[PATCH v3] of: overlay: user space synchronization

[frowand . list]

From: Frank Rowand <frank.rowand@xxxxxxxx>

When an overlay is applied or removed, the live devicetree visible in
/proc/device-tree/, aka /sys/firmware/devicetree/base/, reflects the
changes.  There is no method for user space to determine whether the
live devicetree was modified by overlay actions.

Provide a sysfs file, /sys/firmware/devicetree/tree_version,  to allow
user space to determine if the live devicetree has remained unchanged
while a series of one or more accesses of /proc/device-tree/ occur.

The use of both (1) dynamic devicetree modifications and (2) overlay
apply and removal are not supported during the same boot cycle.  Thus
non-overlay dynamic modifications are not reflected in the value of
tree_version.

Example shell use:

	# set done false (1)
	done=1

	# keep trying until we can make it through the loop without
	# live tree being changed by an overlay changeset during the
	# 'critical region'
	while [ $done = 1 ] ; do

	   # loop while live tree is locked for overlay changes
	   pre_version=$(cat /sys/firmware/devicetree/tree_version)
	   while [ $(( ${pre_version} & 1 )) != 0 ] ; do
	      # echo is optional, sleep value can be tuned
	      echo "${pre_version}  tree locked, sleeping 2 seconds"
	      sleep 2;
	      pre_version=$(cat /sys/firmware/devicetree/tree_version)
	   done

	   # 'critical region'
	   # access /proc/device-tree/ one or more times

	   # check that overlay did not change DT during critical region
	   post_version=$(cat /sys/firmware/devicetree/tree_version)
	   if [ ${post_version} = ${pre_version} ] ; then
	      # set done true (0)
	      done=0
	   fi

	done

Signed-off-by: Frank Rowand <frank.rowand@xxxxxxxx>
---

Changes since version 2:
  - sysfs-firmware-ofw: add comments to shell script example
  - sysfs-firmware-ofw: add more information about the behavior of
    tree_version and how its value changes
  - correct error in patch comment shell script by copying the shell
    script from the patch body

 Documentation/ABI/testing/sysfs-firmware-ofw | 98 ++++++++++++++++++++++++++--
 drivers/of/base.c                            | 66 +++++++++++++++++++
 drivers/of/of_private.h                      |  2 +
 drivers/of/overlay.c                         | 12 ++++
 4 files changed, 171 insertions(+), 7 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-firmware-ofw b/Documentation/ABI/testing/sysfs-firmware-ofw
index edcab3ccfcc0..20860de3ca4c 100644
--- a/Documentation/ABI/testing/sysfs-firmware-ofw
+++ b/Documentation/ABI/testing/sysfs-firmware-ofw
@@ -1,4 +1,10 @@
-What:		/sys/firmware/devicetree/*
+What:		/sys/firmware/devicetree/
+Date:		November 2013
+Contact:	Frank Rowand <frowand.list@xxxxxxxxx>, devicetree@xxxxxxxxxxxxxxx
+Description:
+		Top level Open Firmware, aka devicetree, sysfs directory.
+
+What:		/sys/firmware/devicetree/base
 Date:		November 2013
 Contact:	Grant Likely <grant.likely@xxxxxxx>, devicetree@xxxxxxxxxxxxxxx
 Description:
@@ -6,11 +12,6 @@ Description:
 		hardware, the device tree structure will be exposed in this
 		directory.
 
-		It is possible for multiple device-tree directories to exist.
-		Some device drivers use a separate detached device tree which
-		have no attachment to the system tree and will appear in a
-		different subdirectory under /sys/firmware/devicetree.
-
 		Userspace must not use the /sys/firmware/devicetree/base
 		path directly, but instead should follow /proc/device-tree
 		symlink. It is possible that the absolute path will change
@@ -20,13 +21,96 @@ Description:
 		filesystem support, and has largely the same semantics and
 		should be compatible with existing userspace.
 
