Re: Question about Hot device Identity

["Kevin P. Fleming" <kpfleming@cox.net>]

Guo, Min wrote:

> I can not find the file dev in the /sys/class/usb, does it means that 
> Udev does not support my USB CDROM? If not, which file is behalf of 
> my USB CDROM device?
> 

I think you've missed the point here. There is no such thing as a 
"generic" USB device that could have a dev file for it in 
/sys/class/usb. For a device to have a dev file, it must have some 
sort of device-specific driver talking to it. If that driver exists 
and is loaded, it will (or should) have a class of its own under 
/sys/class, and the dev file will be there.

In the case of a USB CD-ROM, that is handled by the usb-storage 
driver, but that driver _still_ does not create a dev file, because it 
does not directly expose itself to userspace (there is no way to talk 
to the usb-storage driver itself). usb-storage connects the device the 
  SCSI midlayer, which then attaches the sd (SCSI disk) driver to the 
device. _That_ driver is actually a real device that the user can 
interact with, so it creates a dev file in the appropriate place, 
namely /sys/block. You can think of /sys/block as a special-case 
class, and consider it equivalent to /sys/class/block (although it's 
more than that).

The only devices that will ever have "dev" files in /sys for them are 
those that can actually can communicated with from userspace. There is 
no need for userspace to treat your USB CD-ROM as a USB CD-ROM, 
instead it just considers it to be a generic disk (block) device that 
holds read-only media. I don't see any reason this device will ever 
generate any directories under /sys/class/... with dev files in them, 
they're not necessary.

But, this has nothing to do with whether udev can support your device 
or not and create a node in /dev for it. udev should be able to do 
exactly that, but I don't know whether it currently looks only at 
/sys/class (which I think is the case) or whether it also looks at 
/sys/block.



-------------------------------------------------------
This SF.Net email is sponsored by: INetU
Attention Web Developers & Consultants: Become An INetU Hosting Partner.
Refer Dedicated Servers. We Manage Them. You Get 10% Monthly Commission!
INetU Dedicated Managed Hosting http://www.inetu.net/partner/index.php
_______________________________________________
Linux-hotplug-devel mailing list  http://linux-hotplug.sourceforge.net
Linux-hotplug-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel