 6d 75
descriptor 1:    6d 20 32 35 36 30 20 78 20 31 34 34 30 0a 64 65 66 61
descriptor 2:    75 6c 74 20 63 6f 6e 6e 65 63 74 65 64 20 70 72 69 6d
descriptor 3:    61 72 79 20 38 30 30 78 36 30 30 2b 30 2b 30 20 30 6d
descriptor 4:    6d 20 78 20 30 6d 6d 0a 20 20 20 38 30 30 78 36 30 30
extensions:      20
checksum:        20

No header found
Manufacturer: NQ@ Model 696d Serial Number 1970104686
EDID version: 56.48
Analog display, Input voltage level: 0.714/0.286 V
Configurable signal levels
Sync: 
Maximum image size: 32 cm x 120 cm
Gamma: 1.32
DPMS levels: Off
Non-RGB color display
Default (sRGB) color space is primary color space
First detailed timing is preferred timing
Established timings supported:
  720x400@88Hz
  640x480@60Hz
  640x480@67Hz
  640x480@75Hz
  832x624@75Hz
Standard timings supported:
  632x632@108Hz
  504x378@116Hz
  504x504@114Hz
  632x632@108Hz
  600x600@92Hz
  1120x840@93Hz
  1208x906@101Hz
  1120x840@113Hz
Detailed mode: Clock 83.010 MHz, 1584 mm x 1034 mm
                818  850 1667 2407 hborder 101
               1840 1859 1863 3920 vborder 102
               -hsync -vsync analog composite side by side interleaved
Detailed mode: Clock 277.650 MHz, 1892 mm x 32 mm
               1652 2009 2620 2452 hborder 114
               1647 1670 1690 5341 vborder 105
               -hsync +vsync bipolar analog composite side by side interleaved
Detailed mode: Clock 292.810 MHz, 816 mm x 43 mm
                889  943 1503 2969 hborder 32
               1840 1875 1923 3936 vborder 48
               -hsync +vsync bipolar analog composite side by side interleaved
Detailed mode: Clock 83.010 MHz, 1840 mm x 2096 mm
                888  920 1720  920 hborder 54
                109  143  143 2778 vborder 48
               -hsync -vsync digital composite field sequential L/R
Has 32 extension blocks
Checksum: 0x20 (should be 0x69)
EDID block does not conform at all!
    Block has broken checksum
    Bad year of manufacture
    Bad week of manufacture
    Manufacturer name field contains garbage

Wow, these last words were not very kind to my monitor.
$sudo lshw -C display
*-display UNCLAIMED     
       description: VGA compatible controller
       product: Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list
       configuration: latency=0
       resources: memory:e0000000-efffffff memory:f7e20000-f7e3ffff ioport:e000(size=256) memory:f7e00000-f7e1ffff

I tried to change the screen resolution. I think it was previously running at 2560x1440.
$xrandr --output default --mode 2560x1440_60.00 --pos 0x0 --rotate normal
$xrandr --addmode default "2560x1440_60.00"

$xrandr --output default --mode 2560x1440_60.00
 xrandr: Failed to get size of gamma for output default
 xrandr: Configure crtc 0 failed

However I don't know my crtc number so I can't go further on that. I don't know at that point if my problem is to add screen resolution or change driver. 

A:

16.04 has no proprietary AMD drivers, which is most likely why you can't get a QHD resolution. Even if you could, I wouldn't recommend it, since it will probably be very laggy.
If you can, you should just downgrade to 14.04, which is supported by fglrx. If you accidentally take an HWE upgrade, then you can downgrade Xorg to get fglrx back. Messed up updates. Managed to get to desktop, but can't install Fglrx drivers back

