path: root/Documentation/driver-model
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authorGrant Likely <grant.likely@secretlab.ca>2009-03-06 14:05:39 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2009-06-15 21:30:23 -0700
commitb22813b373749d0878e7140e9a6eadf182298709 (patch)
tree835af1d5dc204db9c36514cb21000b32abea27d4 /Documentation/driver-model
parent3959214f971417f4162926ac52ad4cd042958caa (diff)
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>
Diffstat (limited to 'Documentation/driver-model')
1 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/driver-model/device.txt b/Documentation/driver-model/device.txt
index a7cbfff40d07..a124f3126b0d 100644
--- a/Documentation/driver-model/device.txt
+++ b/Documentation/driver-model/device.txt
@@ -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),
+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");