Messages in this thread Patch in this message | | | From | Bart Van Assche <> | Date | Tue, 16 Aug 2011 22:00:07 +0200 | Subject | [PATCH] docs/driver-model: Document device.groups |
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Many drivers use device_create_file() where device.groups should be used instead. This patch documents that and also removes the comments about device classes since these should not be used in new code.
Signed-off-by: Bart Van Assche <bvanassche@acm.org> Cc: Greg Kroah-Hartman <gregkh@suse.de> Cc: Randy Dunlap <rdunlap@xenotime.net> --- Documentation/driver-model/device.txt | 78 +++++++++++++++------------------ 1 files changed, 35 insertions(+), 43 deletions(-)
diff --git a/Documentation/driver-model/device.txt b/Documentation/driver-model/device.txt index bdefe72..e064ca5 100644 --- a/Documentation/driver-model/device.txt +++ b/Documentation/driver-model/device.txt @@ -45,63 +45,55 @@ struct device_attribute { const char *buf, size_t count); };
-Attributes of devices can be exported via drivers using a simple -procfs-like interface. +Attributes of devices can be exported by a device driver through sysfs.
Please see Documentation/filesystems/sysfs.txt for more information on how sysfs works.
+As explained in Documentation/kobject.txt, it is crucial that device +attributes are created before the KOBJ_ADD uevent is generated. The only way +to realize that is by defining an attribute group. + Attributes are declared using a macro called DEVICE_ATTR:
#define DEVICE_ATTR(name,mode,show,store)
Example:
-DEVICE_ATTR(power,0644,show_power,store_power); +static DEVICE_ATTR(type, 0444, show_type, NULL); +static DEVICE_ATTR(power, 0644, show_power, store_power);
-This declares a structure of type struct device_attribute named -'dev_attr_power'. This can then be added and removed to the device's -directory using: +This declares two structures of type struct device_attribute with respective +names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be +organized as follows into a group:
-int device_create_file(struct device *device, struct device_attribute * entry); -void device_remove_file(struct device * dev, struct device_attribute * attr); +static struct attribute *dev_attrs[] = { + &dev_attr_type.attr, + &dev_attr_power.attr, + NULL, +};
-Example: +static struct attribute_group dev_attr_group = { + .attrs = dev_attrs, +};
-device_create_file(dev,&dev_attr_power); -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 +static const struct attribute_group *dev_attr_groups[] = { + &dev_attr_group, + NULL, };
-Then in the module init function is would do: - mydriver_class = class_create(THIS_MODULE, "my_attrs"); - mydriver_class.dev_attr = mydriver_attribs; +This array of groups can then be associated with a device by setting the +group pointer in struct device before device_register() is invoked: + + dev->groups = dev_attr_groups; + device_register(dev); + +The device_register() function will use the 'groups' pointer to create the +device attributes and the device_unregister() function will use this pointer +to remove the device attributes.
-And assuming 'dev' is the struct device passed into the probe hook, the driver -probe function would do something like: - device_create(&mydriver_class, dev, chrdev, &private_data, "my_name"); +Note: several documents about writing Linux device drivers recommend to use +the functions device_create_file() and device_remove_file() to manipulate +device attributes. That advice is incorrect unfortunately. It is important to +realize that if attributes are added after a device is registered userspace +won't get notified and userspace will not know about the new attributes. -- 1.7.3.4
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