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authorDavid Brownell <david-b@pacbell.net>2007-11-15 19:24:01 +0100
committerJean Delvare <khali@hyperion.delvare>2007-11-15 19:24:01 +0100
commit907135aaa0cc120a347222c8f274ecc5ca0db641 (patch)
tree0572c3fc649030ffee737907228a9bfb6094a63a /drivers/i2c/i2c-dev.c
parent99fee6d7e5748d96884667a4628118f7fc130ea0 (diff)
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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: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Jean Delvare <khali@linux-fr.org>
Diffstat (limited to 'drivers/i2c/i2c-dev.c')
-rw-r--r--drivers/i2c/i2c-dev.c60
1 files changed, 59 insertions, 1 deletions
diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c
index 5a15e50..7360f9c 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)
{
@@ -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;