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authorInaky Perez-Gonzalez <inaky@linux.intel.com>2007-02-22 16:37:53 -0800
committerGreg Kroah-Hartman <gregkh@suse.de>2007-02-23 15:03:46 -0800
commit672027a35795ec95f516fdc702ba8900d55a9eef (patch)
tree76d4a091a53edfef3ff1868ede58dbbb90e086fe
parent822c7ef48b06332168530284781640856621d9f4 (diff)
USB: add rationale on why usb descriptor structures have to be packed
Add argumentation in defense of using __attribute__((packed)) in USB descriptors authored by Dave Brownell. Necessary as in some cases it seems superfluous. Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
-rw-r--r--include/linux/usb/ch9.h22
1 files changed, 18 insertions, 4 deletions
diff --git a/include/linux/usb/ch9.h b/include/linux/usb/ch9.h
index 245c72531228..1122a6c2c1a3 100644
--- a/include/linux/usb/ch9.h
+++ b/include/linux/usb/ch9.h
@@ -1,8 +1,9 @@
/*
- * 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
@@ -14,6 +15,19 @@
*
* 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