Date:   Fri, 11 Aug 2023 22:57:52 +0530
From:   Naresh Solanki <naresh.solanki@...ements.com>
To:     Zev Weiss <zev@...ilderbeest.net>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] regulator: userspace-consumer: Add regulator event support

Hi Zev,

On Fri, 4 Aug 2023 at 17:32, Zev Weiss <zev@...ilderbeest.net> wrote:
>
> 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@...ilderbeest.net> 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@...ements.com>
> >> >---
> >> > 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.
I guess many hwmon properties on input are readonly & its possible to
send sysfs_notify on the properties.
Like in
https://github.com/torvalds/linux/blob/master/drivers/hwmon/hwmon.c#L668

>
> Would a uevent-based mechanism maybe be a better fit for the problem
> you're trying to solve?
If the application also needs uevent then that can be added as done in hwmon.
>
> >>
> >> >+      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.
For Documentation/ABI, 'sysfs-driver-regulator-output' below. let me know
if this looks ok.
What:           /sys/bus/platform/drivers/regulator-output/*/events
Date:           August 2023
Contact:        Naresh Solanki <naresh.solanki@...ements.com>
Description:    Provided regulator events.

                Read provides various events the regulator associated with the
                driver has encountered. All REGULATOR_EVENT_* are
                defined in include/linux/regulator/consumer.h

                e.g.
                cat /sys/bus/platform/drivers/regulator-output/ssb_rssd32/events
                0x0
> >>
> >> 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).
Yes they have to be moved to include/uapi in that case.
>
> >>
> >> >
> >> > 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
> >> >

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