-		The contents of /sys/firmware/devicetree/ is a
+		The /sys/firmware/devicetree/base directory is the root
+		node of the devicetree.
+
+		The contents of /sys/firmware/devicetree/base is a
 		hierarchy of directories, one per device tree node. The
 		directory name is the resolved path component name (node
 		name plus address). Properties are represented as files
 		in the directory. The contents of each file is the exact
 		binary data from the device tree.
 
+What:		/sys/firmware/devicetree/tree_version
+Date:		October 2018
+KernelVersion:	4.20
+Contact:	Frank Rowand <frowand.list@xxxxxxxxx>, devicetree@xxxxxxxxxxxxxxx
+Description:
+		When an overlay is applied or removed, the live devicetree
+		visible in /proc/device-tree/, aka
+		/sys/firmware/devicetree/base/, reflects the changes.
+
+		tree_version provides a way for user space to determine if the
+		live devicetree has remained unchanged while a series of one
+		or more accesses of /proc/device-tree/ occur.  If tree_version
+		is odd then the devicetree is locked for an overlay update.
+
+		Details about the value of tree_version:
+
+		   - tree_version is never decremented
+
+		   - tree_version is incremented twice for each overlay
+		     changeset that is applied or removed
+
+		   - When the tree is locked to apply or remove an overlay
+		     changeset, tree_version is incremented once and its
+		     value becomes odd
+
+		   - When the overlay changeset apply or remove is completed
+		     (or an error aborts the overlay changeset apply or
+		     remove), the tree is unlocked and tree_version is
+		     incremented a second time, and its value becomes even
+
+		   - the granularity of tree_version is based on overlay
+		     changesets; it will never be smaller than a changeset,
+		     but may in the future be extended to cover apply and
+		     remove of non-overlay changesets
+
+		   - the value of tree_version is initially 0 at the start
+		     of kernel boot, but may increase during the boot process,
+		     for example due to devicetree unittest activity
+
+		   - tree_version is the ascii representation of a kernel
+		     unsigned 32 bit int variable, thus when incremented
+		     from 4294967295 it wraps around to 0
+
+		The use of both dynamic devicetree modifications and overlay
+		apply and removal are not supported during the same boot
+		cycle.  Thus non-overlay dynamic modifications are not
+		reflected in the value of tree_version.
+
+		Example shell use of tree_version:
+
+		# set done false (1)
+		done=1
+
+		# keep trying until we can make it through the loop without
+		# live tree being changed by an overlay changeset during the
+		# 'critical region'
+		while [ $done = 1 ] ; do
+
+		   # loop while live tree is locked for overlay changes
+		   pre_version=$(cat /sys/firmware/devicetree/tree_version)
+		   while [ $(( ${pre_version} & 1 )) != 0 ] ; do
+		      # echo is optional, sleep value can be tuned
+		      echo "${pre_version}  tree locked, sleeping 2 seconds"
+		      sleep 2;
+		      pre_version=$(cat /sys/firmware/devicetree/tree_version)
+		   done
+
+		   # 'critical region'
+		   # access /proc/device-tree/ one or more times
+
+		   # check that overlay did not change DT during critical region
+		   post_version=$(cat /sys/firmware/devicetree/tree_version)
+		   if [ ${post_version} = ${pre_version} ] ; then
+		      # set done true (0)
+		      done=0
+		   fi
+
+		done
+
+
 What:		/sys/firmware/fdt
 Date:		February 2015
 KernelVersion:	3.19
diff --git a/drivers/of/base.c b/drivers/of/base.c
index 466e3c8582f0..ec0428e47577 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -151,9 +151,71 @@ int of_free_phandle_cache(void)
 late_initcall_sync(of_free_phandle_cache);
 #endif
 
