arm alignment faults...

[Warner Losh]

On Jun 28, 2014, at 10:52 PM, Adrian Chadd <adrian at freebsd.org> wrote:

> On 28 June 2014 21:01, John-Mark Gurney <jmg at funkthat.com> wrote:
>> Adrian Chadd wrote this message on Sat, Jun 28, 2014 at 20:44 -0700:
>>> On 28 June 2014 20:38, John-Mark Gurney <jmg at funkthat.com> wrote:
>>>> So, one of the little projects I'd like to see is the removal of
>>>> ETHER_ALIGN from the tree..  This bogosity can (and does) cause the use
>>>> of bouncing durning DMA ops on all ethernet frames...
>> 
>> Now that I think about it, total removal may not be necessary, just
>> the requirement to use it...  If the ethernet dma engine can do half
>> word aligned dma, then there would be benifit on those to keep
>> ETHER_ALIGN...
>> 
>>> Well, as long as you're not doing it by forcing the various CPUs to
>>> handle unaligned accesses.
>> 
>> Hard to do on armv4 which I don't believe supports unaligned access...
>> 
>>> The cost of those unaligned accesses on some CPUs that support them is
>>> not trivial. We benchmarked some of the ARM cores at Qualcomm back
>>> when looking to migrate stuff to ARM and it wasn't very quick.
>> 
>> I plan on fixing the TCP/IP stack to copy data to an aligned buffer
>> (maybe only if the original buffer isn't aligned) on the stack when
>> __NO_STRICT_ALIGNMENT is not defined...  I can't see how copying the
>> entire packet is cheaper than copying 20 bytes or so...
> 
> There's lots of other stupid corner cases that screw you.
> 
> VLAN headers add extra bytes.
> 
> 802.11 headers can offset things depending upon the 802.11 frame type
> (3-addr, 4-addr, vlan, no vlan, etc.)
> 
> There's no guarantee all ethernet DMA engines can do the alignment as
> required. :(

The ate driver for Atmel’s AT91RM9200 is one such beast.

Warner
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