iio:trigger: Experimental kthread tight loop trigger (thread only)

This patch is in response to that of
Gregor Boirie <gregor.boirie@parrot.com>
who proposed using a tight kthread within a device driver (be it with the
support factored out into a helper library) in order to basically spin as
fast as possible.

It is meant as a talking point rather than a formal proposal of the code
(though we are heading towards that I think).
Also gives people some working code to mess around with.

I proposed that this could be done with a trigger with a few constraints
and this is the proof (be it ugly) of that.

There are some constraints though, some of which we would want to relax
if this were to move forward.

* Will only run the thread part of the registered pollfunc.  This is to
  avoid the overhead of jumping in and out of interrupt context.  Is the
  overhead significant?  Not certain but feels like it should be!

* This limitation precludes any device that 'must' do some work in
  interrupt context.  However, that is true of few if any drivers and
  I suspect that any that do will be restricted to using triggers they
  provide themselves.  Usually we have a top half mainly to grab a
  timestamp as soon after the dataready type signal as possible.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>

drivers/iio/trigger/Kconfig

@@ -24,6 +24,18 @@ config IIO_INTERRUPT_TRIGGER
 	  To compile this driver as a module, choose M here: the
 	  module will be called iio-trig-interrupt.
 
+config IIO_TIGHTLOOP_TRIGGER
+	tristate "A kthread based hammering loop trigger"
+	depends on IIO_SW_TRIGGER
+	help
+	  An experimental trigger, used to allow sensors to be sampled as fast
+	  as possible under the limitations of whatever else is going on.
+	  Uses a tight loop in a kthread.  Will only work with lower half only
+	  trigger consumers.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called iio-trig-loop.
+
 config IIO_SYSFS_TRIGGER
 	tristate "SYSFS trigger"
 	depends on SYSFS

drivers/iio/trigger/Makefile

@@ -7,3 +7,4 @@
 obj-$(CONFIG_IIO_HRTIMER_TRIGGER) += iio-trig-hrtimer.o
 obj-$(CONFIG_IIO_INTERRUPT_TRIGGER) += iio-trig-interrupt.o
 obj-$(CONFIG_IIO_SYSFS_TRIGGER) += iio-trig-sysfs.o
+obj-$(CONFIG_IIO_TIGHTLOOP_TRIGGER) += iio-trig-loop.o

drivers/iio/trigger/iio-trig-loop.c

@@ -0,0 +1,143 @@
+/*
+ * Copyright 2016 Jonathan Cameron <jic23@kernel.org>
+ *
+ * Licensed under the GPL-2.
+ *
+ * Based on a mashup of the hrtimer trigger and continuous sampling proposal of
+ * Gregor Boirie <gregor.boirie@parrot.com>
+ *
+ * Note this is still rather experimental and may eat babies.
+ *
+ * Todo
+ * * Protect against connection of devices that 'need' the top half
+ *   handler.
+ * * Work out how to run top half handlers in this context if it is
+ *   safe to do so (timestamp grabbing for example)
+ *
+ * Tested against a max1363. Used about 33% cpu for the thread and 20%
+ * for generic_buffer piping to /dev/null. Watermark set at 64 on a 128
+ * element kfifo buffer.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+#include <linux/irq_work.h>
+#include <linux/kthread.h>
+#include <linux/freezer.h>
+
+#include <linux/iio/iio.h>
+#include <linux/iio/trigger.h>
+#include <linux/iio/sw_trigger.h>
+
+struct iio_loop_info {
+	struct iio_sw_trigger swt;
+	struct task_struct *task;
+};
+
+static struct config_item_type iio_loop_type = {
+	.ct_owner = THIS_MODULE,
+};
+
+static int iio_loop_thread(void *data)
+{
+	struct iio_trigger *trig = data;
+
+	set_freezable();
+
+	do {
+		iio_trigger_poll_chained(trig);
+	} while (likely(!kthread_freezable_should_stop(NULL)));
+
+	return 0;
+}
+
+static int iio_loop_trigger_set_state(struct iio_trigger *trig, bool state)
+{
+	struct iio_loop_info *loop_trig = iio_trigger_get_drvdata(trig);
+
+	if (state) {
+		loop_trig->task = kthread_run(iio_loop_thread,
+					      trig, trig->name);
+		if (unlikely(IS_ERR(loop_trig->task))) {
+			dev_err(&trig->dev,
+				"failed to create trigger loop thread\n");
+			return PTR_ERR(loop_trig->task);
+		}
+	} else {
+		kthread_stop(loop_trig->task);
+	}
+
+	return 0;
+}
+
+static const struct iio_trigger_ops iio_loop_trigger_ops = {
+	.set_trigger_state = iio_loop_trigger_set_state,
+	.owner = THIS_MODULE,
+};
+
+static struct iio_sw_trigger *iio_trig_loop_probe(const char *name)
+{
+	struct iio_loop_info *trig_info;
+	int ret;
+
+	trig_info = kzalloc(sizeof(*trig_info), GFP_KERNEL);
+	if (!trig_info)
+		return ERR_PTR(-ENOMEM);
+
+	trig_info->swt.trigger = iio_trigger_alloc("%s", name);
+	if (!trig_info->swt.trigger) {
+		ret = -ENOMEM;
+		goto err_free_trig_info;
+	}
+
+	iio_trigger_set_drvdata(trig_info->swt.trigger, trig_info);
+	trig_info->swt.trigger->ops = &iio_loop_trigger_ops;
+
+	ret = iio_trigger_register(trig_info->swt.trigger);
+	if (ret)
+		goto err_free_trigger;
+
+	iio_swt_group_init_type_name(&trig_info->swt, name, &iio_loop_type);
+
+	return &trig_info->swt;
+
+err_free_trigger:
+	iio_trigger_free(trig_info->swt.trigger);
+err_free_trig_info:
+	kfree(trig_info);
+
+	return ERR_PTR(ret);
+}
+
+static int iio_trig_loop_remove(struct iio_sw_trigger *swt)
+{
+	struct iio_loop_info *trig_info;
+
+	trig_info = iio_trigger_get_drvdata(swt->trigger);
+
+	iio_trigger_unregister(swt->trigger);
+	iio_trigger_free(swt->trigger);
+	kfree(trig_info);
+
+	return 0;
+}
+
+static const struct iio_sw_trigger_ops iio_trig_loop_ops = {
+	.probe = iio_trig_loop_probe,
+	.remove = iio_trig_loop_remove,
+};
+
+static struct iio_sw_trigger_type iio_trig_loop = {
+	.name = "loop",
+	.owner = THIS_MODULE,
+	.ops = &iio_trig_loop_ops,
+};
+
+module_iio_sw_trigger_driver(iio_trig_loop);
+
+MODULE_AUTHOR("Jonathan Cameron <jic23@kernel.org>");
+MODULE_DESCRIPTION("Loop based trigger for the iio subsystem");
+MODULE_LICENSE("GPL v2");
+MODULE_ALIAS("platform:iio-trig-loop");