[Bug 201611] [patch] Add devfs_get_cdevpriv_from_file(9)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=201611

            Bug ID: 201611
           Summary: [patch] Add devfs_get_cdevpriv_from_file(9)
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Keywords: patch
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: aritger at nvidia.com
          Keywords: patch

Created attachment 158831
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=158831&action=edit
patch to implement devfs_get_cdevpriv_from_file(9)

In the NVIDIA GPU driver stack for FreeBSD, we have a variety of cases
where one user-space driver component calls our kernel-space driver
component to allocate an object (e.g., a block of video memory) and then
needs to share the object with another user-space driver component.

We would like to share these objects between user-space driver components
by passing a file descriptor over a UNIX domain socket.  E.g.,

    Process A:
    - alloc video memory
    - fd = open("/dev/nvidiactl");
    - call kernel driver to associate video memory with fd
    - pass fd to process B
    - close fd

    Process B:
    - receive fd from process A
    - call kernel driver to retrieve video memory assocated with fd
    - close fd

But to do this, we would like to be able to map from a file descriptor to
the per-open file descriptor data (registered with devfs_set_cdevpriv(9)
and normally retrieved using devfs_get_cdevpriv(9)).

If I understand correctly, this can be done with (pseudo-code):

    struct file *fp;
    struct cdev_privdata *p;
    struct driver_per_open *popen;

    fget(curthread, fd, &fp);

    p = fp->f_cdevpriv;
    popen = p->cdpd_data;

    /* use popen... */

    fdrop(fp, curthread);

However, it looks like struct cdev_privdata is not intended to be accessed
by drivers.

Would it be reasonable to extend the devfs_cdevpriv(9) family of
functions with a function that can retrieve the driver specific data,
given a struct file?  That way, drivers could do:

    struct file *fp;
    struct driver_per_open *popen;

    fget(curthread, fd, &fp);

    devfs_get_cdevpriv_from_file(&popen, fp);

    /* use popen... */

    fdrop(fp, curthread);

The attached patch implements devfs_get_cdevpriv_from_file().

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