PCI: endpoint: Add EP function driver to provide NTB functionality

Add a new endpoint function driver to provide NTB functionality using
multiple PCIe endpoint instances.

[arnd@arndb.de: Select configfs dependency]
[yebin10@huawei.com: Fix unused but set variables]
[geert+renesas@glider.be: Explain NTB in PCI_EPF_NTB help text]

Link: https://lore.kernel.org/r/20210201195809.7342-14-kishon@ti.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/endpoint/functions/Kconfig

@@ -12,3 +12,16 @@ config PCI_EPF_TEST
 	   for PCI Endpoint.
 
 	   If in doubt, say "N" to disable Endpoint test driver.
+
+config PCI_EPF_NTB
+	tristate "PCI Endpoint NTB driver"
+	depends on PCI_ENDPOINT
+	select CONFIGFS_FS
+	help
+	  Select this configuration option to enable the Non-Transparent
+	  Bridge (NTB) driver for PCI Endpoint. NTB driver implements NTB
+	  controller functionality using multiple PCIe endpoint instances.
+	  It can support NTB endpoint function devices created using
+	  device tree.
+
+	  If in doubt, say "N" to disable Endpoint NTB driver.

drivers/pci/endpoint/functions/Makefile

@@ -4,3 +4,4 @@
 #
 
 obj-$(CONFIG_PCI_EPF_TEST)		+= pci-epf-test.o
+obj-$(CONFIG_PCI_EPF_NTB)		+= pci-epf-ntb.o