usb: gadget: configfs: OS Extended Compatibility descriptors support

Add handling of OS Extended Compatibility descriptors from configfs interface.
Hosts which expect the "OS Descriptors" ask only for configurations @ index 0,
but linux-based USB devices can provide more than one configuration.
This patch adds marking one of gadget's configurations the configuration
to be reported at index 0, regardless of the actual sequence of usb_add_config
invocations used for adding the configurations. The configuration is selected
by creating a symbolic link pointing to it from the "os_desc" directory
located at the top of a gadget's directory hierarchy.

One kind of "OS Descriptors" are "Extended Compatibility Descriptors",
which need to be specified per interface. This patch adds interface.<n>
directory in function's configfs directory to represent each interface
defined by the function. Each interface's directory contains two attributes:
"compatible_id" and "sub_compatible_id", which represent 8-byte
strings to be reported to the host as the "Compatible ID" and "Sub Compatible
ID".

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/configfs-usb-gadget b/Documentation/ABI/testing/configfs-usb-gadget
index 0e7b786f24ac..5c0b3e6eb981 100644
--- a/Documentation/ABI/testing/configfs-usb-gadget
+++ b/Documentation/ABI/testing/configfs-usb-gadget
@@ -62,6 +62,19 @@ KernelVersion:	3.11
 Description:
                 This group contains functions available to this USB gadget.
 
+What:           /config/usb-gadget/gadget/functions/<func>.<inst>/interface.<n>
+Date:           May 2014
+KernelVersion:  3.16
+Description:
+                This group contains "Feature Descriptors" specific for one
+                gadget's USB interface or one interface group described
+                by an IAD.
+
+                The attributes:
+
+                compatible_id           - 8-byte string for "Compatible ID"
+                sub_compatible_id       - 8-byte string for "Sub Compatible ID"
+
 What:           /config/usb-gadget/gadget/strings
 Date:           Jun 2013
 KernelVersion:  3.11