From 285a054d6dda7f5eb1a6a8ae4753a5cf0d812cf2 Mon Sep 17 00:00:00 2001
From: lumia-emu <lumia-emu@local>
Date: Sun, 13 Sep 2026 10:25:22 +0000
Subject: [PATCH] b21: logger S8 (pop.w -> bx lr)

---
 tools/sbl_patch.py | 69 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/tools/sbl_patch.py b/tools/sbl_patch.py
index 0778060..218e2f4 100644
--- a/tools/sbl_patch.py
+++ b/tools/sbl_patch.py
@@ -661,6 +661,75 @@ def main():
 
     save(d, 'img0_b20.elf')
 
+    # b21 = b18 + logger S8 (0x8034810 pop.w -> bx lr):
+    # the logger is a naked pop.w {r2-r8,sb,sl,pc} (0x28) with no push;
+    # called via bl from scaffold-frame workers, it eats 0x28 of live
+    # stack per call (slots->fill->zeros->null-loop->IMEM overflow).
+    # S8 turns it into a no-op return (bx lr, 2B; tail bytes unreachable):
+    # return value ignored (callers set r0=0 after), workers unwind via
+    # their own epilogue pops. Logging already empty (UART stubs).
+    d = bytearray(base)
+
+    for va, (a, b) in BLX2BL.items():
+
+        set4(d, va, b)
+
+    for va, exp, new in [
+
+        (0x802F664, 'e4f77efc', 'c046c046'),
+
+        (0x802F672, 'e6f7bbfa', 'c046c046'),
+
+        (0x802F678, 'e9f7fef9', 'c046c046'),
+
+        (0x801B71E, '9847fee7', 'c046c046'),
+
+        (0x8008396, 'fef780f9', 'c046c046'),
+
+        (0x8016184, 'fdf790fc', 'c046c046'),
+
+        (0x801B76C, '280b0508', '00000708'),
+
+        (0x8017AFC, 'bde8fc87', 'eef74fb8'),
+
+        (0x802F690, '9847fee7', 'c046c046'),
+
+        (0x8013BAC, 'f8bd0a4a', 'f3f730b8'),
+
+    ]:
+
+        assert hx(d, va) == exp, (hex(va), hx(d, va))
+
+        set4(d, va, new)
+
+    # Full bl idiom at 0x8006682 (bl + pop, 6B) -> NOPs (see b18).
+    o = off(0x8006682)
+
+    assert d[o:o + 6].hex() == '00f001f807bd', hx(d, 0x8006682, 6)
+
+    d[o:o + 6] = bytes.fromhex('c046c046c046')
+
+    o = off(0x8013AA8)
+
+    assert d[o:o + 4].hex() == '07f026be', hx(d, 0x8013AA8)
+
+    d[o:o + 4] = bytes.fromhex('c046c046')
+
+    o = off(0x8013BA6)
+
+    assert d[o:o + 2].hex() == 'b888', (hex(0x8013BA6), hx(d, 0x8013BA6, 2))
+
+    d[o:o + 2] = bytes.fromhex('ff20')
+
+    # Logger S8: pop.w entry (4B bde8fc87) -> bx lr (first 2B).
+    o = off(0x8034810)
+
+    assert d[o:o + 4].hex() == 'bde8fc87', hx(d, 0x8034810)
+
+    d[o:o + 2] = bytes.fromhex('7047')
+
+    save(d, 'img0_b21.elf')
+