RE: [PATCH 31/68] uas: Pack iu struct definitions

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> From: linux-usb-owner@xxxxxxxxxxxxxxx [mailto:linux-usb-owner@xxxxxxxxxxxxxxx] On Behalf Of David Laight
> Sent: Monday, November 18, 2013 1:36 AM
> 
> > It doesn't look like the networking subsystem (which seems to be the
> > code you're most familiar with) uses packed, but plenty of other
> > subsystems do.
> 
> I'm necessarily trying to change the world here :-) But ...
> 
> I suspect a few places have got caught out where 32bit values
> get 16bit aligned and 64bit values are only 32bit aligned.
> eg the x86 bios partition table and (IIRC) some scsi commands.
> 
> There are also many places where C structures are used to map
> device registers (for memory mapped access) - these must be
> marked 'packed'.
> 
> There are also small (and probably measurable in some network
> benchmark) unnecessary costs in marking items 'packed' on systems
> where unaligned accesses aren't supported.
> The networking stack goes to great lengths to avoid misaligned
> buffers. AFAICT very few of the 'packed' structures in the usb
> headers will ever get misaligned.
> 
> Even if a structure has (say) a 64bit item that is only 32bit
> aligned then the single field can be given an alignment attribute
> so that two 32bit accesses are generated - rather than the eight
> 8bit ones that 'packed' required.

I think you're overlooking the fact that the usb structures can come in
over the wire and end up in a buffer somewhere. If there are several of
these back-to-back (as in the configuration descriptor), then the start
of the individual structs won't necessarily be aligned as you expect them
to be.

-- 
Paul

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