staging:iio:sysfs-trigger: Use irq_work to properly active trigger

Since iio_trigger_poll() calls generic_handle_irq() it need to be called from
hardirq context. The sysfs trigger is kicked from userspace, so it is obviously
not possible to fulfill this requirement by calling iio_trigger_poll directly.
As a workaround commit 1f785681 ("staging:iio:trigger sysfs userspace trigger
rework.") added iio_trigger_poll_chained() which uses handle_nested_irq instead
of generic_handle_irq. This in itself is a hack and only works by chance.
handle_nested_irq is intended to be called from the threaded interrupt handler
of the parent IRQ. Using handle_nested_irq is also problematic since it will
only call the threaded handler of the IRQ. But quite a few IIO drivers rely on
their hardirq handler being called or undefined behaviour will occur.

This patch uses the irq_work framework to schedule the call to
iio_trigger_poll() from hardirq context, which fixes the issues described above.

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/trigger/Kconfig

···
 config IIO_SYSFS_TRIGGER
 	tristate "SYSFS trigger"
 	depends on SYSFS
+	depends on HAVE_IRQ_WORK
+	select IRQ_WORK
 	help
 	  Provides support for using SYSFS entry as IIO triggers.
 	  If unsure, say N (but it's safe to say "Y").

drivers/staging/iio/trigger/iio-trig-sysfs.c

···
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/list.h>
+#include <linux/irq_work.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/trigger.h>
 
 struct iio_sysfs_trig {
 	struct iio_trigger *trig;
+	struct irq_work work;
 	int id;
 	struct list_head l;
 };
···
 	.release = &iio_trigger_sysfs_release,
 };
 
+static void iio_sysfs_trigger_work(struct irq_work *work)
+{
+	struct iio_sysfs_trig *trig = container_of(work, struct iio_sysfs_trig,
+							work);
+
+	iio_trigger_poll(trig->trig, 0);
+}
+
 static ssize_t iio_sysfs_trigger_poll(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t count)
 {
 	struct iio_trigger *trig = to_iio_trigger(dev);
-	iio_trigger_poll_chained(trig, 0);
+	struct iio_sysfs_trig *sysfs_trig = trig->private_data;
+
+	irq_work_queue(&sysfs_trig->work);
 
 	return count;
 }
···
 	t->trig->dev.groups = iio_sysfs_trigger_attr_groups;
 	t->trig->ops = &iio_sysfs_trigger_ops;
 	t->trig->dev.parent = &iio_sysfs_trig_dev;
+	t->trig->private_data = t;
+
+	init_irq_work(&t->work, iio_sysfs_trigger_work);
 
 	ret = iio_trigger_register(t->trig);
 	if (ret)