What's the purpose of subsys_interface in linux/device.h?

On Fri, Dec 16, 2016 at 8:57 AM, Perr Zhang [off-list ref] wrote:

 ---- On Fri, 16 Dec 2016 00:26:24 +0800 Alexander Kapshuk [off-list ref] wrote ----
 >
 > Having looked at the pieces of source code below, it appears that
 > subsys_interface is the means to expose specific functionality of a
 > subsystem/class of devices, as the commentary below states.
 > include/linux/device.h:331,350
 > /**
 >  * struct subsys_interface - interfaces to device functions
 >  * @name:       name of the device function
 >  * @subsys:     subsytem of the devices to attach to
 >  * @node:       the list of functions registered at the subsystem
 >  * @add_dev:    device hookup to device function handler
 >  * @remove_dev: device hookup to device function handler
 >  *
 >  * Simple interfaces attached to a subsystem. Multiple interfaces can
 >  * attach to a subsystem and its devices. Unlike drivers, they do not
 >  * exclusively claim or control devices. Interfaces usually represent
 >  * a specific functionality of a subsystem/class of devices.
 >  */
 > struct subsys_interface {
 > const char *name;
 > struct bus_type *subsys;
 > struct list_head node;
 > int (*add_dev)(struct device *dev, struct subsys_interface *sif);
 > void (*remove_dev)(struct device *dev, struct subsys_interface *sif);
 > };
 >
 > Here's sample usage:
 > arch/tile/kernel/sysfs.c:211,216
 > static struct subsys_interface hv_stats_interface = {
 > .name = "hv_stats",
 > .subsys = &cpu_subsys,
 > .add_dev = hv_stats_device_add,
 > .remove_dev = hv_stats_device_remove,
 > };
 >
 > And the add_dev method, whose main purpose seems to be creating a
 > sysfs file for a particular device:
 > arch/tile/kernel/sysfs.c:189,199
 > static int hv_stats_device_add(struct device *dev, struct subsys_interface *sif)
 > {
 > int err, cpu = dev->id;
 >
 > if (!cpu_online(cpu))
 > return 0;
 >
 > err = sysfs_create_file(&dev->kobj, &dev_attr_hv_stats.attr);
 >
 > return err;
 > }
 >
 > Here's an example of the hv_stats_interface being registered with the subsystem:
 > arch/tile/kernel/sysfs.c:261
 > err = subsys_interface_register(&hv_stats_interface);
 >
 > Hopefully, other members of this list will offer a better informed
 > explanation than that given here.

Sorry for sending email repeatedly.
I have also read the code:
device_add() ->  bus_add_device() -> device_add_attrs(bus, dev) -> device_create_file(dev, &bus->dev_attrs[i])
Since the attribute file/functionality could be added through bus->dev_attrs, why not to do it in this way?

My understanding is that an interface is like a protocol that has been
put in place to enable a particular set of functions within a given
subsystem or device model.

I am sending a copy of this email to the list as well, so you can
hopefully get a better informed reply to your query.