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Driver Core: Warn driver authors about adding device attributes 

Add a blurb to the driver-model documentation about how (not) to add
extra attributes to a struct device at driver probe time.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Grant Likely 2009-03-06 14:05:39 -07:00 committed by Greg Kroah-Hartman
parent 3959214f97
commit b22813b373
1 changed files with 32 additions and 0 deletions
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32
Documentation/driver-model/device.txt
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@ -162,3 +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");