can: mcp251xfd: mcp251xfd_get_tef_len(): work around erratum DS80000789E 6.

can: mcp251xfd: mcp251xfd_get_tef_len(): work around erratum DS80000789E 6.

Commit b8e0ddd36ce9 ("can: mcp251xfd: tef: prepare to workaround
broken TEF FIFO tail index erratum") introduced
mcp251xfd_get_tef_len() to get the number of unhandled transmit events
from the Transmit Event FIFO (TEF).

As the TEF has no head index, the driver uses the TX-FIFO's tail index
instead, assuming that send frames are completed.

When calculating the number of unhandled TEF events, that commit
didn't take mcp2518fd erratum DS80000789E 6. into account. According
to that erratum, the FIFOCI bits of a FIFOSTA register, here the
TX-FIFO tail index might be corrupted.

However here it seems the bit indicating that the TX-FIFO is
empty (MCP251XFD_REG_FIFOSTA_TFERFFIF) is not correct while the
TX-FIFO tail index is.

Assume that the TX-FIFO is indeed empty if:
- Chip's head and tail index are equal (len == 0).
- The TX-FIFO is less than half full.
  (The TX-FIFO empty case has already been checked at the
   beginning of this function.)
- No free buffers in the TX ring.

If the TX-FIFO is assumed to be empty, assume that the TEF is full and
return the number of elements in the TX-FIFO (which equals the number
of TEF elements).

If these assumptions are false, the driver might read to many objects
from the TEF. mcp251xfd_handle_tefif_one() checks the sequence numbers
and will refuse to process old events.

Reported-by: Renjaya Raga Zenta <renjaya.zenta@formulatrix.com>
Closes: https://patch.msgid.link/CAJ7t6HgaeQ3a_OtfszezU=zB-FqiZXqrnATJ3UujNoQJJf7GgA@mail.gmail.com
Fixes: b8e0ddd36ce9 ("can: mcp251xfd: tef: prepare to workaround broken TEF FIFO tail index erratum")
Tested-by: Renjaya Raga Zenta <renjaya.zenta@formulatrix.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20241126-mcp251xfd-fix-length-calculation-v2-1-c2ed516ed6ba@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>