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SubjectRe: [PATCH v2] of: overlay: user space synchronization
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Hi Alan,

Thanks for all the suggestions!


On 10/16/18 13:04, Alan Tull wrote:
> 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:
>>

I'll add:
# not done

>> done=1
>>
>
> Suggest a few comments, such as:

Yes to adding all of the suggested comments below. They should make
the shell code more readable.


> + # 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)?

Maybe a bad habit on my part, but since a program return value of 0 is
true and non-zero if false, I use 0 for true and 1 for false in shell
scripts. It confuses my C-based brain, but is consistent within the
script.


>>
>> 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

Thanks, I missed removing that in the comment.


>> 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

I'll add (a little verbose, but clarity is key here):
# set done true

>> 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.

I'll add this.


>> +
>> + 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.

I'll make the same changes here.

-Frank


>> + 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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