net: dsa: sja1105: send multiple spi_messages instead of using cs_change

net: dsa: sja1105: send multiple spi_messages instead of using cs_change

The sja1105 driver has been described by Mark Brown as "not using the
[ SPI ] API at all idiomatically" due to the use of cs_change:
https://patchwork.kernel.org/project/netdevbpf/patch/20210520135031.2969183-1-olteanv@gmail.com/

According to include/linux/spi/spi.h, the chip select is supposed to be
asserted for the entire length of a SPI message, as long as cs_change is
false for all member transfers. The cs_change flag changes the following:

(i) When a non-final SPI transfer has cs_change = true, the chip select
    should temporarily deassert and then reassert starting with the next
    transfer.
(ii) When a final SPI transfer has cs_change = true, the chip select
     should remain asserted until the following SPI message.

The sja1105 driver only uses cs_change for its first property, to form a
single SPI message whose layout can be seen below:

                                             this is an entire, single spi_message
           _______________________________________________________________________________________________
          /                                                                                               \
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
          | hdr_xfer[0] | chunk_xfer[0] | hdr_xfer[1] | chunk_xfer[1] |     | hdr_xfer[n] | chunk_xfer[n] |
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
cs_change      false          true           false           true                false          false

           ____________________________  _____________________________       _____________________________
CS line __/                            \/                             \ ... /                             \__

The fact of the matter is that spi_max_message_size() has an ambiguous
meaning if any non-final transfer has cs_change = true.

If the SPI master has a limitation in that it cannot keep the chip
select asserted for more than, say, 200 bytes (like the spi-sc18is602),
the normal thing for it to do is to implement .max_transfer_size and
.max_message_size, and limit both to 200: in the "worst case" where
cs_change is always false, then the controller can, indeed, not send
messages larger than 200 bytes.

But the fact that the SPI controller's max_message_size does not
necessarily mean that we cannot send messages larger than that.
Notably, if the SPI master special-cases the transfers with cs_change
and treats every chip select toggling as an entirely new transaction,
then a SPI message can easily exceed that limit. So there is a
temptation to ignore the controller's reported max_message_size when
using cs_change = true in non-final transfers.

But that can lead to false conclusions. As Mark points out, the SPI
controller might have a different kind of limitation with the max
message size, that has nothing at all to do with how long it can keep
the chip select asserted.
For example, that might be the case if the device is able to offload the
chip select changes to the hardware as part of the data stream, and it
packs the entire stream of commands+data (corresponding to a SPI
message) into a single DMA transfer that is itself limited in size.

So the only thing we can do is avoid ambiguity by not using cs_change at
all. Instead of sending a single spi_message, we now send multiple SPI
messages as follows:

                  spi_message 0                 spi_message 1                       spi_message n
           ____________________________   ___________________________        _____________________________
          /                            \ /                           \      /                             \
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
          | hdr_xfer[0] | chunk_xfer[0] | hdr_xfer[1] | chunk_xfer[1] |     | hdr_xfer[n] | chunk_xfer[n] |
          +-------------+---------------+-------------+---------------+ ... +-------------+---------------+
cs_change      false          true           false           true                false          false

           ____________________________  _____________________________       _____________________________
CS line __/                            \/                             \ ... /                             \__

which is clearer because the max_message_size limit is now easier to
enforce. What is transmitted on the wire stays, of course, the same.

Additionally, because we send no more than 2 transfers at a time, we now
avoid dynamic memory allocation too, which might be seen as an
improvement by some.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>