#9457 BLOC 1.5-F11: XO-1.5 OS30 boot scripts aren't quite right

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#9457: XO-1.5 OS30 boot scripts aren't quite right
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           Reporter:  wad        |       Owner:  dsd          
               Type:  defect     |      Status:  new          
           Priority:  blocker    |   Milestone:  1.5-F11      
          Component:  initramfs  |     Version:  not specified
         Resolution:             |    Keywords:               
        Next_action:  design     |    Verified:  0            
Deployment_affected:             |   Blockedby:  9558         
           Blocking:             |  
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Comment(by dsd):

 OK, not quite as easy as I hoped. The problem is trying to distinguish
 between internal and external cards from the initramfs.

 If there is no internal card and you boot from an external one, then that
 external one becomes mmcblk0.

 udev can't tell the difference between the 2, so it's not like we can
 write a udev rule to rename it to mmcblk0. (or at least I can't think of
 how)

 I see 2 possible paths forward.

 1. Modify sdhci driver to expose port information in sysfs. I figure that
 sdhci itself must realise that there's nothing in the "first" slot but
 there is one in the 2nd, so if it could export a "slot" attribute then we
 could pick that up in a udev rule.

 2. Use OFW's chosen information as a hint but use guessing as a fallback.
 For example, if OFW says the bootpath is /pci/sd at c/disk at 2 (external SD)
 then we try to find the root at /dev/mmcblk1p2 (this is the name it would
 have if both SD cards are present). but if that device can't be found,
 then fall back on /dev/mmcblk0p2
 (this approach could have repercussions, given that there's no definitive
 way to know when sdhci has finished enumerating all the buses. so that
 check could run too early, before the cards and partitions have been
 detected)

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Ticket URL: <http://dev.laptop.org/ticket/9457#comment:16>
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