can: length: fix bitstuffing count

can: length: fix bitstuffing count

[ Upstream commit 9fde4c557f78ee2f3626e92b4089ce9d54a2573a ]

The Stuff Bit Count is always coded on 4 bits [1]. Update the Stuff
Bit Count size accordingly.

In addition, the CRC fields of CAN FD Frames contain stuff bits at
fixed positions called fixed stuff bits [2]. The CRC field starts with
a fixed stuff bit and then has another fixed stuff bit after each
fourth bit [2], which allows us to derive this formula:

  FSB count = 1 + round_down(len(CRC field)/4)

The length of the CRC field is [1]:

  len(CRC field) = len(Stuff Bit Count) + len(CRC)
                 = 4 + len(CRC)

with len(CRC) either 17 or 21 bits depending of the payload length.

In conclusion, for CRC17:

  FSB count = 1 + round_down((4 + 17)/4)
            = 6

and for CRC 21:

  FSB count = 1 + round_down((4 + 21)/4)
            = 7

Add a Fixed Stuff bits (FSB) field with above values and update
CANFD_FRAME_OVERHEAD_SFF and CANFD_FRAME_OVERHEAD_EFF accordingly.

[1] ISO 11898-1:2015 section 10.4.2.6 "CRC field":

  The CRC field shall contain the CRC sequence followed by a recessive
  CRC delimiter. For FD Frames, the CRC field shall also contain the
  stuff count.

  Stuff count

  If FD Frames, the stuff count shall be at the beginning of the CRC
  field. It shall consist of the stuff bit count modulo 8 in a 3-bit
  gray code followed by a parity bit [...]

[2] ISO 11898-1:2015 paragraph 10.5 "Frame coding":

  In the CRC field of FD Frames, the stuff bits shall be inserted at
  fixed positions; they are called fixed stuff bits. There shall be a
  fixed stuff bit before the first bit of the stuff count, even if the
  last bits of the preceding field are a sequence of five consecutive
  bits of identical value, there shall be only the fixed stuff bit,
  there shall not be two consecutive stuff bits. A further fixed stuff
  bit shall be inserted after each fourth bit of the CRC field [...]

Fixes: 85d99c3e2a13 ("can: length: can_skb_get_frame_len(): introduce function to get data length of frame in data link layer")
Suggested-by: Thomas Kopp <Thomas.Kopp@microchip.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Thomas Kopp <Thomas.Kopp@microchip.com>
Link: https://lore.kernel.org/all/20230611025728.450837-2-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>