#12216 NORM 13.1.0: XO-1.75/XO4 don't support 1.8 volt SD cards (was: arm doesn't support 1.8 volt card)

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#12216: XO-1.75/XO4 don't support 1.8 volt SD cards
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           Reporter:  jvonau     |       Owner:  cjb                     
               Type:  defect     |      Status:  assigned                
           Priority:  normal     |   Milestone:  13.1.0                  
          Component:  kernel     |     Version:  Software Build 12.1.0-21
         Resolution:             |    Keywords:                          
        Next_action:  never set  |    Verified:  0                       
Deployment_affected:             |   Blockedby:                          
           Blocking:             |  
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Comment(by wad):

 Why doesn't someone ask me, if they don't know ?

 On XO-1.75:
 Only +3.3V operation of the SD interfaces is supported, but they
 definitely are NOT hardwired.  Power is switched.

 The external SD slot power switch is controlled by EN_SD2_PWR#.

 Both the internal eMMC and the internal SD slot share an MMC interface.
 Only one can be used at a time, selected by a jumper when the internal SD
 slot is installed.    As such, they share a power switch, controlled by
 EN_SD1_PWR#.

 However, as with the XO-1.5, these power control lines do NOT go to a GPIO
 accessible by the main processor.   They are instead connected to the
 embedded controller.   They can be turned on and off by sending commands
 to the EC.   I believe the EC firmware already implements these commands,
 but Linux was not wired up properly to use them.

 This method of control was done because we encountered some SD cards which
 violated the SD spec and lost data if they were turned off immediately
 after a data transfer.   Having the EC control their power allows it to
 maintain power to the SD card for a second after the main processor is
 turned off, fixing our random data loss problems.   I believe this breaks
 the simplistic DT model of how things are turned on/off...

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Ticket URL: <http://dev.laptop.org/ticket/12216#comment:8>
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