Re: alignment faults in 3.6

Rob Herring [off-list ref] writes:

On 10/11/2012 07:40 AM, Eric Dumazet wrote:

quoted

On Thu, 2012-10-11 at 12:28 +0000, Arnd Bergmann wrote:

quoted

Rob Herring as the original reporter has dropped off the Cc list, adding
him back.

I assume that the calxeda xgmac driver is the culprit then. It uses
netdev_alloc_skb() rather than netdev_alloc_skb_ip_align() in
xgmac_rx_refill but it is not clear whether it does so intentionally
or by accident.

This in fact does work and eliminates the unaligned traps. However, not
all h/w can do IP aligned DMA (i.MX FEC for example), so I still think
this is a questionable optimization by the compiler. We're saving 1 load
instruction here for data that is likely already in the cache. It may be
legal per the ABI, but the downside of this optimization is much greater
than the upside.

The compiler is working *exactly* as it should.  Merging the loads saves
cycles *and* code size.  Many of these added up can make a real difference.

When writing code, you must follow all the rules, whether you like them
or not.  Without rules, the compiler would be very limited in the
optimisations it could perform.

Unfortunately, new optimisations occasionally uncover broken code
violating some constraint or other.  When this happens, the correct
course of action is to fix the code, not cripple the compiler.

-- 
Måns Rullgård
mans@mansr.com