[PATCH 3/5] Watchdog: add a scope value to the watchdog feature

[Juergen Borleis <jbe@pengutronix.de>]

Sometimes the SoC internal watchdogs are inappropriate to restart the
machine in a reliable manner. This change should help to handle more than
one watchdog unit by adding a scope parameter. The framework always
prefers the watchdog with the widest scope. For example a watchdog
which is able to restart the whole machine (SoC + external devices) gets
precedence over a watchdog which can restart the SoC only.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
---
 Documentation/user/user-manual.rst |  1 +
 Documentation/user/watchdog.rst    | 74 ++++++++++++++++++++++++++++++++++++++
 common/restart.c                   |  6 ++++
 drivers/watchdog/im28wd.c          |  1 +
 drivers/watchdog/imxwd.c           |  1 +
 drivers/watchdog/wd_core.c         | 22 ++++++++++--
 include/watchdog.h                 |  3 ++
 7 files changed, 106 insertions(+), 2 deletions(-)
 create mode 100644 Documentation/user/watchdog.rst

diff --git a/Documentation/user/user-manual.rst b/Documentation/user/user-manual.rst
index 0d6daee..8a32469 100644
--- a/Documentation/user/user-manual.rst
+++ b/Documentation/user/user-manual.rst
@@ -30,6 +30,7 @@ Contents:
    system-setup
    reset-reason
    system-reset
+   watchdog
 
 * :ref:`search`
 * :ref:`genindex`
diff --git a/Documentation/user/watchdog.rst b/Documentation/user/watchdog.rst
new file mode 100644
index 0000000..d8e6e76
--- /dev/null
+++ b/Documentation/user/watchdog.rst
@@ -0,0 +1,74 @@
+.. _watchdog_usage:
+
+Using a watchdog
+----------------
+
+Watchdogs are commonly used to prevent bad software from hanging the whole
+system for ever. Sometimes a simple approach can help to make a system work
+if hanging failures are happen very seldom: just restart the system and do
+everything again in the same way as it was done when the system starts the
+last time.
+
+But using a watchdog should always be the 'last resort' to keep a machine
+working. The focus should still be on finding and fixing the bug ;)
+
+A more complex way to use a watchdog in a real-word example is when the state
+frameworks comes into play. Then the watchdog's task isn't only to keep the
+machine working. It also monitors the whole health of the machine including
+hardware and software. Especially something like a firmware update can go
+wrong: a wrong firmware was programmed (wrong release or for a different machine),
+programming was incomplete due to a user intervention or power fail and so on.
+
+In this case the watchdog does not just restart the system if the software hangs,
+it also provides 'additional' information about the firmware by this restart.
+The barebox's state framework is now able to run some kind of state machine to handle
+firmware updates in a correct manner: trying the new firmware once and if it fails falling
+back to the previous firmware (if available) for example.
+
+Refer the :ref:`reset_reason` how to detect the reason why the bootloader runs.
+This information can be used by the barebox's state framework.
+
+.. _watchdog_restart_pitfalls:
+
+Watchdog Pitfalls
+~~~~~~~~~~~~~~~~~
+
+If a watchdog triggers a machine restart it suffers from the same issues like
+a regular user triggered system machine restart. Refer :ref:`system_reset_pitfalls`
+for further details.
+So keep this in mind when you select an available watchdog on your machine for
+this task. And if you are a hardware designer keep this in mind even more, and
+provide a reliable restart source for the software developers and to keep their
+headache low.
+
+Watchdogs from the developers point of view
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Watchdogs gets registered in barebox with a scope. When you register your own
+watchdog driver, check its hardware scope carefully and use one of the
+definitions from the file ``include/restart.h``.
+
+The list of defined scopes (defined in file ``include/restart.h``):
+
+* ``FEATURE_SCOPE_UNKNOWN``: completely useless watchdog, maybe a last resort..
+* ``FEATURE_SCOPE_CPU``: this watchdog is able to restart the core CPU only.
+  Regarding to the issues in :ref:`system_reset_pitfalls` this kind of watchdog
+  seems more or less useless.
+* ``FEATURE_SCOPE_SOC``: this watchdog is able to restart the whole SoC. Regarding
+  to the issues in :ref:`system_reset_pitfalls` this scope is more reliable, but
+  depends on the machine and its hardware design if is able to bring the machine
+  back into life under every circumstance.
+* ``FEATURE_SCOPE_MACHINE``: it is able to restart the whole machine and does
+  the same like a real ``POR`` does. Best scope and always reliable.
+
+The selected scope is very important because barebox will always use
+the watchdog with the best available scope.
+
+But that is true only for watchdogs used in barebox and as long barebox is
+running.
+
+If an operating system runs later on, it is the task of this OS to use a watchdog
+with a correct scope. Otherwise it suffers from the :ref:`system_reset_pitfalls`
+as well. This is even more important if the state framework is used. That means
+barebox and the operating system must use the same watchdog in order to check
+and change the states correctly.
diff --git a/common/restart.c b/common/restart.c
index cfdc463..507a39a 100644
--- a/common/restart.c
+++ b/common/restart.c
@@ -106,6 +106,12 @@ void restart_remove_handler(void (*func)(struct device_d*), struct device_d *dev
 }
 EXPORT_SYMBOL(restart_remove_handler);
 
