Re: trap handler for unaligned memory read/write

[Jun Sun <jsun@mvista.com>]

As usual, I am replying to my own question.  Maybe I was asking to soon
... :-)

I found some drivers offer an copy-only-tiny-buffer which will copy
small packets (for what purpose?).  Apparently after the copying the
misalignment disappears.  So MIPS, you just set a high copy_break_size
so that a copying always happens.

In fact this is what is done for other non-x86 architectures in tulip
drivers.

Jun

Jun Sun wrote:
> 
> I was trying to run some PCI ether drivers and always got bus error, at
> least when I use ipconfig bootp code.
> 
> However, the problem seems to be generic.
> 
> Ethernet device writes a whole packet in the memory.  Driver and network
> stack code often directly dereference a pointer in to the packet.
> However, the ether header is 14 byte long.  If you align packet from the
> beginning, then IP header will be off-aligned.
> 
> Any suggestions?
> 
> If this is a valid problem, I think the long term solution should be in
> network code, which should not assume they can dereference on an
> unaligned address.
> 
> For short-term solutions, we can have trap handler that supports the
> unaligned read/write.  Does anybody know if there is such a trap handler
> for MIPS?
> 
> Thanks.
> 
> Jun