#12045 BLOC CL4-fir: Choice of UART for mach-mmp

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#12045: Choice of UART for mach-mmp
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 Reporter:  cjb      |                 Owner:  wmb at firmworks.com
     Type:  defect   |                Status:  new              
 Priority:  blocker  |             Milestone:  CL4-firmware     
Component:  kernel   |               Version:  not specified    
 Keywords:           |           Next_action:  design           
 Verified:  0        |   Deployment_affected:                   
Blockedby:           |              Blocking:                   
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 The kernel code for MMP* is currently inflexible in that it demands the
 console UART be hardcoded for a given kernel image.  But we want to boot
 the same kernel image on XO-1.75 and XO-4, and they use different UARTs.
 (The UART3 used by XO-1.75 is not used on XO-4.)

 Tegra (another DT platform) uses a cookie in RAM to encode which UART
 should be used.  We should have OFW do the same, probably with a "magic
 cookie,uart address" combination.  If we find the cookie then we use that
 address, else we use UART3 to retain compatibility with old 1.75 firmware.

 Until this happens, we can't make a single kernel image that provides
 serial console on XO-1.75 and XO-4, so it's important.  Moreover, if we
 leave UART3 (for XO-1.75) as the default, it crashes XO-4 because the
 UART3 clock is not turned on.

 Assigning first to Mitch to provide the cookie in OFW, then to kernel.

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Ticket URL: <http://dev.laptop.org/ticket/12045>
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