On Jul 29, 2013, at 11:58 AM, Hans Petter Selasky wrote:

> The aligned will make sure that the structure gets padded properly to the 
> size specified. Only on ARM/MIPS etc, structures get automatically aligned 
> according to the element in the structure requiring the greatest alignment.

I'd turn this around and say only on x86 do structures not get aligned this 
way. On any riscy architecture, unaligned accesses are expensive, which is why 
the ABI there mandates this.

> I've test-compiled the USB WLAN drivers, and the aligned makes a difference. 
> The problem is that the radiotap header skews some following elements, so 
> that they are no longer aligned. The radiotap header itself is packed, and 
> this is not a problem.

This suggests a bigger problem. How is the radiotap header being popualted? Is 
it by DMA or programmatically? If by DMA then we have cache line sharing, which 
is bad. If by program, why is it packed to start with?

Warner

> --HPS
> 
>  
> -----Original message-----
> > From:Warner Losh <i...@bsdimp.com>
> > Sent: Monday 29th July 2013 17:04
> > To: Adrian Chadd <adr...@freebsd.org>
> > Cc: Hans Petter Selasky <hans.petter.sela...@bitfrost.no>; freebsd-arm 
> > <freebsd-...@freebsd.org>; freebsd-wireless@freebsd.org
> > Subject: Re: My WLI-UC-GNM up crash
> > 
> > Aren't structures already aligned to 4 bytes when placed inside other 
> > structures (unless marked __packed)?
> > 
> > Warner
> > 
> > On Jul 28, 2013, at 11:50 AM, Adrian Chadd wrote:
> > 
> > > As long as that results in the radiotap structures being 4 or 8 byte
> > > padded when it's embedded in the softc - then yes, indeed.
> > > 
> > > Xiao, can you try?
> > > 
> > > 
> > > -adrian
> > > 
> > > On 28 July 2013 03:35, Hans Petter Selasky <h...@bitfrost.no> wrote:
> > >> Hi,
> > >> 
> > >> Can you try the attached patch?
> > >> 
> > >> --HPS
> > > _______________________________________________
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> > 
> > 

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