+#define show_one(object)						\
+	static ssize_t show_##object					\
+	(struct kobject *kobj, struct attribute *attr, char *buf)	\
+	{								\
+		return sprintf(buf, "%u\n", object);			\
+	}
+
+struct global_attr {
+	struct attribute attr;
+	ssize_t (*show)(struct kobject *kobj,
+			struct attribute *attr, char *buf);
+	ssize_t (*store)(struct kobject *a, struct attribute *b,
+			 const char *c, size_t count);
+};
+
+#define define_one_global_ro(_name)		\
+static struct global_attr attr_##_name =	\
+__ATTR(_name, 0444, show_##_name, NULL)
+
+/*
+ * tree_version must be unsigned so that at maximum value it wraps
+ * from an odd value (4294967295) to an even value (0).
+ */
+static u32 tree_version;
+
+show_one(tree_version);
+
+define_one_global_ro(tree_version);
+
+static struct attribute *of_attributes[] = {
+	&attr_tree_version.attr,
+	NULL
+};
+
+static const struct attribute_group of_attr_group = {
+	.attrs = of_attributes,
+};
+
+/*
+ * internal documentation
+ * tree_version_increment() - increment base version
+ *
+ * Before starting overlay apply or overlay remove, call this function.
+ * After completing overlay apply or overlay remove, call this function.
+ *
+ * caller must hold of_mutex
+ *
+ * Userspace can use the value of this variable to determine whether the
+ * devicetree has changed while accessing the devicetree.  An odd value
+ * means a modification is underway.  An even value means no modification
+ * is underway.
+ *
+ * The use of both (1) dynamic devicetree modifications and (2) overlay apply
+ * and removal are not supported during the same boot cycle.  Thus non-overlay
+ * dynamic modifications are not reflected in the value of tree_version.
+ */
+void tree_version_increment(void)
+{
+	tree_version++;
+}
+
 void __init of_core_init(void)
 {
 	struct device_node *np;
+	int ret;
 
 	of_populate_phandle_cache();
 
@@ -172,6 +234,10 @@ void __init of_core_init(void)
 	/* Symlink in /proc as required by userspace ABI */
 	if (of_root)
 		proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
+
+	ret = sysfs_create_group(&of_kset->kobj, &of_attr_group);
+	if (ret)
+		pr_err("sysfs_create_group of_attr_group failed: %d\n", ret);
 }
 
 static struct property *__of_find_property(const struct device_node *np,
diff --git a/drivers/of/of_private.h b/drivers/of/of_private.h
index 216175d11d3d..adf89f6bad81 100644
--- a/drivers/of/of_private.h
+++ b/drivers/of/of_private.h
@@ -31,6 +31,8 @@ struct alias_prop {
 extern struct list_head aliases_lookup;
 extern struct kset *of_kset;
 
+void tree_version_increment(void);
+
 #if defined(CONFIG_OF_DYNAMIC)
 extern int of_property_notify(int action, struct device_node *np,
 			      struct property *prop, struct property *old_prop);
diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
index eda57ef12fd0..53b1810b1e03 100644
--- a/drivers/of/overlay.c
+++ b/drivers/of/overlay.c
@@ -770,6 +770,9 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
 	of_overlay_mutex_lock();
 	mutex_lock(&of_mutex);
 
+	/* live tree may change after this point, user space synchronization */
+	tree_version_increment();
+
 	ret = of_resolve_phandles(tree);
 	if (ret)
 		goto err_free_tree;
@@ -832,6 +835,9 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
 	free_overlay_changeset(ovcs);
 
 out_unlock:
+	/* live tree change complete, user space synchronization */
+	tree_version_increment();
+
 	mutex_unlock(&of_mutex);
 	of_overlay_mutex_unlock();
 
@@ -1028,6 +1034,9 @@ int of_overlay_remove(int *ovcs_id)
 
 	mutex_lock(&of_mutex);
 
+	/* live tree may change after this point, user space synchronization */
+	tree_version_increment();
+
 	ovcs = idr_find(&ovcs_idr, *ovcs_id);
 	if (!ovcs) {
 		ret = -ENODEV;
@@ -1083,6 +1092,9 @@ int of_overlay_remove(int *ovcs_id)
 	free_overlay_changeset(ovcs);
 
 out_unlock:
+	/* live tree change complete, user space synchronization */
+	tree_version_increment();
+
 	mutex_unlock(&of_mutex);
 
 out:
-- 
Frank Rowand <frank.rowand@xxxxxxxx>