Boot from SDCard about lumia950xlpkg HOT 7 CLOSED

It may be possible to add boot from sd card support. But be aware you won't be able to chainload NT directly from a SD card without writing code to fake it as an emmc in the UEFI. SD card controller only shows up when certain drivers are installed in Windows on Qualcomm devices. Windows needs disk access to access drivers. If you don't boot windows from a ramdisk and don't add fake emmc emulation, chances are you'll get a 0x7B bugcheck.

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MicroSD is populated in UEFI and should be usable. The UI for boot device selection (which will be tracked in a separate issue) has not yet been implemented.

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@gus33000 So WinPE is likely possible, then what about booting windows using wimboot / booting from vhd?

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wimboot is the name for a technology working with file diffing against a main install.wim located in the recovery partition. It requires disk access.

VHD boot requires disk access as well.

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From what I understand, a few things would need to change on the UEFI side:

• Implement a runtime DXE to support the SD controller that works with the BDS phase
  and talks EFI_BLOCK_IO_PROTOCOL for the window boot manager.
• Update ACPI code to expose this controller at boot time (if it isn't already?)

My system-level knowledge of Windows is limited, but given that the windows boot manager and windows os loader both use the EFI Block IO protocol while chain loading, wouldn't we only need to modify the SD controller driver in windows to present itself as a SERVICE_BOOT_START driver?

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@b-man there's a quick way to verify the last one, just boot Windows RT (coz it's 32bit and can be loaded by wp uefi) on sdcard with sd controller related drivers set to load on boot.

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Linux is more likely utilizing SD card boot.

@b-man Stock Qualcomm UEFI does expose SD card with the protocol, but it will bugcheck later when ExitBootServices is called - SDBus.sys does not support booting from it (at least right now). BTW, it refused to boot from eMMC HS400 too (I configure eMMC to HS200 here). I have not expose it in this implementation yet.

The controller has a dependency to TLMM GPIO (to indicate SD card presence). It may be possible to remove the detection and report it as always present (the patch for DB410c booting from LK did so). I have not tried it yet.

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