[PATCH v2 08/10] fdtfactor: Add -S/--strip-local-fixups option

[Douglas Anderson]

By default, `fdtfactor` preserves `/__local_fixups__` on `base.dtb` so
that when `fdt_overlay_apply()` merges a `.dtbo` overlay, the
reconstructed device tree retains full `/__local_fixups__` information
(for instance, if an OS kernel wants to know which property cells
contain phandles at runtime).

When `-L` was passed to `dtc` solely to help `fdtfactor` locate phandle
cells at build time and `/__local_fixups__` is not needed at runtime,
retaining `/__local_fixups__` in `base.dtb` wastes space.

Add `-S` / `--strip-local-fixups` to omit `/__local_fixups__` from
`base.dtb` (and therefore from the merged device trees produced by
`fdt_overlay_apply()`). On a set of 47 Qualcomm SC7180 Trogdor
Chromebook board DTBs, `fdtfactor -S` reduces total uncompressed size
from 7.69 MB to 634 KB (91.8% savings).

Add a test case in `tests/run_tests.sh` verifying that `fdtfactor -S`
followed by `fdtoverlay` reproduces DTBs compiled with `-@` (without
`-L`) via `dtbs_equal_unordered`.

Assisted-by: Gemini:gemini-pro-prerelease
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

Changes in v2:
- Split -S/--strip-local-fixups into a separate opt-in patch (David
  Gibson)

 README.md          |  9 +++++++++
 fdtfactor          | 34 ++++++++++++++++++++++++++--------
 tests/run_tests.sh | 22 ++++++++++++++++++++++
 3 files changed, 57 insertions(+), 8 deletions(-)

diff --git a/README.md b/README.md
index 446135e7c02c..9b2115ebbc67 100644
--- a/README.md
+++ b/README.md
@@ -118,6 +118,15 @@ Options:
 * `-o`, `--outdir`: Output directory for generated `.dtbo` files and
   `--base-dtb` (default: alongside each input `.dtb` for `.dtbo` files,
   current directory for `--base-dtb`).
+* `-S`, `--strip-local-fixups`: Omit `/__local_fixups__` from the generated
+  `base.dtb` and final reconstructed device trees. By default, `fdtfactor`
+  preserves `/__local_fixups__` on `base.dtb` so that when `fdt_overlay_apply()`
+  merges a `.dtbo` overlay, the reconstructed device tree retains full
+  `/__local_fixups__` information (e.g., for an OS kernel that wants to
+  know which property cells contain phandles at runtime). When `-L` was
+  passed to `dtc` solely to help `fdtfactor` locate phandles at build time
+  and `/__local_fixups__` is not needed at runtime, passing `-S` strips it
+  from `base.dtb` to save space.
 * `-s`, `--stats`: Print size savings summary.
 
 ### TODO
diff --git a/fdtfactor b/fdtfactor
index f1a5b35d5ee9..ebe1e3d44382 100755
--- a/fdtfactor
+++ b/fdtfactor
@@ -424,7 +424,8 @@ def _build_base_tree(dtbs):
     return base_root
 
 
-def _serialize_base_dtb(base_root, base_dtb_symbols, rsvmap, boot_cpuid_phys):
+def _serialize_base_dtb(base_root, base_dtb_symbols, rsvmap, boot_cpuid_phys,
+                        strip_local_fixups=False):
     """Materialize phandles, `/__symbols__`, and `/__local_fixups__` and flatten.
 
     Args:
@@ -433,6 +434,8 @@ def _serialize_base_dtb(base_root, base_dtb_symbols, rsvmap, boot_cpuid_phys):
         base_dtb_symbols: Dict mapping common symbol labels to target paths.
         rsvmap: Sequence of `(address, size)` memory reservation tuples.
         boot_cpuid_phys: Physical boot CPU ID for the FDT header.
+        strip_local_fixups: If True, omit `/__local_fixups__` from the
+            generated `base.dtb`.
 
