(In reply to comment #17)
> With the extra output the display on -> standby (off) transition now looks
> like:
> 
> [  227.434722] nouveau D[     I2C][0000:05:00.0] AUXCH(3): 8: 0x00000600 1
> [  227.434736] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000002
> [  227.434738] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  227.434741] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  227.434743] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  227.434877] nouveau D[     I2C][0000:05:00.0] AUXCH(3): 00 0x01108000
> 0x10000000
> [  227.434884] nouveau  [   PDISP][0000:05:00.0] nv50_sor_mthd: 0000 0184
> [  227.434886] nouveau  [   PDISP][0000:05:00.0]        0200 0102
> [  227.434888] nouveau  [   PDISP][0000:05:00.0]        0002 01c1
> [  227.434891] nouveau  [   PDISP][0000:05:00.0]        0006 0284
> [  227.434893] nouveau  [   PDISP][0000:05:00.0]        0002 0284
> [  227.434913] nouveau  [   PDISP][0000:05:00.0] nv50_sor_mthd: 0006 0184
> [  227.434915] nouveau  [   PDISP][0000:05:00.0]        0200 0102
> [  227.434917] nouveau  [   PDISP][0000:05:00.0]        0002 01c1
> [  227.434919] nouveau  [   PDISP][0000:05:00.0]        0006 0284
> [  227.434921] nouveau  [   PDISP][0000:05:00.0]        0002 0284
> 
> and the (unsuccessful) off -> on transition looks like:
> 
> [  277.955374] nouveau D[     I2C][0000:05:00.0] AUXCH(3): 8: 0x00000600 1
> [  277.955387] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000001
> [  277.955389] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  277.955392] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  277.955394] nouveau D[     I2C][0000:05:00.0] AUXCH(3): wr 0x00000000
> [  277.955528] nouveau D[     I2C][0000:05:00.0] AUXCH(3): 00 0x01108000
> 0x10000000
> [  277.957570] nouveau  [   PDISP][0000:05:00.0] nv50_sor_mthd: 0000 0184
> [  277.957574] nouveau  [   PDISP][0000:05:00.0]        0200 0102
> [  277.957576] nouveau  [   PDISP][0000:05:00.0]        0002 01c1
> [  277.957578] nouveau  [   PDISP][0000:05:00.0]        0006 0284
> [  277.957580] nouveau  [   PDISP][0000:05:00.0]        0002 0284
> [  277.957600] nouveau  [   PDISP][0000:05:00.0] nv50_sor_mthd: 0006 0184
> [  277.957602] nouveau  [   PDISP][0000:05:00.0]        0200 0102
> [  277.957604] nouveau  [   PDISP][0000:05:00.0]        0002 01c1
> [  277.957607] nouveau  [   PDISP][0000:05:00.0]        0006 0284
> [  277.957609] nouveau  [   PDISP][0000:05:00.0]        0002 0284
> 
> I'll attach the full log soon (pressed for time ATM).

I see what's going on, I'll push a patch in the next little while.  Thank you.