Problem with non aligned DMA in usbnet on ARM

Hi,

Martin Fuzzey a ?crit :

Hi all,

So what is the correct fix for this problem?
The options seem to be:
1) Remove skb_reserve() from usbnet
But this will cause the ip header to be non aligned on platforms that
don't have the DMA alignment requirement.

And cause lot's of unaligned access for the IP stack. This will slow 
down arm platform...

2) Define NET_IP_ALIGN to 0 for ARM
Seems a bit intrusive to me.

Again this will generate lot's of unaligned access.

3) Change the HCD to copy if not aligned
I'd like to avoid that.

4) Change usbnet to use seperate buffers for the usb side and skb.
Copy between them so both remain aligned
Seems unlikely that the gain further up the stack from ensuring the ip
header is aligned will offset the cost of the copy.

Any ideas?

I got the same problem on the gadget side : 
http://article.gmane.org/gmane.linux.usb.general/28700

A solution could be the hcd driver to tell it doesn't support unaligned 
transfert and in this case the usb stack/usb driver align it.

It is best to solve in usb driver, because for example in Asix case the 
driver already do copy (see asix_rx_fixup/asix_tx_fixup).


Matthieu

PS : what hcd driver do you use ?