From: https://kernel.ubuntu.com/~kernel-ppa/mainline/v3.15-rc2-trusty/CHANGES
I see the folowing changes for radeon:

Alex Deucher (13):
      drm/radeon/dp: handle zero sized i2c over aux transactions (v2)
      drm/dp/i2c: send bare addresses to properly reset i2c connections (v4)
      drm/dp/i2c: Update comments about common i2c over dp assumptions (v3)
      drm/radeon/dp: switch to the common i2c over aux code
      drm/radeon: fix audio pin counts for DCE6+ (v2)
      drm/radeon: disable mclk dpm on R7 260X
      drm/radeon: fix runpm handling on APUs (v4)
      drm/radeon: update CI DPM powertune settings
      drm/radeon: add support for newer mc ucode on SI (v2)
      drm/radeon: add support for newer mc ucode on CI (v2)
      drm/radeon: re-enable mclk dpm on R7 260X asics
      drm/radeon/si: make sure mc ucode is loaded before checking the size
      drm/radeon/ci: make sure mc ucode is loaded before checking the size

Christian König (2):
      drm/radeon: apply more strict limits for PLL params v2
      drm/radeon: improve PLL params if we don't match exactly v2

Christoph Jaeger (2):
      ...
      drm/radeon: fix VCE fence command

Quentin Casasnovas (1):
      drm/radeon: memory leak on bo reservation failure. v2

The:
Christian König (2):
      drm/radeon: apply more strict limits for PLL params v2
      drm/radeon: improve PLL params if we don't match exactly v2
rings a bell for me, because I believe someone somewhere was saying that a 
kernel parameter
speaking of PLL was fixing the problem... but people did not find PLL 
parameters for radeon,
so people did not follow about that comment.

From:
https://www.x.org/releases/current/doc/man/man4/radeon.4.xhtml
I see:
Option "LVDSProbePLL" "boolean"
When BIOS panel information isn’t available (like on PowerBooks), it may still 
be necessary to
use the firmware-provided PLL values for the panel or flickering will happen. 
This option will
force probing of the current value programmed in the chip when X is launched in 
that case.
This is only useful for LVDS panels (laptop internal panels). The default is on.
Also:
Option "DefaultTMDSPLL" "boolean"
Use the default driver provided TMDS PLL values rather than the ones provided 
by the BIOS.
This option has no effect on Mac cards.
Enable this option if you are having problems with a DVI monitor using the 
internal TMDS controller.
The default is off.

Also:
      drm/radeon: update CI DPM powertune settings
ring an other bell, because I think I remember someone saying something about 
DPM in some message.
Sorry for not being more specific.

A similar, older fixed closed bug:
https://bugzilla.kernel.org/show_bug.cgi?id=26552

** Bug watch added: Linux Kernel Bug Tracker #26552
   https://bugzilla.kernel.org/show_bug.cgi?id=26552

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https://bugs.launchpad.net/bugs/1791312

Title:
  ubuntu 18.04 flickering screen with Radeon X1600

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  my laptop: HP Compaq nx9420
  my grafic card: 
  01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] 
RV530/M56-P [Mobility Radeon X1600] (prog-if 00 [VGA controller])

  attachment:
  the bug-apport-file

  Description:
  I start ubuntu 18.04 LTS (Kernel 4.15.0.33) in textmode;
  -> as soon as the kernel-module radeon.ko is loaded the screen starts 
flickering
  when starting the grafic-mode the flickering continues; so the screen is 
unusable.

  when setting the option "radeon.modeset=0" (thus: not use radeon drivers) the 
screen does not flicker
  but the resolution is limited to 1400x1050 (instead of natural 1680x1050)
  so I can not use this as work-around

  when using the old ubuntu 14.04.05 LTS (Kernel 3.13.0-157)
  the screen is o.k. NO flickering in textmode, nice grafic, NO flickering 
  xrandr tells: 1680x1050 60.1

  so I must stay on old ubuntu 14.04.05 ???

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