[PATCH net-next v2 3/3] checkpatch: check for comment explaining rgmii(|-rxid|-txid) PHY modes

[Matthias Schiffer <matthias.schiffer@ew.tq-group.com>]

Historically, the RGMII PHY modes specified in Device Trees have been
used inconsistently, often referring to the usage of delays on the PHY
side rather than describing the board; many drivers still implement this
incorrectly.

Require a comment in Devices Trees using these modes (usually mentioning
that the delay is realized on the PCB), so we can avoid adding more
incorrect uses (or will at least notice which drivers still need to be
fixed).

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
---
 Documentation/dev-tools/checkpatch.rst |  9 +++++++++
 scripts/checkpatch.pl                  | 12 ++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/Documentation/dev-tools/checkpatch.rst b/Documentation/dev-tools/checkpatch.rst
index 76bd0ddb00416..d5c47e560324f 100644
--- a/Documentation/dev-tools/checkpatch.rst
+++ b/Documentation/dev-tools/checkpatch.rst
@@ -495,6 +495,15 @@ Comments
 
     See: https://lore.kernel.org/lkml/20131006222342.GT19510@leaf/
 
+  **UNCOMMENTED_RGMII_MODE**
+    Historically, the RGMII PHY modes specified in Device Trees have been
+    used inconsistently, often referring to the usage of delays on the PHY
+    side rather than describing the board.
+
+    PHY modes "rgmii", "rgmii-rxid" and "rgmii-txid" modes require the clock
+    signal to be delayed on the PCB; this unusual configuration should be
+    described in a comment. If they are not (meaning that the delay is realized
+    internally in the MAC or PHY), "rgmii-id" is the correct PHY mode.
 
 Commit message
 --------------
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 664f7b7a622c2..f597734d83cc0 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3741,6 +3741,18 @@ sub process {
 			}
 		}
 
+# Check for RGMII phy-mode with delay on PCB
+		if ($realfile =~ /\.(dts|dtsi|dtso)$/ &&
+		    $line =~ /^\+\s*(phy-mode|phy-connection-type)\s*=\s*"/ &&
+		    !ctx_has_comment($first_line, $linenr)) {
+			my $prop = $1;
+			my $mode = get_quoted_string($line, $rawline);
+			if ($mode =~ /^"rgmii(?:|-rxid|-txid)"$/) {
+				WARN("UNCOMMENTED_RGMII_MODE",
+				     "$prop $mode without comment -- delays on the PCB should be described, otherwise use \"rgmii-id\"\n" . $herecurr);
+			}
+		}
+
 # check for using SPDX license tag at beginning of files
 		if ($realline == $checklicenseline) {
 			if ($rawline =~ /^[ \+]\s*\#\!\s*\//) {
-- 
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