Re: [PATCH] regulator: userspace-consumer: Add regulator event support

From: Zev Weiss
Date: Fri Aug 04 2023 - 08:02:08 EST


On Fri, Aug 04, 2023 at 01:59:44AM PDT, Naresh Solanki wrote:
Hi Zev,


On Fri, 4 Aug 2023 at 02:15, Zev Weiss <zev@xxxxxxxxxxxxxxxxx> wrote:

On Thu, Aug 03, 2023 at 04:12:25AM PDT, Naresh Solanki wrote:
>Add sysfs attribute to track regulator events received from regulator
>notifier block handler.
>

Hi Naresh,

Could you provide a bit more detail on how this is intended to be used?
Some of the details (more below) seem a bit odd to me...
My application registers a event callback on the 'events' to track regulator
events
Reference:
https://github.com/9elements/pwrseqd/blob/main/src/VoltageRegulatorSysfs.cpp#L258

>Signed-off-by: Naresh Solanki <Naresh.Solanki@xxxxxxxxxxxxx>
>---
> drivers/regulator/userspace-consumer.c | 52 +++++++++++++++++++++++++-
> 1 file changed, 51 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/regulator/userspace-consumer.c b/drivers/regulator/userspace-consumer.c
>index 97f075ed68c9..a0b980022993 100644
>--- a/drivers/regulator/userspace-consumer.c
>+++ b/drivers/regulator/userspace-consumer.c
>@@ -29,6 +29,10 @@ struct userspace_consumer_data {
>
> int num_supplies;
> struct regulator_bulk_data *supplies;
>+
>+ struct kobject *kobj;
>+ struct notifier_block nb;
>+ unsigned long events;
> };
>
> static ssize_t name_show(struct device *dev,
>@@ -89,12 +93,30 @@ static ssize_t state_store(struct device *dev, struct device_attribute *attr,
> return count;
> }
>
>+static DEFINE_MUTEX(events_lock);
>+
>+static ssize_t events_show(struct device *dev,
>+ struct device_attribute *attr, char *buf)
>+{
>+ struct userspace_consumer_data *data = dev_get_drvdata(dev);
>+ unsigned long e;
>+
>+ mutex_lock(&events_lock);
>+ e = data->events;
>+ data->events = 0;

...particularly this bit -- a read operation on a read-only file (and
especially one with 0644 permissions) having side-effects (clearing the
value it accesses) seems on the face of it like fairly surprising
behavior. Is this a pattern that's used elsewhere in any other sysfs
files?
These are regulator events & are valid when it occurs.
Userspace application is intended to consume them as soon as the
event is notified by kernel sysfs_notify.


Sure, but that doesn't really address what I was concerned about -- as written this is a read operation on a read-only file (0444, not 0644 as I mistakenly wrote above) that nevertheless alters the state of an internal kernel data structure. Can you point to any other sysfs attributes that behave like that? I can't think of one offhand, and I'd be reluctant to establish the precedent.

Would a uevent-based mechanism maybe be a better fit for the problem you're trying to solve?


>+ mutex_unlock(&events_lock);
>+
>+ return sprintf(buf, "0x%lx\n", e);
>+}
>+
> static DEVICE_ATTR_RO(name);
> static DEVICE_ATTR_RW(state);
>+static DEVICE_ATTR_RO(events);

New sysfs attributes should be documented in Documentation/ABI, which
this appears to be missing.
Sure I can check.

However, it looks like this would expose the values of all the
REGULATOR_EVENT_* constants as a userspace-visible ABI -- is that
something we really want to do?
Yes.

Given that the REGULATOR_EVENT_* constants are defined in headers under include/linux and not include/uapi, it doesn't seem like they were intended to be used as part of a userspace-visible interface. If they're going to be, I think they should be moved to the uapi directory so that applications can use the proper definitions from the kernel instead of manually replicating it on their own (but I suspect we should probably find a different approach instead).


>
> static struct attribute *attributes[] = {
> &dev_attr_name.attr,
> &dev_attr_state.attr,
>+ &dev_attr_events.attr,
> NULL,
> };
>
>@@ -115,12 +137,28 @@ static const struct attribute_group attr_group = {
> .is_visible = attr_visible,
> };
>
>+static int regulator_userspace_notify(struct notifier_block *nb,
>+ unsigned long event,
>+ void *ignored)
>+{
>+ struct userspace_consumer_data *data =
>+ container_of(nb, struct userspace_consumer_data, nb);
>+
>+ mutex_lock(&events_lock);
>+ data->events |= event;
>+ mutex_unlock(&events_lock);
>+

Using a single global mutex (events_lock) to protect a single member of
a per-device struct looks weird. Unless there's something subtle going
on that I'm not seeing, it seems like the lock should be a member of the
data struct instead of global, and since no blocking operations happen
under it could it just be a spinlock? Or since it's just some simple
updates to a single variable, why not just use an atomic_t and skip the
lock entirely?
Intent is that only one thread at a time is to be allowed to access/modify
the data->events variable to prevent potential data corruption and
race conditions. Sure can change it to spinlock or atomic_t.


>+ sysfs_notify(data->kobj, NULL, dev_attr_events.attr.name);
>+
>+ return NOTIFY_OK;
>+}
>+
> static int regulator_userspace_consumer_probe(struct platform_device *pdev)
> {
> struct regulator_userspace_consumer_data tmpdata;
> struct regulator_userspace_consumer_data *pdata;
> struct userspace_consumer_data *drvdata;
>- int ret;
>+ int i, ret;
>
> pdata = dev_get_platdata(&pdev->dev);
> if (!pdata) {
>@@ -153,6 +191,7 @@ static int regulator_userspace_consumer_probe(struct platform_device *pdev)
> drvdata->num_supplies = pdata->num_supplies;
> drvdata->supplies = pdata->supplies;
> drvdata->no_autoswitch = pdata->no_autoswitch;
>+ drvdata->kobj = &pdev->dev.kobj;
>
> mutex_init(&drvdata->lock);
>
>@@ -186,6 +225,13 @@ static int regulator_userspace_consumer_probe(struct platform_device *pdev)
> }
> drvdata->enabled = !!ret;
>
>+ drvdata->nb.notifier_call = regulator_userspace_notify;
>+ for (i = 0; i < drvdata->num_supplies; i++) {
>+ ret = devm_regulator_register_notifier(drvdata->supplies[i].consumer, &drvdata->nb);
>+ if (ret)
>+ goto err_enable;
>+ }
>+
> return 0;
>
> err_enable:
>@@ -197,6 +243,10 @@ static int regulator_userspace_consumer_probe(struct platform_device *pdev)
> static int regulator_userspace_consumer_remove(struct platform_device *pdev)
> {
> struct userspace_consumer_data *data = platform_get_drvdata(pdev);
>+ int i;
>+
>+ for (i = 0; i < data->num_supplies; i++)
>+ devm_regulator_unregister_notifier(data->supplies[i].consumer, &data->nb);
>
> sysfs_remove_group(&pdev->dev.kobj, &attr_group);
>
>
>base-commit: 4fb53b2377c364e3753d6e293913b57dad68e98b
>--
>2.41.0
>