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I just upgraded to Ubuntu 17.04 and on trying to connect more than one
external monitor, the system either goes to a black screen (recoverable
from a system terminal) or will switch on one external monitor but not
both. This was working with the ati proprietary drivers in 16.10, but
these appear not to be supported in 17.04.

This is the what I think is the relevant output of dmesg | egrep 'drm|radeon' 
(the errors that come up after I try to connect a second external monitor):
[  557.739832] [drm:atombios_crtc_mode_fixup [radeon]] *ERROR* unable to 
allocate a PPLL
[  557.739850] [drm:drm_crtc_helper_set_config [drm_kms_helper]] *ERROR* failed 
to set mode on [CRTC:40:crtc-2]
[  584.375631] [drm:atombios_crtc_mode_fixup [radeon]] *ERROR* unable to 
allocate a PPLL
[  584.375651] [drm:drm_crtc_helper_set_config [drm_kms_helper]] *ERROR* failed 
to set mode on [CRTC:40:crtc-2]
[  589.582640] [drm:atombios_crtc_mode_fixup [radeon]] *ERROR* unable to 
allocate a PPLL
[  589.582656] [drm:drm_crtc_helper_set_config [drm_kms_helper]] *ERROR* failed 
to set mode on [CRTC:40:crtc-2]

I've replicated the error with a totally new user account, so it doesn't
appear to be an old account settings problem.

Any advice would be welcome.

  Installed: 1:7.9.0-0ubuntu1
  Candidate: 1:7.9.0-0ubuntu1
  Version table:
 *** 1:7.9.0-0ubuntu1 500
        500 zesty/main amd64 Packages
        100 /var/lib/dpkg/status

** Affects: xserver-xorg-video-ati (Ubuntu)
     Importance: Undecided
         Status: New

More than one external monitor not supported
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