From 59bcac0b1a48c7821d335bc52b9098da9e125932 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Mon, 18 Oct 2021 11:21:42 +0200
Subject: [PATCH] watchdog: always prefer /dev/watchdog0 over /dev/watchdog

---
 man/systemd-system.conf.xml | 4 ++--
 src/shared/watchdog.c       | 9 ++++++++-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/man/systemd-system.conf.xml b/man/systemd-system.conf.xml
index 2307a8f5c9..3805a010e2 100644
--- a/man/systemd-system.conf.xml
+++ b/man/systemd-system.conf.xml
@@ -144,7 +144,7 @@
         depending on hardware capabilities).</para>
 
         <para>If <varname>RuntimeWatchdogSec=</varname> is set to a non-zero value, the watchdog hardware
-        (<filename>/dev/watchdog</filename> or the path specified with <varname>WatchdogDevice=</varname> or
+        (<filename>/dev/watchdog0</filename> or the path specified with <varname>WatchdogDevice=</varname> or
         the kernel option <varname>systemd.watchdog-device=</varname>) will be programmed to automatically
         reboot the system if it is not contacted within the specified timeout interval. The system manager
         will ensure to contact it at least once in half the specified timeout interval. This feature requires
@@ -182,7 +182,7 @@
 
         <listitem><para>Configure the hardware watchdog device that the
         runtime and shutdown watchdog timers will open and use. Defaults
-        to <filename>/dev/watchdog</filename>. This setting has no
+        to <filename>/dev/watchdog0</filename>. This setting has no
         effect if a hardware watchdog is not available.</para></listitem>
       </varlistentry>
 
diff --git a/src/shared/watchdog.c b/src/shared/watchdog.c
index 8a551fb5c8..c23a4cbac4 100644
--- a/src/shared/watchdog.c
+++ b/src/shared/watchdog.c
@@ -10,6 +10,7 @@
 #include "errno-util.h"
 #include "fd-util.h"
 #include "log.h"
+#include "path-util.h"
 #include "string-util.h"
 #include "time-util.h"
 #include "watchdog.h"
@@ -127,7 +128,13 @@ static int open_watchdog(void) {
         if (watchdog_fd >= 0)
                 return 0;
 
-        fn = watchdog_device ?: "/dev/watchdog";
+        /* Let's prefer new-style /dev/watchdog0 (i.e. kernel 3.5+) over classic /dev/watchdog. The former
+         * has the benefit that we can easily find the matching directory in sysfs from it, as the relevant
+         * sysfs attributes can only be found via /sys/dev/char/<major>:<minor> if the new-style device
+         * major/minor is used, not the old-style. */
+        fn = !watchdog_device || path_equal(watchdog_device, "/dev/watchdog") ?
+                "/dev/watchdog0" : watchdog_device;
+
         watchdog_fd = open(fn, O_WRONLY|O_CLOEXEC);
         if (watchdog_fd < 0)
                 return log_debug_errno(errno, "Failed to open watchdog device %s, ignoring: %m", fn);
-- 
2.25.1