docs: explain firmware payload discovery requirements

Building.md

@@ -15,10 +15,15 @@ The device profile defines the firmware DSC, defines, preflight commands,
 and output path. Host requirements and input preparation are documented in
 [firmware integration](https://unixnu.com/porting/uefi-firmware).
 
-Provide the board firmware inputs and compiled FDT described by the device
-profile before packaging:
+The firmware embeds `UniXNU`, `UniXNUDxe`, `DarwinBoot` and the shared
+`UniXNURamDiskDxe`, with DiskIoDxe and FAT support. Keep each INF in the DSC
+and FDF. The RAM disk driver publishes the bootloader's initrd so UniXNU can
+read `EFI/UniXNU/unixnu.toml` and list the payload entries.
+
+Provide the selected device's payload inputs and compiled firmware FDT before packaging:
 
 ```sh
+./unixnu build payload --unique-chip-id <hex> --trust-cache <file> --data-ramdisk <seed>
 ./unixnu bootimg --uefi ../uefi-sheng --dtb /path/to/sheng.dtb
 ```
 

Platforms/Xiaomi/shengPkg/PORTING.md

@@ -42,6 +42,11 @@ UniXNU supplies the application package and registrar. DarwinBoot loads the
 XNU kernel and ADT sidecar. UniXNU-sheng selects board firmware and boot-image
 parameters. UniXNU-sm8550-common supplies shared XNU platform operations.
 
+`UniXNURamDiskDxe` comes from UniXNU's shared package. It publishes the
+FDT initrd as Block I/O; DiskIoDxe and FAT expose the payload filesystem.
+`ShengBootMemoryDxe` adds DRAM within the board's mapping limits.
+Both modules are listed in the platform DSC and firmware volume.
+
 Build through the UniXNU resolver using [Building.md](../../../Building.md).
 Keep architecture, device-tree, and packaging documentation in the
 [UniXNU site](https://unixnu.com/porting/uefi-firmware).

README.md

@@ -13,6 +13,15 @@ DarwinBoot loader. UniXNU-sm8550-common supplies shared XNU platform support.
 - [Platform details](Platforms/Xiaomi/shengPkg/PORTING.md)
 - [Platform status](Status.md)
 
+## Payload firmware
+
+Build through `./unixnu build firmware --uefi ../uefi-sheng` from UniXNU.
+The DSC and FDF include `UniXNURamDiskDxe`, DiskIoDxe and FAT support so the
+boot-image payload appears as a filesystem. `UniXNURamDiskDxe` is a shared
+module in UniXNU's `package/`, and uses the Mu-Silicium FDT handoff.
+`ShengBootMemoryDxe` supplies the board's memory expansion policy.
+See [required firmware integration](https://unixnu.com/porting/uefi-firmware#required-integration).
+
 ## Credits
 
 [Microsoft](https://github.com/microsoft/) for their [Project Mu](https://microsoft.github.io/mu/) which Powers this Project. <br>