include: one folder per subsystem, and includes that name it

Sixty-five headers sat side by side in include/, so `#include "irq.h"`
said nothing about where irq.h was or what it belonged to, and errno.h,
stat.h and fcntl.h at the top shadowed the system's for any host tool
built with -I./include. They are in folders now -- arch/ core/ fs/
dev/ gfx/ net/ uapi/ (and ldls/ for the Linux driver layer) -- and
every include names the folder, as Linux's do: "core/proc.h".

Done mechanically and checked the hard way: the kernel built from this
tree disassembles identically to the one built from the tree before
it. The full image boots, and userland is the same set of programs.
Paths in docs and comments follow; docs/guides/layout.md has a table
of what is where, and tools/fix-includes.py rewrites old-style
includes from the folders in the tree, for code written before this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

tools/fix-includes.py

@@ -0,0 +1,93 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Rewrite #include "x.h" to the folder x.h lives in under include/.
+#
+# include/ is split by subsystem -- arch/, core/, fs/, dev/, gfx/, net/,
+# uapi/, ldls/ -- and every include names the folder, as Linux's do. Code
+# written before the split, or by habit, says #include "proc.h"; this turns it
+# into #include "core/proc.h". The folder each header is in is read from the
+# tree, so moving a header and running this again is the whole procedure.
+#
+#   python3 tools/fix-includes.py FILE...       rewrite these files
+#   python3 tools/fix-includes.py --modified    every file git says is changed
+#   python3 tools/fix-includes.py --check FILE  print what would change, write
+#                                               nothing; exit 1 if anything would
+#
+# Only quoted includes are touched. <errno.h> in a host tool is the system's
+# errno.h and stays that way, and a file with a same-named header next to it
+# keeps its own.
+
+import os
+import re
+import subprocess
+import sys
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+INC = os.path.join(ROOT, "include")
+
+where = {}
+for sub in sorted(os.listdir(INC)):
+    d = os.path.join(INC, sub)
+    if os.path.isdir(d):
+        for h in os.listdir(d):
+            if h.endswith(".h"):
+                where[h] = sub
+
+pat = re.compile(r'(#\s*include\s*")((?:\.\./)*)(include/)?([A-Za-z0-9_]+\.h)"')
+
+
+def fix(path, text):
+    def one(m):
+        pre, dots, incdir, h = m.groups()
+        if h not in where:
+            return m.group(0)
+        if incdir:
+            return '%s%sinclude/%s/%s"' % (pre, dots, where[h], h)
+        if dots or os.path.exists(os.path.join(os.path.dirname(path), h)):
+            return m.group(0)
+        return '%s%s/%s"' % (pre, where[h], h)
+    return pat.sub(one, text)
+
+
+def modified():
+    out = subprocess.run(["git", "-C", ROOT, "status", "--porcelain"],
+                         capture_output=True, text=True, check=True).stdout
+    files = []
+    for line in out.splitlines():
+        p = line[3:].split(" -> ")[-1]
+        if p.endswith((".c", ".h", ".S", ".inc")):
+            files.append(os.path.join(ROOT, p))
+    return files
+
+
+def main(argv):
+    check = "--check" in argv
+    args = [a for a in argv if not a.startswith("--")]
+    files = modified() if "--modified" in argv else args
+    if not files:
+        print(__doc__ or "usage: fix-includes.py [--check] [--modified] FILE...")
+        return 2
+    changed = 0
+    for f in files:
+        if not os.path.isfile(f):
+            continue
+        with open(f, encoding="utf-8", errors="surrogateescape") as fh:
+            old = fh.read()
+        new = fix(f, old)
+        if new == old:
+            continue
+        changed += 1
+        if check:
+            for a, b in zip(old.splitlines(), new.splitlines()):
+                if a != b:
+                    print("%s: %s  ->  %s" % (os.path.relpath(f), a.strip(), b.strip()))
+        else:
+            with open(f, "w", encoding="utf-8", errors="surrogateescape") as fh:
+                fh.write(new)
+            print("fixed", os.path.relpath(f))
+    return 1 if check and changed else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1:]))