> From: Hans de Goede [mailto:hdegoede@xxxxxxxxxx] > On 11/15/2013 04:29 PM, David Laight wrote: > >> From: Of Hans de Goede > >> > >> The iu struct definitions are usb packet definitions, so no alignment should > >> happen. Notice that assuming 32 bit alignment this does not make any > >> difference at all. > >> > >> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx> > >> --- > >> include/linux/usb/uas.h | 10 +++++----- > >> 1 file changed, 5 insertions(+), 5 deletions(-) > >> > >> diff --git a/include/linux/usb/uas.h b/include/linux/usb/uas.h > >> index 772b66b..3fc8e8b 100644 > >> --- a/include/linux/usb/uas.h > >> +++ b/include/linux/usb/uas.h > >> @@ -9,7 +9,7 @@ struct iu { > >> __u8 iu_id; > >> __u8 rsvd1; > >> __be16 tag; > >> -}; > >> +} __attribute__((__packed__)); > > > > Won't this cause the compiler to generate multiple byte accesses > > for the 'tag' field in systems that don't support misaligned > > accesses. > > The tag field is aligned to a multiple of its size, so that should not > happen, unless some system wants 32 bits alignment for 16 bit variables. That isn't going to be true. It might even be against the C standard. In any case such a system wouldn't be able to define __be16 (unless the compiler implemented it using byte accesses). ... > Without packed struct members will never be mis-aligned in memory as the > compiler will simply add padding. packed is used when the struct represent > data coming from an outside source, ie a file or a socket, or in this case > an usb data packet. What would be useful is an attribute that errorred if any implicit padding were added. If you are worried about padding add a compile-time test on the size of the structure. David -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
