Message-Id: <1539649621-5518-1-git-send-email-frowand.list@gmail.com>
Date:   Mon, 15 Oct 2018 17:27:01 -0700
From:   frowand.list@...il.com
To:     Rob Herring <robh+dt@...nel.org>, pantelis.antoniou@...sulko.com,
        Pantelis Antoniou <panto@...oniou-consulting.com>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        geert@...ux-m68k.org, Alan Tull <atull@...nel.org>
Subject: [PATCH v2] of: overlay: user space synchronization

From: Frank Rowand <frank.rowand@...y.com>

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 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:

	done=1

	while [ $done = 1 ] ; do

		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}  locked, sleeping 2 seconds"
 			sleep 2;
 			pre_version=$(cat /sys/firmware/devicetree/tree_version)
 			version=${pre_version}
		done

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

		post_version=$(cat /sys/firmware/devicetree/tree_version)

		if [ ${post_version} = ${pre_version} ] ; then
			done=0
		fi

	done

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

updates since v1:
  - removed unneeded variable "version" from shell script
  - explicitly state that an odd value of tree_version means the
    devicetree is locked for an overlay update not just in the
    source, but also in Documentation/ABI/testing/sysfs-firmware-ofw
  - document that tree_version is reported as an ascii number, the
    internal representation, and the resultant wrap around behavior

 Documentation/ABI/testing/sysfs-firmware-ofw | 67 +++++++++++++++++++++++++---
 drivers/of/base.c                            | 66 +++++++++++++++++++++++++++
 drivers/of/of_private.h                      |  2 +
 drivers/of/overlay.c                         | 12 +++++
 4 files changed, 140 insertions(+), 7 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-firmware-ofw b/Documentation/ABI/testing/sysfs-firmware-ofw
index edcab3ccfcc0..d2346eb61924 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@...il.com>, devicetree@...r.kernel.org
+Description:
+		Top level Open Firmware, aka devicetree, sysfs directory.
+
+What:		/sys/firmware/devicetree/base
 Date:		November 2013
 Contact:	Grant Likely <grant.likely@....com>, devicetree@...r.kernel.org
 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 +21,65 @@ 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@...il.com>, devicetree@...r.kernel.org
+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.
+
+		The contents of tree_version is the ascii representation of
+		a kernel unsigned 32 bit int variable, thus when incremented
+		from 4294967295 it will wrap 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:
+
+		done=1
+
+		while [ $done = 1 ] ; do
+
+		   pre_version=$(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=$(cat /sys/firmware/devicetree/tree_version)
+		   done
+
+		   # 'critical region'
+		   # access /proc/device-tree/ one or more times
+
+		   post_version=$(cat /sys/firmware/devicetree/tree_version)
+
+		   if [ ${post_version} = ${pre_version} ] ; then
+		      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@...y.com>

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