Hi, 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. And on such a system we want the compiler to generate byte accesses, to avoid throwing an unaligned memory access error. We cannot allow the compiler to add padding to avoid such unaligned accesses, since this struct is send as a blob over usb, and the sending device will not insert any padding, iow this struct represent data as it is on the wire, not how the data would (optimally) be represented in host memory.
I'd have thought that 'packed' should only be specified on structures that are likely to be misaligned in memory.
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. Regards, Hans -- 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
