tools: ynl: don't skip regeneration from make targets

Commit 2b7ac0c87d ("tools: ynl-gen: don't touch the output file if
content is the same") is working too well. It was added so that
ynl-regen -f doesn't make us rebuild half of the kernel, if there
are no actual changes in any generated code.

When ynl-gen-c is called by make, however, we're better off trusting
make's tracking and overwrite the file. Otherwise if output is identical
we won't update file timestamps and make will retry code gen on every
invocation.

Link: https://lore.kernel.org/r/20231129193622.2912353-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -1164,8 +1164,9 @@ class RenderInfo:
 
 
 class CodeWriter:
-    def __init__(self, nlib, out_file=None):
+    def __init__(self, nlib, out_file=None, overwrite=True):
         self.nlib = nlib
+        self._overwrite = overwrite
 
         self._nl = False
         self._block_end = False
@@ -1186,8 +1187,9 @@ class CodeWriter:
             return
         # Avoid modifying the file if contents didn't change
         self._out.flush()
-        if os.path.isfile(self._out_file) and filecmp.cmp(self._out.name, self._out_file, shallow=False):
-            return
+        if not self._overwrite and os.path.isfile(self._out_file):
+            if filecmp.cmp(self._out.name, self._out_file, shallow=False):
+                return
         with open(self._out_file, 'w+') as out_file:
             self._out.seek(0)
             shutil.copyfileobj(self._out, out_file)
@@ -2516,6 +2518,8 @@ def main():
     parser.add_argument('--header', dest='header', action='store_true', default=None)
     parser.add_argument('--source', dest='header', action='store_false')
     parser.add_argument('--user-header', nargs='+', default=[])
+    parser.add_argument('--cmp-out', action='store_true', default=None,
+                        help='Do not overwrite the output file if the new output is identical to the old')
     parser.add_argument('--exclude-op', action='append', default=[])
     parser.add_argument('-o', dest='out_file', type=str, default=None)
     args = parser.parse_args()
@@ -2543,7 +2547,7 @@ def main():
         print(f'Message enum-model {parsed.msg_id_model} not supported for {args.mode} generation')
         os.sys.exit(1)
 
-    cw = CodeWriter(BaseNlLib(), args.out_file)
+    cw = CodeWriter(BaseNlLib(), args.out_file, overwrite=(not args.cmp_out))
 
     _, spec_kernel = find_kernel_root(args.spec)
     if args.mode == 'uapi' or args.header:

tools/net/ynl/ynl-regen.sh

@@ -30,8 +30,8 @@ for f in $files; do
     fi
 
     echo -e "\tGEN ${params[2]}\t$f"
-    $TOOL --mode ${params[2]} --${params[3]} --spec $KDIR/${params[0]} \
-	  $args -o $f
+    $TOOL --cmp-out --mode ${params[2]} --${params[3]} \
+	  --spec $KDIR/${params[0]} $args -o $f
 done
 
 popd >>/dev/null