USB hotplug

USB hotplug

[Tom]

I'm trying to make sure that I fully understand how the uevents sent
to userspace from the kernel occur when a device is hotplugged.

When looking through the drivers/usb/usb-skeleton.c driver, the init
function calls usb_register(), which eventually leads to
kobject_uevent(.., KOBJ_ADD) being called. However, this is just
called when the driver is loaded, and not when a device is hotplugged,
correct?

There is also a call in drivers/usb/usb-skeleton.c probe() to
usb_register_dev(). My understanding is that probe() gets called when
the USB core thinks it has a device that the driver can handle. Is
this the point at which the uevent needs to be sent to notify
userspace that a device has been hotplugged? (e.g.,
video_register_device ends up triggering a kobject_uevent call when
probe is called. However, I don't see usb_register_dev() resulting in
kobject_uevent being called.)

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Re: USB hotplug

[Greg KH]

On Mon, Nov 05, 2007 at 09:20:04PM -0800, Tom wrote:
> I'm trying to make sure that I fully understand how the uevents sent
> to userspace from the kernel occur when a device is hotplugged.
> 
> When looking through the drivers/usb/usb-skeleton.c driver, the init
> function calls usb_register(), which eventually leads to
> kobject_uevent(.., KOBJ_ADD) being called. However, this is just
> called when the driver is loaded, and not when a device is hotplugged,
> correct?

No, you also get a hotplug call when the device is added to the system.

Run udevmonitor and plug in a device and see all of the events that you
get, and which order they come in.

> There is also a call in drivers/usb/usb-skeleton.c probe() to
> usb_register_dev(). My understanding is that probe() gets called when
> the USB core thinks it has a device that the driver can handle.

Yes.

> Is this the point at which the uevent needs to be sent to notify
> userspace that a device has been hotplugged? (e.g.,
> video_register_device ends up triggering a kobject_uevent call when
> probe is called. However, I don't see usb_register_dev() resulting in
> kobject_uevent being called.)

It happens in device_create().

thanks,

greg k-h

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