chrdev: allocate dynamic chardevs in all unused holes

This is a duct-tape-and-chewing-gum solution to the problem
with the major numbers running out when allocating major
numbers dynamically.

To avoid collisions in the major space, we supply a bitmap with
"holes" that exist in the lower range of major numbers [0-254]
and pick numbers from there, beginning with the unused char
device 8 and moving up through 26, 40, 60-63, 93-94, 102,
120-127, 159, 213-215, 222-223 and 234-254.

The algorithm will behave like the old dynamic assignment
to begin with: dynamic majors will be assigned starting with
254, 253, ... but when it reaches 234 it will make a jump
and assign 223 and so on.

It will also FAIL if we actually fill up all free major
numbers. This seems to me like the reasonable thing to do
since the other numbers are, after all, reserved.

This also deletes the comment /* temporary */ which must be
one of the biggest lies ever.

This also updates the Documentation/devices.txt document to
reflect that all these numbers are used for dynamic assignment.

Reported-by: Ying Huang <ying.huang@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v4->v5:
- Create a new macro, BITS32() in <linux/bitops.> that allow
  us to make an intuitive bitmask for the existing major
  number. This reuses GENMASK(), clamps the arguments and
  switch them around: to me it is atleast most intuitive to
  have the lower bit before the higher one.
- Use BITS32() and BIT_MASK() to define the bitlist for
  available devices.
- Assign dynamic majors from the top down (254, 253...) like
  the old code was doing. I tried assigning bottom up
  (8, 20,...) but that gave rise to "interesting" phenomena
  on Intels test servers running random QEMU: character
  devices in the lower range would stop probing properly,
  even if they have a major number not colliding with the
  dynamic assignment. At one time IDE tape (major 37) and
  one time ISDN (major 45). No clue as to why.
ChangeLog v3->v4:
- Create the BITS() macro in a separate patch. This was
  more tangled up than I thought, as the nice build servers
  quickly told me.
ChangeLog v2->v3:
- Of course I had a dangling hunk for pr_dbg()->pr_debug()
  in my working tree. Mea culpa.
- Fix the redefinition of BITS() in select.c to use the
  more precise name FDS_BITS() over the generic plural.
ChangeLog v1->v2:
- Follow-up on the previous RFC patch, this uses Torvald's
  suggested bitmap approach to allocate devices instead of
  a list of free numbers.
- As a result of using find_first_zero_bit(), the major
  numbers are assigned from low to high instead from high
  to low. It's a bit scarier but I guess drivers using
  dynamic numbers should be all right with it, I'm more
  worried about userspaces expecting dynamic majors to
  be in the [234,254] range. Input welcome, maybe I'm
  just chicken.
- This still needs to be applied on top of the previous
  fix to start warning about going below major 234. If
  you prefer to just get this patch and get rid of the
  problem then tell me.
4 files changed