+void watchdog_init_global_info(int *scope)
+{
+	globalvar_add_simple_enum("system.wd.scope", scope,
+				scope_names, ARRAY_SIZE(scope_names));
+}
+
 static int reset_feature_init(void)
 {
 	globalvar_add_simple_enum("system.reset", &reset_source, reset_src_names,
diff --git a/drivers/watchdog/im28wd.c b/drivers/watchdog/im28wd.c
index c824a25..918d37a 100644
--- a/drivers/watchdog/im28wd.c
+++ b/drivers/watchdog/im28wd.c
@@ -197,6 +197,7 @@ static int imx28_wd_probe(struct device_d *dev)
 	if (IS_ERR(priv->regs))
 		return PTR_ERR(priv->regs);
 	priv->wd.set_timeout = imx28_watchdog_set_timeout;
+	priv->wd.scope = FEATURE_SCOPE_SOC;
 
 	if (!(readl(priv->regs + MXS_RTC_STAT) & MXS_RTC_STAT_WD_PRESENT)) {
 		rc = -ENODEV;
diff --git a/drivers/watchdog/imxwd.c b/drivers/watchdog/imxwd.c
index 3cbae09..60772b1 100644
--- a/drivers/watchdog/imxwd.c
+++ b/drivers/watchdog/imxwd.c
@@ -185,6 +185,7 @@ static int imx_wd_probe(struct device_d *dev)
 	}
 	priv->ops = ops;
 	priv->wd.set_timeout = imx_watchdog_set_timeout;
+	priv->wd.scope = FEATURE_SCOPE_SOC;
 	priv->dev = dev;
 
 	if (IS_ENABLED(CONFIG_WATCHDOG_IMX)) {
diff --git a/drivers/watchdog/wd_core.c b/drivers/watchdog/wd_core.c
index 3d0cfc6..3116761 100644
--- a/drivers/watchdog/wd_core.c
+++ b/drivers/watchdog/wd_core.c
@@ -13,22 +13,31 @@
  */
 
 #include <common.h>
+#include <init.h>
 #include <command.h>
 #include <errno.h>
 #include <linux/ctype.h>
 #include <watchdog.h>
+#include <globalvar.h>
+#include <restart.h>
 
 /*
  * Note: this simple framework supports one watchdog only.
  */
 static struct watchdog *watchdog;
+static int watchdog_scope;
+
+void watchdog_init_global_info(int*);
 
 int watchdog_register(struct watchdog *wd)
 {
-	if (watchdog != NULL)
-		return -EBUSY;
+	/* ignore a lower or same priority, it isn't a failure */
+	if (wd->scope <= watchdog_scope)
+		return 0;
 
 	watchdog = wd;
+	watchdog_scope = wd->scope;
+
 	return 0;
 }
 EXPORT_SYMBOL(watchdog_register);
@@ -39,6 +48,8 @@ int watchdog_deregister(struct watchdog *wd)
 		return -ENODEV;
 
 	watchdog = NULL;
+	watchdog_scope = FEATURE_SCOPE_UNKNOWN;
+
 	return 0;
 }
 EXPORT_SYMBOL(watchdog_deregister);
@@ -55,3 +66,10 @@ int watchdog_set_timeout(unsigned timeout)
 	return watchdog->set_timeout(watchdog, timeout);
 }
 EXPORT_SYMBOL(watchdog_set_timeout);
+
+static int watchdog_capability_init(void)
+{
+	watchdog_init_global_info(&watchdog_scope);
+	return 0;
+}
+coredevice_initcall(watchdog_capability_init);
diff --git a/include/watchdog.h b/include/watchdog.h
index 7e37b7c..e7047bb 100644
--- a/include/watchdog.h
+++ b/include/watchdog.h
@@ -13,8 +13,11 @@
 #ifndef INCLUDE_WATCHDOG_H
 # define INCLUDE_WATCHDOG_H
 
+#include <restart.h>
+
 struct watchdog {
 	int (*set_timeout)(struct watchdog *, unsigned);
+	enum f_scope scope;
 };
 
 #ifdef CONFIG_WATCHDOG
-- 
2.1.4


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