Re: Finding the slot in the ioconf table a module attaches to?

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

On Wed, Feb 01, 2023 at 11:14:42 -0800, Brian Buhrow wrote:

> 	hello.  Okay.  That is helpful.  Passing -1 in as the cmajor
> number to the devsw_attach() function does, in fact, assign a
> reasonable major number which seems to work.  I use the
> cdevsw_lookup_major() function to retrieve the assigned number and
> print it for the user.

devsw_attach updates &cmajor with the assigned number if you passed
NODEVMAJOR (-1) in it, so you don't even need to look it up
separately.  We also have in-kernel convenience "MAKEDEV".

E.g., paraphrasing a bit, vbox guest additions module does:

    bmajor = cmajor = NODEVMAJOR;
    error = devsw_attach("vboxguest", NULL, &bmajor,
                         &vboxguest_cdevsw, &cmajor);
    if (error)
        ...

    error = do_sys_mknod(curlwp, "/dev/vboxguest",
                         S_IFCHR | 0666, makedev(cmajor, 0),
                         &retval, UIO_SYSSPACE);
    if (error == EEXIST) {
        error = 0;
    	/*
    	 * Since NetBSD doesn't yet have a major reserved for
    	 * vboxguest, the (first free) major we get will
    	 * change when new devices are added, so an existing
    	 * /dev/vboxguest may now point to some other device,
    	 * creating confusion (tripped me up a few times).
    	 */
    	aprint_normal("vboxguest: major %d:"
    	              " check existing /dev/vboxguest\n", cmajor);
    }

(The comment is no longer true, as we do have a reserved major for vbox now).

-uwe