[Trac #1388] setting dcon to "dcon sleep" mode does not shut off panel in pre-b3

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#1388: setting dcon to "dcon sleep" mode does not shut off panel in pre-b3
----------------------+-----------------------------------------------------
 Reporter:  dilinger  |       Owner:  wad    
     Type:  defect    |      Status:  new    
 Priority:  blocker   |   Milestone:  BTest-3
Component:  hardware  |    Keywords:         
----------------------+-----------------------------------------------------
 Writing 0x2 to the DCON Display Mode register (1) should enable DCON Sleep
 Mode.  Quoth the spec:

 {{{
 One key to the power efficiency of the DCON is its ability to enter a low
 power state in which
 the LCD is completely turned off, and the local SDRAM frame buffer is set
 to self-refresh mode
  this mode is known as DCON Sleep Mode (or just Sleep mode).
 }}}

 On b2, writing this bit would turn off the panel.  On pre-b3, the panel
 remains on.  A quick way to reproduce this in OFW is:

 {{{
 ok  dev screen
 ok  2 mode!
 }}}

 The panel *should* shut off; it does not.  Instead, the backlight shuts
 off (which it should), video blanking is enabled (hence the black screen),
 but the panel remains powered up.

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Ticket URL: <http://dev.laptop.org/ticket/1388>
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