Commit 907135aa authored by David Brownell's avatar David Brownell Committed by Jean Delvare
Browse files

i2c-dev: "how does it work" comments 



This adds some "how does this work" comments to the i2c-dev driver,
plus separators between the three main components:

  - The parallel list of i2c_adapters ("i2c_dev_list"), each of which
    gets a "struct i2c_dev" and a /dev/i2c-X character special file.

  - An i2cdev_driver gets adapter add/remove notifications, which are
    used to maintain that list of adapters.

  - Special file operations, which let userspace talk either directly to
    the adapter (for i2c_msg operations) or through cached addressing info
    using an anonymous i2c_client (never registered anywhere).

Plus there's the usual module load/unload record keeping.

After making sense of this code, I think that the anonymous i2c_client
is pretty shady.  But since it's never registered, using this code with
a system set up for "new style" I2C drivers is no more complicated than
always using the I2C_SLAVE_FORCE ioctl (instead of I2C_SLAVE).

Signed-off-by: default avatarDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: default avatarJean Delvare <khali@linux-fr.org>
parent 99fee6d7

drivers/i2c/i2c-dev.c

+59 −1
Original line number Diff line number Diff line
@@ -38,6 +38,15 @@ 


static struct i2c_driver i2cdev_driver;



/*

 * An i2c_dev represents an i2c_adapter ... an I2C or SMBus master, not a

 * slave (i2c_client) with which messages will be exchanged.  It's coupled

 * with a character special file which is accessed by user mode drivers.

 *

 * The list of i2c_dev structures is parallel to the i2c_adapter lists

 * maintained by the driver model, and is updated using notifications

 * delivered to the i2cdev_driver.

 */

struct i2c_dev {

	struct list_head list;

	struct i2c_adapter *adap;
@@ -103,6 +112,25 @@ static ssize_t show_adapter_name(struct device *dev, 
}

static DEVICE_ATTR(name, S_IRUGO, show_adapter_name, NULL);



/* ------------------------------------------------------------------------- */



/*

 * After opening an instance of this character special file, a file

 * descriptor starts out associated only with an i2c_adapter (and bus).

 *

 * Using the I2C_RDWR ioctl(), you can then *immediately* issue i2c_msg

 * traffic to any devices on the bus used by that adapter.  That's because

 * the i2c_msg vectors embed all the addressing information they need, and

 * are submitted directly to an i2c_adapter.  However, SMBus-only adapters

 * don't support that interface.

 *

 * To use read()/write() system calls on that file descriptor, or to use

 * SMBus interfaces (and work with SMBus-only hosts!), you must first issue

 * an I2C_SLAVE (or I2C_SLAVE_FORCE) ioctl.  That configures an anonymous

 * (never registered) i2c_client so it holds the addressing information

 * needed by those system calls and by this SMBus interface.

 */



static ssize_t i2cdev_read (struct file *file, char __user *buf, size_t count,

                            loff_t *offset)

{
@@ -172,6 +200,16 @@ static int i2cdev_ioctl(struct inode *inode, struct file *file, 
	switch ( cmd ) {

	case I2C_SLAVE:

	case I2C_SLAVE_FORCE:

		/* NOTE:  devices set up to work with "new style" drivers

		 * can't use I2C_SLAVE, even when the device node is not

		 * bound to a driver.  Only I2C_SLAVE_FORCE will work.

		 *

		 * Setting the PEC flag here won't affect kernel drivers,

		 * which will be using the i2c_client node registered with

		 * the driver model core.  Likewise, when that client has

		 * the PEC flag already set, the i2c-dev driver won't see

		 * (or use) this setting.

		 */

		if ((arg > 0x3ff) ||

		    (((client->flags & I2C_M_TEN) == 0) && arg > 0x7f))

			return -EINVAL;
@@ -386,6 +424,13 @@ static int i2cdev_open(struct inode *inode, struct file *file) 
	if (!adap)

		return -ENODEV;



	/* This creates an anonymous i2c_client, which may later be

	 * pointed to some address using I2C_SLAVE or I2C_SLAVE_FORCE.

	 *

	 * This client is ** NEVER REGISTERED ** with the driver model

	 * or I2C core code!!  It just holds private copies of addressing

	 * information and maybe a PEC flag.

	 */

	client = kzalloc(sizeof(*client), GFP_KERNEL);

	if (!client) {

		i2c_put_adapter(adap);
@@ -394,7 +439,6 @@ static int i2cdev_open(struct inode *inode, struct file *file) 
	snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);

	client->driver = &i2cdev_driver;



	/* registered with adapter, passed as client to user */

	client->adapter = adap;

	file->private_data = client;
@@ -422,6 +466,14 @@ static const struct file_operations i2cdev_fops = { 
	.release	= i2cdev_release,

};



/* ------------------------------------------------------------------------- */



/*

 * The legacy "i2cdev_driver" is used primarily to get notifications when

 * I2C adapters are added or removed, so that each one gets an i2c_dev

 * and is thus made available to userspace driver code.

 */



static struct class *i2c_dev_class;



static int i2cdev_attach_adapter(struct i2c_adapter *adap)
@@ -486,6 +538,12 @@ static struct i2c_driver i2cdev_driver = { 
	.detach_client	= i2cdev_detach_client,

};



/* ------------------------------------------------------------------------- */



/*

 * module load/unload record keeping

 */



static int __init i2c_dev_init(void)

{

	int res;

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