thunderbolt: Add DMA traffic test driver

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>

drivers/thunderbolt/Kconfig

@@ -31,4 +31,17 @@ config USB4_KUNIT_TEST
 	bool "KUnit tests"
 	depends on KUNIT=y
 
+config USB4_DMA_TEST
+	tristate "DMA traffic test driver"
+	depends on DEBUG_FS
+	help
+	  This allows sending and receiving DMA traffic through loopback
+	  connection. Loopback connection can be done by either special
+	  dongle that has TX/RX lines crossed, or by simply connecting a
+	  cable back to the host. Only enable this if you know what you
+	  are doing. Normal users and distro kernels should say N here.
+
+	  To compile this driver a module, choose M here. The module will be
+	  called thunderbolt_dma_test.
+
 endif # USB4

drivers/thunderbolt/Makefile

@@ -7,3 +7,6 @@ thunderbolt-objs += nvm.o retimer.o quirks.o
 thunderbolt-${CONFIG_ACPI} += acpi.o
 thunderbolt-$(CONFIG_DEBUG_FS) += debugfs.o
 thunderbolt-${CONFIG_USB4_KUNIT_TEST} += test.o
+
+thunderbolt_dma_test-${CONFIG_USB4_DMA_TEST} += dma_test.o
+obj-$(CONFIG_USB4_DMA_TEST) += thunderbolt_dma_test.o