From 672027a35795ec95f516fdc702ba8900d55a9eef Mon Sep 17 00:00:00 2001 From: Inaky Perez-Gonzalez Date: Thu, 22 Feb 2007 16:37:53 -0800 Subject: 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 Signed-off-by: Greg Kroah-Hartman --- include/linux/usb/ch9.h | 22 ++++++++++++++++++---- 1 file 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 -- cgit v1.2.2