Re: char-major-6

[Dave Sherohman <esper@sherohman.org>]

On Thu, Sep 12, 2002 at 08:13:30PM -0400, Daniel Barclay wrote:
> > $ ls -l /dev | grep " 6,"
> > crw-rw----    1 root     lp         6,   0 Nov 30  2000 lp0
> ...
> 
> 
> Where is it defined in the kernel that character major device number 6 is 
> parallel port support?  

It isn't.

> - Is that definition only within the parallel-port support module?  
> - Is that definition known only once the module is loaded?

Sort of.  The parallel port support code (whether built into the
kernel or loaded as an external module) registers itself as being
capable of handling requests for char-major-6.  When something is
sent to /dev/lp0, the kernel checks to see if there's any code in
memory that can handle char-major-6 devices.  If there is, the data
gets passed to that code.  If there isn't, then it looks to see if
there's a not-yet-loaded module to provide that capability.  At no
time does the kernel know that char-major-6 = parallel port.  (If it
did, adding support for new devices would require changing the core
kernel.  This way, you can just register a handler and go - although
you do have to watch out so nobody else uses the same device mode and
number as you until/unless you can get Linus to officially recognize
your code.)

> - Can the kernel (or the module loader) be informed about the
>   definition (the mapping from the number to the module name)
>   so that that module can be loaded when the device is accessed?
>   If so, how?

Yep.  That's what /etc/modules.conf does.

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