     Returns:
         Serialized `.dtb` `bytes` for the common base DTB.
@@ -462,11 +465,12 @@ def _serialize_base_dtb(base_root, base_dtb_symbols, rsvmap, boot_cpuid_phys):
         for label, target_path in base_dtb_symbols.items():
             sym_node.set_str(label, target_path or '/')
 
-    # Always create `/__local_fixups__` on the base DTB so `fdt_overlay_apply()`
-    # updates it with the overlay's phandle fixups when merging.
-    lf_root = out_root.create_path('/__local_fixups__')
-    for path, prop_name, offs in lf_entries:
-        lf_root.create_path(path).set_u32_list(prop_name, offs)
+    if not strip_local_fixups:
+        # Create `/__local_fixups__` on the base DTB so `fdt_overlay_apply()`
+        # updates it with the overlay's phandle fixups when merging.
+        lf_root = out_root.create_path('/__local_fixups__')
+        for path, prop_name, offs in lf_entries:
+            lf_root.create_path(path).set_u32_list(prop_name, offs)
 
     return out_root.flatten(mem_rsv=rsvmap, boot_cpuid_phys=boot_cpuid_phys)
 
@@ -610,11 +614,13 @@ def _build_board_dtbo(dtb, base_root, base_dtb_symbols,
     return dtbo_root.flatten()
 
 
-def create_base_dtb_and_overlays(dtbs):
+def create_base_dtb_and_overlays(dtbs, strip_local_fixups=False):
     """Compute the base DTB and per-board DTBO blobs from parsed DTBs.
 
     Args:
         dtbs: Sequence of `ParsedDtb` instances (at least two).
+        strip_local_fixups: If True, omit `/__local_fixups__` from the
+            generated `base.dtb`.
 
     Returns:
         Tuple of `(base_dtb_blob, dtbo_blobs)` where `base_dtb_blob` is the
@@ -629,6 +635,7 @@ def create_base_dtb_and_overlays(dtbs):
     base_dtb_blob = _serialize_base_dtb(
         base_root, base_dtb_symbols,
         dtbs[0].mem_rsv, dtbs[0].boot_cpuid_phys,
+        strip_local_fixups=strip_local_fixups,
     )
     dtbo_blobs = [
         _build_board_dtbo(
@@ -691,6 +698,14 @@ def parse_args(argv=None):
             'for --base-dtb)'
         ),
     )
+    parser.add_argument(
+        '-S', '--strip-local-fixups',
+        action='store_true',
+        help=(
+            'Omit /__local_fixups__ from the generated base .dtb and '
+            'reconstructed device trees'
+        ),
+    )
     parser.add_argument(
         '-s', '--stats',
         action='store_true',
@@ -775,7 +790,10 @@ def factor_dtb_files(args):
         seen_paths.add(abs_out)
         dtbo_paths.append(out_path)
 
-    base_dtb_blob, dtbo_blobs = create_base_dtb_and_overlays(dtbs)
+    base_dtb_blob, dtbo_blobs = create_base_dtb_and_overlays(
+        dtbs,
+        strip_local_fixups=args.strip_local_fixups,
+    )
 
     if args.outdir:
         os.makedirs(args.outdir, exist_ok=True)
diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index e012aff2193b..b0c621e5d1c3 100755
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -1184,6 +1184,28 @@ fdtfactor_tests () {
     run_wrap_error_test $PYTHON $FDTFACTOR -o fdtfactor_legacy.test.dir \
 	fdtfactor_out.test.dir/fdtfactor_board1.test.dtbo \
 	fdtfactor_board2.test.dtb
+
+    # Strip local fixups (-S): verify that merged DTBs match DTBs compiled
+    # with -@ (without -L), and that inputs lacking /__local_fixups__ are
+    # rejected.
+    for i in 1 2 3; do
+	run_dtc_test -b 2 -@ -I dts -O dtb -o fdtfactor_board${i}.nolf.test.dtb \
+	    "$SRCDIR/fdtfactor_board${i}.dts"
+    done
+    run_wrap_error_test $PYTHON $FDTFACTOR -o fdtfactor_strip.test.dir \
+	fdtfactor_board1.nolf.test.dtb fdtfactor_board2.test.dtb
+    rm -rf fdtfactor_strip.test.dir
+    run_wrap_test $PYTHON $FDTFACTOR -S -b base_strip.test.dtb \
+	-o fdtfactor_strip.test.dir \
+	fdtfactor_board1.test.dtb fdtfactor_board2.test.dtb \
+	fdtfactor_board3.test.dtb
+    for i in 1 2 3; do
+	run_wrap_test $FDTOVERLAY -i fdtfactor_strip.test.dir/base_strip.test.dtb \
+	    -o fdtfactor_strip.test.dir/merged${i}.test.dtb \
+	    fdtfactor_strip.test.dir/fdtfactor_board${i}.test.dtbo
+	run_test dtbs_equal_unordered fdtfactor_board${i}.nolf.test.dtb \
+	    fdtfactor_strip.test.dir/merged${i}.test.dtb
+    done
 }
 
 while getopts "vt:me" ARG ; do
-- 
2.56.0.313.g2479322e68-goog