page-types: whitespace alignment

[safe/jmp/linux-2.6] / Documentation / driver-model / device.txt

diff --git a/Documentation/driver-model/device.txt b/Documentation/driver-model/device.txt
index 58cc5dc..a124f31 100644 (file)
--- a/Documentation/driver-model/device.txt
+++ b/Documentation/driver-model/device.txt
@@ -76,6 +76,14 @@ driver_data:   Driver-specific data.
 
 platform_data: Platform data specific to the device.
 
+              Example:  for devices on custom boards, as typical of embedded
+              and SOC based hardware, Linux often uses platform_data to point
+              to board-specific structures describing devices and how they
+              are wired.  That can include what ports are available, chip
+              variants, which GPIO pins act in what additional roles, and so
+              on.  This shrinks the "Board Support Packages" (BSPs) and
+              minimizes board-specific #ifdefs in drivers.
+
 current_state: Current power state of the device.
 
 saved_state:   Pointer to saved state of the device. This is usable by
@@ -119,9 +127,11 @@ void unlock_device(struct device * dev);
 Attributes
 ~~~~~~~~~~
 struct device_attribute {
-        struct attribute        attr;
-        ssize_t (*show)(struct device * dev, char * buf, size_t count, loff_t off);
-        ssize_t (*store)(struct device * dev, const char * buf, size_t count, loff_t off);
+       struct attribute        attr;
+       ssize_t (*show)(struct device *dev, struct device_attribute *attr,
+                       char *buf);
+       ssize_t (*store)(struct device *dev, struct device_attribute *attr,
+                        const char *buf, size_t count);
 };
 
 Attributes of devices can be exported via drivers using a simple
@@ -152,3 +162,35 @@ device_remove_file(dev,&dev_attr_power);
 
 The file name will be 'power' with a mode of 0644 (-rw-r--r--).
 
+Word of warning:  While the kernel allows device_create_file() and
+device_remove_file() to be called on a device at any time, userspace has
+strict expectations on when attributes get created.  When a new device is
+registered in the kernel, a uevent is generated to notify userspace (like
+udev) that a new device is available.  If attributes are added after the
+device is registered, then userspace won't get notified and userspace will
+not know about the new attributes.
+
+This is important for device driver that need to publish additional
+attributes for a device at driver probe time.  If the device driver simply
+calls device_create_file() on the device structure passed to it, then
+userspace will never be notified of the new attributes.  Instead, it should
+probably use class_create() and class->dev_attrs to set up a list of
+desired attributes in the modules_init function, and then in the .probe()
+hook, and then use device_create() to create a new device as a child
+of the probed device.  The new device will generate a new uevent and
+properly advertise the new attributes to userspace.
+
+For example, if a driver wanted to add the following attributes:
+struct device_attribute mydriver_attribs[] = {
+       __ATTR(port_count, 0444, port_count_show),
+       __ATTR(serial_number, 0444, serial_number_show),
+       NULL
+};
+
+Then in the module init function is would do:
+       mydriver_class = class_create(THIS_MODULE, "my_attrs");
+       mydriver_class.dev_attr = mydriver_attribs;
+
+And assuming 'dev' is the struct device passed into the probe hook, the driver
+probe function would do something like:
+       create_device(&mydriver_class, dev, chrdev, &private_data, "my_name");