Date:   Fri, 4 Aug 2023 05:02:02 -0700
From:   Zev Weiss <zev@...ilderbeest.net>
To:     Naresh Solanki <naresh.solanki@...ements.com>
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

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.

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
>> >

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