native_rpi5_entry.S: Pi 5 native (non-UEFI) boot entry stub (FABRIC-3.md §IV.3 item 1)

rpi5_native_start masks x0 down to the documented 32-bit DTB-pointer range
(the firmware's own entry protocol leaves the upper 32 bits unspecified),
stores it into g_rpi5_dtb_ptr for the still-open DTB->BootInfo constructor
(item 2) to read, then switches sp to a dedicated 2 MiB BSS stack -- this
path has no EDK2 boot stack to inherit, unlike every other entry path in
this codebase.

Intentionally halts (wfe/b loop) afterward rather than tail-calling into
item 2's constructor, which doesn't exist yet -- no stub function pretending
to be more than it is.

Not yet linked at the real 0x80000 load address; that needs its own linker
script/build target, not scoped into this item. Compiles and links into the
existing ARCH=aarch64 QEMU/UEFI acceptance build as dead code (ELF kernel
build's KERNEL_ASM wildcards every *.S in arch/aarch64/; nothing there
branches to it), same as rpi5_dtb.c/rpi5_mailbox.c before it.

Verified 3-arch boot to ok>/zuse)ok>.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

src/starkernel/arch/aarch64/native_rpi5_entry.S

@@ -0,0 +1,76 @@
+/*
+ * native_rpi5_entry.S (aarch64) - Raspberry Pi 5 native (non-UEFI) entry
+ * stub (FABRIC-3.md §IV.3 item 1, 2026-09-04).
+ *
+ * **Never run on real hardware** -- the Pi 5 isn't in hand until
+ * 2026-09-17 (FABRIC-3.md §II). Compile-only-verified.
+ *
+ * Entered directly by Pi 5 firmware at the classic bare-metal load address
+ * 0x80000 (item 6, `config.txt`'s `os_check=0`, is still open -- that's
+ * what actually gets a real image loaded there; this file is just the
+ * code that would run once it is). No UEFI, no ACPI -- none of
+ * `boot/uefi_loader.c`'s PE-loader shape applies on this path. Per
+ * FABRIC-3.md §IV.1's own researched entry protocol:
+ *
+ *   x0 = 32-bit DTB pointer, upper 32 bits of the 64-bit register
+ *        UNSPECIFIED -- must mask before use.
+ *   x1-x3 = reserved / zero, unused here.
+ *
+ * This stub's own job, and nothing more yet: mask and capture that DTB
+ * pointer into `g_rpi5_dtb_ptr` (declared in
+ * `include/starkernel/rpi5_native_entry.h`) where the still-open
+ * DTB->BootInfo constructor (§IV.3 item 2) can find it, then establish a
+ * dedicated stack -- this path has no EDK2 boot stack to inherit, there is
+ * no EDK2 at all here. It intentionally halts afterward rather than
+ * tail-calling into a function that does not exist yet; wiring that call
+ * is item 2's own job, not this one's.
+ *
+ * Build-system note: `Makefile.starkernel`'s `KERNEL_ASM` wildcards every
+ * `*.S` file in this directory into the monolithic kernel ELF build (not
+ * the PE/COFF loader build -- that list is explicit, boot.S/isr.S only,
+ * see the Makefile), so this file *does* compile and link into the
+ * existing ARCH=aarch64 QEMU/UEFI acceptance build. Nothing there ever
+ * branches to `rpi5_native_start` -- it is dead code in that build by
+ * design, exactly like `rpi5_dtb.c`/`rpi5_mailbox.c` before it. A real,
+ * separately-linked Pi 5 native boot image (its own linker script placing
+ * this code at 0x80000) does not exist yet -- that is a build-system task,
+ * not scoped into this item.
+ */
+
+    .section .bss
+    .align 8
+    .global g_rpi5_dtb_ptr
+g_rpi5_dtb_ptr:
+    .space 8                       /* uint64_t -- masked DTB pointer */
+
+    .align 4
+g_rpi5_native_stack:
+    .space 0x200000                 /* 2 MiB, same convention as the
+                                      * amd64/riscv64 kernel_entry.S
+                                      * BSS stacks */
+g_rpi5_native_stack_top:
+
+    .section .text
+    .global rpi5_native_start
+rpi5_native_start:
+    /* Mask x0 to its documented 32-bit DTB-pointer range (§IV.1: the
+     * upper 32 bits of the 64-bit register are unspecified by the
+     * firmware's own entry protocol). */
+    mov     x1, #0xffffffff
+    and     x0, x0, x1
+
+    adrp    x2, g_rpi5_dtb_ptr
+    add     x2, x2, :lo12:g_rpi5_dtb_ptr
+    str     x0, [x2]
+
+    /* Establish this path's own dedicated stack. */
+    adrp    x0, g_rpi5_native_stack_top
+    add     x0, x0, :lo12:g_rpi5_native_stack_top
+    mov     sp, x0
+
+    /* §IV.3 item 2 (DTB->BootInfo constructor) does not exist yet --
+     * halt rather than branch to a function that isn't real. Replace
+     * this loop with a tail call once that item lands. */
+.Lhalt:
+    wfe
+    b       .Lhalt