From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755059AbZCFVGK (ORCPT ); Fri, 6 Mar 2009 16:06:10 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753248AbZCFVF4 (ORCPT ); Fri, 6 Mar 2009 16:05:56 -0500 Received: from ag-out-0708.google.com ([72.14.246.251]:54865 "EHLO ag-out-0708.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753007AbZCFVFz (ORCPT ); Fri, 6 Mar 2009 16:05:55 -0500 From: Grant Likely Subject: [RFC] Warn driver authors about adding device attributes To: linux-kernel@vger.kernel.org, gregkh@suse.de, kay.sievers@vrfy.org Date: Fri, 06 Mar 2009 14:05:39 -0700 Message-ID: <20090306210258.24123.50321.stgit@localhost.localdomain> User-Agent: StGIT/0.14.2 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- Kay, Greg; Is this an accurate description of how device drivers should approach adding additional device attributes? Thanks, g. Documentation/driver-model/device.txt | 32 ++++++++++++++++++++++++++++++++ 1 files changed, 32 insertions(+), 0 deletions(-) diff --git a/Documentation/driver-model/device.txt b/Documentation/driver-model/device.txt index a05ec50..d181d15 100644 --- a/Documentation/driver-model/device.txt +++ b/Documentation/driver-model/device.txt @@ -160,3 +160,35 @@ 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");