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i2c-dev: "how does it work" comments

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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: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
This commit is contained in:
David Brownell 2007-11-15 19:24:01 +01:00 committed by Jean Delvare
parent 99fee6d7e5
commit 907135aaa0
1 changed files with 59 additions and 1 deletions
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60
drivers/i2c/i2c-dev.c
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@ -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;