I managed to get the implementation working for us using the following dirty patch for kernel 3.14,
-----------------------------------------------------------------------------------------------------
diff -u -r a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
--- a/drivers/usb/gadget/f_fs.c 2014-06-11 21:02:49.000000000 +0200
+++ b/drivers/usb/gadget/f_fs.c 2015-10-08 11:34:29.177420065 +0200
@@ -1449,8 +1449,9 @@
break;
case HID_DT_HID:
- pr_vdebug("hid descriptor\n");
- if (length != sizeof(struct hid_descriptor))
+ pr_vdebug("hid or sc device class descriptor\n");
+ if (length != sizeof(struct hid_descriptor) &&
+ length != 0x36)
goto inv_length;
break;
-----------------------------------------------------------------------------------------------------
It turns out that our main problem was passing the smart card device class descriptor to FunctionFS as the code in f_fs.c incorrectly identified our descriptor as a HID descriptor since HID_DT_HID == 0x21 which corresponds to the bDescriptorType value for a smart card device class descriptor. It looks like FunctionFS does not make provision for class descriptors specific to certain interfaces.
I also had to remove "USB wakeup signalling" from dwFeatures in our smart card class descriptor to get the implementation working on Windows.
This might help someone someday :-)
Cheers,
Pieter
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