On Fri, Dec 16, 2016 at 8:57 AM, Perr Zhang <strongbox8@zoho.com> wrote:

---- On Fri, 16 Dec 2016 00:26:24 +0800 Alexander Kapshuk <alexander.kapshuk@gmail.com> 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.