1 files changed, 84 insertions, 2 deletions

diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c
index 5a15e50748de..c21ae20ae362 100644
--- a/drivers/i2c/i2c-dev.c
+++ b/drivers/i2c/i2c-dev.c
@@ -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)
 {
@@ -154,6 +182,29 @@ static ssize_t i2cdev_write (struct file *file, const char __user *buf, size_t c
         return ret;
 }
 
+/* This address checking function differs from the one in i2c-core
+   in that it considers an address with a registered device, but no
+   bounded driver, as NOT busy. */
+static int i2cdev_check_addr(struct i2c_adapter *adapter, unsigned int addr)
+{
+        struct list_head *item;
+        struct i2c_client *client;
+        int res = 0;
+
+        mutex_lock(&adapter->clist_lock);
+        list_for_each(item, &adapter->clients) {
+                client = list_entry(item, struct i2c_client, list);
+                if (client->addr == addr) {
+                        if (client->driver)
+                                res = -EBUSY;
+                        break;
+                }
+        }
+        mutex_unlock(&adapter->clist_lock);
+
+        return res;
+}
+
 static int i2cdev_ioctl(struct inode *inode, struct file *file,
                 unsigned int cmd, unsigned long arg)
 {
@@ -172,11 +223,22 @@ 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;
-                if ((cmd == I2C_SLAVE) && i2c_check_addr(client->adapter,arg))
+                if (cmd == I2C_SLAVE && i2cdev_check_addr(client->adapter, arg))
                         return -EBUSY;
+                /* REVISIT: address could become busy later */
                 client->addr = arg;
                 return 0;
         case I2C_TENBIT:
@@ -386,6 +448,13 @@ 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 +463,6 @@ 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 +490,14 @@ 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 +562,12 @@ 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;