On Fri, Dec 16, 2016 at 8:57 AM, Perr Zhang <strongbox8@xxxxxxxx> wrote:
> ---- On Fri, 16 Dec 2016 00:26:24 +0800 Alexander Kapshuk <alexander.kapshuk@xxxxxxxxx> 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.
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