On Tue, 5 Aug 1997, Colin Wood wrote:

> > [ 'u' prefix ioctls are for special use ]
> 
> How useful is this?  I mean, what kind of driver has "special" ioctls
> which no one would use?  Why not just have an internal function to do
> whatever is needed?  (Or am I totally missing the point here?)

Ioctl processing within the kernel happens heirachially.  This means that
when a process issues an ioctl() on a device, several subsystems of the
kernel attempt to handle the ioctl().  Only if none of these subsystems
knows how to deal with the ioctl() is it ever handed to the device.  This
is why it is important to pick an ioctl identifier that will not be
interpreted by some other kernel subsystem.

-J