From grant.likely@secretlab.ca Tue Apr 21 15:08:23 2009 From: Grant Likely Date: Fri, 06 Mar 2009 14:05:39 -0700 Subject: Driver Core: Warn driver authors about adding device attributes To: gregkh@suse.de, kay.sievers@vrfy.org Message-ID: <20090306210258.24123.50321.stgit@localhost.localdomain> From: Grant Likely 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 Cc: Kay Sievers Signed-off-by: Greg Kroah-Hartman --- Documentation/driver-model/device.txt | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) --- 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), + 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");