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SubjectRe: [PATCH v2] of: overlay: user space synchronization
On Mon, Oct 15, 2018 at 7:28 PM <frowand.list@gmail.com> wrote:

Hi Frank,

Thanks for all your work on this!

> From: Frank Rowand <frank.rowand@sony.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
>

Suggest a few comments, such as:

+ # Keep trying until we can make it through the loop without live
tree being changed
+ # by an overlay during the 'critical region'

> while [ $done = 1 ] ; do

Nit: doesn't $done mean 'not done' in this script (assuming True is 1)?

>
> pre_version=$(cat /sys/firmware/devicetree/tree_version)

Suggest:
+ # loop while live tree is locked for overlay changes

> 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}

Unused 'version' variable

> done
>
> # 'critical region'
> # access /proc/device-tree/ one or more times
>
> post_version=$(cat /sys/firmware/devicetree/tree_version)
>

+ # Final check that DT wasn't possibly overlaid during the critical region

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

This threw me off for a moment as I was figuring at first that setting
done = 0 meant that we weren't done, so keep looping.

> fi
>
> done
>
> Signed-off-by: Frank Rowand <frank.rowand@sony.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@gmail.com>, devicetree@vger.kernel.org
> +Description:
> + Top level Open Firmware, aka devicetree, sysfs directory.
> +
> +What: /sys/firmware/devicetree/base
> Date: November 2013
> Contact: Grant Likely <grant.likely@arm.com>, devicetree@vger.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@gmail.com>, devicetree@vger.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.

This explains the intention very clearly and directly.

I think it would help to give more of an idea of its behavior. I
initially had to look at the code to understand what kind of values to
expect and whether the tree_version somehow actually reflected some
version # that I hadn't known about, hidden in DT somewhere. It would
be helpful to have it spelled out what granularity this counter has
and some other things about its behavior, somehow explaining that
* tree_version is a counter that is incremented and never decremented
* tree_version is incremented twice per change
* that the granularity is an overlay, tree_version is not counting
changeset changes or something else
* tree_version is updated (incremented) for both applied and for
removed overlays

Such as:

tree_version is incremented twice for each overlay that is applied or
removed (and never decremented). When the tree is locked for
processing an overlay, tree_version is incremented once and its value
becomes odd. Then when the overlay is done being applied or removed,
the tree is unlocked and tree_version is incremented again, becoming
an even value.

> +
> + 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:
> +

Same comments as in the header about adding some comments and possibly
reconsidering the 'done' variable.

> + 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:
> --

Thanks!
Alan

> Frank Rowand <frank.rowand@sony.com>
>

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