Re: pull request: wireless-next-2.6 2010-09-21

[David Miller <davem@davemloft.net>]

From: Christian Lamparter <chunkeey@googlemail.com>
Date: Wed, 22 Sep 2010 20:54:05 +0200

> Only the structs which deal with hardware (hw.h, eeprom.h, wlan.h)
> or firmware (fwcmd.h, fwdesc.h & wlan.h) interface have the
> __packed attribute. And there are several good reasons.

There is a cost in being lazy and just stabbing __packed to every
structure shared with firmware or hardware.

And that cost is that every larger-than-byte sized element will be
loaded with a disgustingly expensive series of byte loads, shifts,
and masks.

Every access.

This is because GCC is not allowed to assume the alignment of any
structure marked with __packed.  Therefore only byte sized loads are
safe on architectures that require types be aligned to their size.

This really is a performance issue, otherwise I frankly wouldn't
care where you put __packed.

So please minimize the annotations to only the structures that
actually require __packed, not those that "might."