Michael,

The UEFI Driver Model and the Driver Binding Protocol
provide support for this case.  The idea is that a driver
is loaded and started, but when a UEFI Driver is started,
it only registers a Driver Binding Protocol.  Then in the 
BDS phase, the devices required to boot are started using
the UEFI Boot Service ConnectController() and
ConnectController() calls the Driver Binding Protocol(s).

The dependencies between UEFI Drivers are in their Driver 
Binding Protocols which are not used until after all of 
the UEFI Drivers are loaded and started.

Mike

> -----Original Message-----
> From: edk2-devel [mailto:[email protected]] On Behalf Of Michael
> Zimmermann
> Sent: Wednesday, May 17, 2017 9:43 PM
> To: edk2-devel-01 <[email protected]>; Zeng, Star 
> <[email protected]>; Dong,
> Eric <[email protected]>
> Subject: [edk2] UEFI_DRIVER dependencies
> 
> I know that UEFI_DRIVERs don't need or support Depex sections, but
> what if an UEFI_DRIVER depends on a protocol provided by another
> UEFI_DRIVER?
> Since they get loaded automatically because I put them in my
> platform's fdf, it raises the question of the loading order.
> 
> Will they get loaded in the order they're defined? How often will the
> core retry if one of the drivers returns EFI_NOT_READY?
> 
> Thanks,
> Michael
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