Jean-Louis Dupond, thank you for reporting this and helping make Ubuntu better. 
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apport-collect 1374607
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** Tags added: bisect-done

** Changed in: linux (Ubuntu)
   Importance: Undecided => Low

** Changed in: linux (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1374607

Title:
  nouveau - HDMI gives no more signal since 3.15 (optimus)

Status in The Linux Kernel:
  Confirmed
Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  This week I tried HDMI again on my system, and noticed it was completely 
broken.
  Got just no signal on my screen.

  I knew it was working some months (and upgrades) ago.
  After testing I found out the issue was introduced between kernel v3.14 and 
v3.15.

  Situation:
  Insert HDMI cable into my laptop, and no signal on the external display.

  The dmesg fills with errors also:
  [  186.210622] nouveau E[   PDISP][0000:01:00.0] INVALID_STATE [UNK08] chid 0 
mthd 0x0080 data 0x00000000
  [  186.210627] nouveau E[   PDISP][0000:01:00.0] Core:
  [  186.210634] nouveau E[   PDISP][0000:01:00.0]      0x0084: 0x5dc34da1 -> 
0x80000000
  [  186.210640] nouveau E[   PDISP][0000:01:00.0]      0x0088: 0x00000000      
        

  etc.

  After some git bisect playtime, I found out the following commit caused the 
issue:
  
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/gpu/drm/nouveau?id=05c63c2ff23a80b654d6c088ac3ba21628db0173

  So I went back to stable 3.16 (Ubuntu Utopic version), and did a git revert 
for that commit.
  And yep, HDMI output was working again, and no more nouveau errors in dmesg.

  
  My device:
  [   30.823267] nouveau  [  DEVICE][0000:01:00.0] BOOT0  : 0x0c1a00a1
  [   30.823269] nouveau  [  DEVICE][0000:01:00.0] Chipset: GF108 (NVC1)
  [   30.823270] nouveau  [  DEVICE][0000:01:00.0] Family : NVC0

  Laptop dell xps 15 l502x

  So this can be fixed with just reverting this commit. Else this is an
  annoying regression in Utopic.

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