/*
- * This file holds USB constants and structures that are needed for USB
- * device APIs. These are used by the USB device model, which is defined
- * in chapter 9 of the USB 2.0 specification. Linux has several APIs in C
- * that need these:
+ * This file holds USB constants and structures that are needed for
+ * USB device APIs. These are used by the USB device model, which is
+ * defined in chapter 9 of the USB 2.0 specification and in the
+ * Wireless USB 1.0 (spread around). Linux has several APIs in C that
+ * need these:
*
* - the master/host side Linux-USB kernel driver API;
* - the "usbfs" user space API; and
*
* There's also "Wireless USB", using low power short range radios for
* peripheral interconnection but otherwise building on the USB framework.
+ *
+ * Note all descriptors are declared '__attribute__((packed))' so that:
+ *
+ * [a] they never get padded, either internally (USB spec writers
+ * probably handled that) or externally;
+ *
+ * [b] so that accessing bigger-than-a-bytes fields will never
+ * generate bus errors on any platform, even when the location of
+ * its descriptor inside a bundle isn't "naturally aligned", and
+ *
+ * [c] for consistency, removing all doubt even when it appears to
+ * someone that the two other points are non-issues for that
+ * particular descriptor type.
*/
#ifndef __LINUX_USB_CH9_H
#define USB_DT_WIRE_ADAPTER 0x21
#define USB_DT_RPIPE 0x22
-/* conventional codes for class-specific descriptors */
-#define USB_DT_CS_DEVICE 0x21
-#define USB_DT_CS_CONFIG 0x22
-#define USB_DT_CS_STRING 0x23
-#define USB_DT_CS_INTERFACE 0x24
-#define USB_DT_CS_ENDPOINT 0x25
+/* Conventional codes for class-specific descriptors. The convention is
+ * defined in the USB "Common Class" Spec (3.11). Individual class specs
+ * are authoritative for their usage, not the "common class" writeup.
+ */
+#define USB_DT_CS_DEVICE (USB_TYPE_CLASS | USB_DT_DEVICE)
+#define USB_DT_CS_CONFIG (USB_TYPE_CLASS | USB_DT_CONFIG)
+#define USB_DT_CS_STRING (USB_TYPE_CLASS | USB_DT_STRING)
+#define USB_DT_CS_INTERFACE (USB_TYPE_CLASS | USB_DT_INTERFACE)
+#define USB_DT_CS_ENDPOINT (USB_TYPE_CLASS | USB_DT_ENDPOINT)
/* All standard descriptors have these 2 fields at the beginning */
struct usb_descriptor_header {