Hi,
On 11/15/2013 10:09 PM, Greg KH wrote:
On Fri, Nov 15, 2013 at 03:56:31PM -0000, David Laight wrote:
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.
No, Linux relies on the compiler to pack things like this together and
not put padding in, with the __packed__ attribute (don't we have a
__packed macro instead of the __attribute__() thing?)
Yes we've a __packed macro, but the norm in the usb subsys seems to be to use
__attribute__ ((__packed__)), see include/[uapi/]linux/usb/*.h
Regards,
Hans
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