usb: gadget: OS String support

There is a custom (non-USB IF) extension to the USB standard:

http://msdn.microsoft.com/library/windows/hardware/gg463182

They grant permission to use the specification - there is
"Microsoft OS Descriptor Specification License Agreement"
under the link mentioned above, and its Section 2 "Grant
of License", letter (b) reads:

"Patent license. Microsoft hereby grants to You a nonexclusive,
royalty-free, nontransferable, worldwide license under Microsoft’s
patents embodied solely within the Specification and that are owned
or licensable by Microsoft to make, use, import, offer to sell,
sell and distribute directly or indirectly to Your Licensees Your
Implementation. You may sublicense this patent license to Your
Licensees under the same terms and conditions."

The said extension is maintained by Microsoft for Microsoft.

Yet it is fairly common for various devices to use it, and a
popular proprietary operating system expects devices to provide
"OS descriptors", so Linux-based USB gadgets whishing to be able
to talk to a variety of operating systems should be able to provide
the "OS descriptors".

This patch adds optional support for gadgets whishing to expose
the so called "OS String" under index 0xEE of language 0.
The contents of the string is generated based on the qw_sign
array and b_vendor_code.

Interested gadgets need to set the cdev->use_os_string flag,
fill cdev->qw_sign with appropriate values and fill cdev->b_vendor_code
with a value of their choice.

This patch does not however implement responding to any vendor-specific
USB requests.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/composite.c

@@ -21,6 +21,22 @@
 #include <linux/usb/composite.h>
 #include <asm/unaligned.h>
 
+/**
+ * struct usb_os_string - represents OS String to be reported by a gadget
+ * @bLength: total length of the entire descritor, always 0x12
+ * @bDescriptorType: USB_DT_STRING
+ * @qwSignature: the OS String proper
+ * @bMS_VendorCode: code used by the host for subsequent requests
+ * @bPad: not used, must be zero
+ */
+struct usb_os_string {
+	__u8	bLength;
+	__u8	bDescriptorType;
+	__u8	qwSignature[OS_STRING_QW_SIGN_LEN];
+	__u8	bMS_VendorCode;
+	__u8	bPad;
+} __packed;
+
 /*
  * The code in this file is utility code, used to build a gadget driver
  * from one or more "function" drivers, one or more "configuration"
@@ -961,6 +977,19 @@ static int get_string(struct usb_composite_dev *cdev,
 		return s->bLength;
 	}
 
+	if (cdev->use_os_string && language == 0 && id == OS_STRING_IDX) {
+		struct usb_os_string *b = buf;
+		b->bLength = sizeof(*b);
+		b->bDescriptorType = USB_DT_STRING;
+		compiletime_assert(
+			sizeof(b->qwSignature) == sizeof(cdev->qw_sign),
+			"qwSignature size must be equal to qw_sign");
+		memcpy(&b->qwSignature, cdev->qw_sign, sizeof(b->qwSignature));
+		b->bMS_VendorCode = cdev->b_vendor_code;
+		b->bPad = 0;
+		return sizeof(*b);
+	}
+
 	list_for_each_entry(uc, &cdev->gstrings, list) {
 		struct usb_gadget_strings **sp;
 

include/linux/usb/composite.h

@@ -335,11 +335,17 @@ static inline struct usb_composite_driver *to_cdriver(
 	return container_of(gdrv, struct usb_composite_driver, gadget_driver);
 }
 
+#define OS_STRING_QW_SIGN_LEN		14
+#define OS_STRING_IDX			0xEE
+
 /**
  * struct usb_composite_device - represents one composite usb gadget
  * @gadget: read-only, abstracts the gadget's usb peripheral controller
  * @req: used for control responses; buffer is pre-allocated
  * @config: the currently active configuration
+ * @qw_sign: qwSignature part of the OS string
+ * @b_vendor_code: bMS_VendorCode part of the OS string
+ * @use_os_string: false by default, interested gadgets set it
  *
  * One of these devices is allocated and initialized before the
  * associated device driver's bind() is called.
@@ -372,6 +378,11 @@ struct usb_composite_dev {
 
 	struct usb_configuration	*config;
 
+	/* OS String is a custom (yet popular) extension to the USB standard. */
+	u8				qw_sign[OS_STRING_QW_SIGN_LEN];
+	u8				b_vendor_code;
+	unsigned int			use_os_string:1;
+
 	/* private: */
 	/* internals */
 	unsigned int			suspended:1;