iio: core: wrap IIO device into an iio_dev_opaque object

There are plenty of bad designs we want to discourage or not have to review
manually usually about accessing private (marked as [INTERN]) fields of
'struct iio_dev'.

Sometimes users copy drivers that are not always the best examples.

A better idea is to hide those fields into the framework.
For 'struct iio_dev' this is a 'struct iio_dev_opaque' which wraps a public
'struct iio_dev' object.

In the next series, some fields will be moved to this new struct, each with
it's own rework.

This rework will not be complete-able for a while, as many fields need some
drivers to be reworked in order to finalize them (e.g. 'indio_dev->mlock').

But some fields can already be moved, and in time, all of them may get
there (in the 'struct iio_dev_opaque' object).

Since a lot of drivers also call 'iio_priv()', in order to preserve
fast-paths (where this matters), the public iio_dev object will have a
'priv' field that will have the pointer to the private information already
computed. The reference returned by this field should be guaranteed to be
cacheline aligned.

The opaque parts will be moved into the 'include/linux/iio/iio-opaque.h'
header. Should the hidden information be required for some debugging or
some special needs, it can be made available via this header.
Otherwise, only the IIO core files should include this file.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -25,6 +25,7 @@
 #include <linux/debugfs.h>
 #include <linux/mutex.h>
 #include <linux/iio/iio.h>
+#include <linux/iio/iio-opaque.h>
 #include "iio_core.h"
 #include "iio_core_trigger.h"
 #include <linux/iio/sysfs.h>
@@ -1473,6 +1474,8 @@ static void iio_device_unregister_sysfs(struct iio_dev *indio_dev)
 static void iio_dev_release(struct device *device)
 {
 	struct iio_dev *indio_dev = dev_to_iio_dev(device);
+	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
+
 	if (indio_dev->modes & INDIO_ALL_TRIGGERED_MODES)
 		iio_device_unregister_trigger_consumer(indio_dev);
 	iio_device_unregister_eventset(indio_dev);
@@ -1481,7 +1484,7 @@ static void iio_dev_release(struct device *device)
 	iio_buffer_put(indio_dev->buffer);
 
 	ida_simple_remove(&iio_ida, indio_dev->id);
-	kfree(indio_dev);
+	kfree(iio_dev_opaque);
 }
 
 struct device_type iio_device_type = {
@@ -1495,10 +1498,11 @@ struct device_type iio_device_type = {
  **/
 struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv)
 {
+	struct iio_dev_opaque *iio_dev_opaque;
 	struct iio_dev *dev;
 	size_t alloc_size;
 
-	alloc_size = sizeof(struct iio_dev);
+	alloc_size = sizeof(struct iio_dev_opaque);
 	if (sizeof_priv) {
 		alloc_size = ALIGN(alloc_size, IIO_ALIGN);
 		alloc_size += sizeof_priv;
@@ -1506,11 +1510,14 @@ struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv)
 	/* ensure 32-byte alignment of whole construct ? */
 	alloc_size += IIO_ALIGN - 1;
 
-	dev = kzalloc(alloc_size, GFP_KERNEL);
-	if (!dev)
+	iio_dev_opaque = kzalloc(alloc_size, GFP_KERNEL);
+	if (!iio_dev_opaque)
 		return NULL;
 
-	dev->dev.parent = parent;
+	dev = &iio_dev_opaque->indio_dev;
+	dev->priv = (char *)iio_dev_opaque +
+		ALIGN(sizeof(struct iio_dev_opaque), IIO_ALIGN);
+
 	dev->dev.groups = dev->groups;
 	dev->dev.type = &iio_device_type;
 	dev->dev.bus = &iio_bus_type;
@@ -1524,7 +1531,7 @@ struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv)
 	if (dev->id < 0) {
 		/* cannot use a dev_err as the name isn't available */
 		pr_err("failed to get device id\n");
-		kfree(dev);
+		kfree(iio_dev_opaque);
 		return NULL;
 	}
 	dev_set_name(&dev->dev, "iio:device%d", dev->id);

include/linux/iio/iio-opaque.h

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _INDUSTRIAL_IO_OPAQUE_H_
+#define _INDUSTRIAL_IO_OPAQUE_H_
+
+/**
+ * struct iio_dev_opaque - industrial I/O device opaque information
+ * @indio_dev:			public industrial I/O device information
+ */
+struct iio_dev_opaque {
+	struct iio_dev			indio_dev;
+};
+
+#define to_iio_dev_opaque(indio_dev)		\
+	container_of(indio_dev, struct iio_dev_opaque, indio_dev)
+
+#endif

include/linux/iio/iio.h

@@ -522,6 +522,8 @@ struct iio_buffer_setup_ops {
  * @flags:		[INTERN] file ops related flags including busy flag.
  * @debugfs_dentry:	[INTERN] device specific debugfs dentry.
  * @cached_reg_addr:	[INTERN] cached register address for debugfs reads.
+ * @priv:		[DRIVER] reference to driver's private information
+ *			**MUST** be accessed **ONLY** via iio_priv() helper
  */
 struct iio_dev {
 	int				id;
@@ -571,6 +573,7 @@ struct iio_dev {
 	char				read_buf[20];
 	unsigned int			read_buf_len;
 #endif
+	void				*priv;
 };
 
 const struct iio_chan_spec
@@ -698,9 +701,10 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
 #define IIO_ALIGN L1_CACHE_BYTES
 struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);
 
+/* The information at the returned address is guaranteed to be cacheline aligned */
 static inline void *iio_priv(const struct iio_dev *indio_dev)
 {
-	return (char *)indio_dev + ALIGN(sizeof(struct iio_dev), IIO_ALIGN);
+	return indio_dev->priv;
 }
 
 static inline struct iio_dev *iio_priv_to_dev(void *priv)