This driver allows sending DMA traffic over XDomain connection.
Specifically over a loopback connection using either a Thunderbolt/USB4
cable that is connected back to the host router port, or a special
loopback dongle that has RX and TX lines crossed. This can be useful at
manufacturing floor to check whether Thunderbolt/USB4 ports are
functional.

The driver exposes debugfs directory under the XDomain service that can
be used to configure the driver, start the test and check the results.

If a loopback dongle is used the steps to send and receive 1000 packets
can be done like:

  # modprobe thunderbolt_dma_test
  # echo 1000 > /sys/kernel/debug/thunderbolt/<service_id>/dma_test/packets_to_receive
  # echo 1000 > /sys/kernel/debug/thunderbolt/<service_id>/dma_test/packets_to_send
  # echo 1 > /sys/kernel/debug/thunderbolt/<service_id>/dma_test/test
  # cat /sys/kernel/debug/thunderbolt/<service_id>/dma_test/status

When a cable is connected back to host then there are two Thunderbolt
services, one is configured for receiving (does not matter which one):

  # modprobe thunderbolt_dma_test
  # echo 1000 > /sys/kernel/debug/thunderbolt/<service_a>/dma_test/packets_to_receive
  # echo 1 > /sys/kernel/debug/thunderbolt/<service_a>/dma_test/test

The other one for sending:

  # echo 1000 > /sys/kernel/debug/thunderbolt/<service_b>/dma_test/packets_to_send
  # echo 1 > /sys/kernel/debug/thunderbolt/<service_b>/dma_test/test

Results can be read from both services status attributes.
Signed-off-by: Isaac Hazan <isaac.hazan@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>