Re: carl9170.ko RX - DMA, TX - COPY

[Christian Lamparter <chunkeey@googlemail.com>]

On Wednesday 04 May 2011 18:52:58 Andrew V. Stepanov wrote:
> I think it is possible to align SKB frames in carl9170.ko driver.
> 
> rt2800usb do such think in next way:
> 
> void rt2x00queue_align_frame(struct sk_buff *skb)
> {
> »···unsigned int frame_length = skb->len;
> »···unsigned int align = ALIGN_SIZE(skb, 0);
> 
> »···if (!align)
> »···»···return;
> 
> »···skb_push(skb, align);
> »···memmove(skb->data, skb->data + align, frame_length);
> »···skb_trim(skb, frame_length);
> }
> 
> What do you think about this?
No that's wrong, here's the comment from rt2x00queue.c

"When DMA allocation is required we should guarantee to the
 driver that the DMA is aligned to a 4-byte boundary.
 However some drivers require L2 padding to pad the payload
 rather then the header. This could be a requirement for
 PCI and USB devices, while header alignment only is valid
 for PCI devices."

Do you see the difference there; header alignment is only needed
for PCI whereas rt2800usb hw/fw allows just the L2 data to be
padded and sadly AR9170 hw does not have this feature. 

Also, someone <https://patchwork.kernel.org/patch/98997/> added
bounce buffers to musb some time ago, so why not use this approach
instead? The advantage is that you won't need to modify any other
drivers when you will (inevitable) replace the device with a
better one, right?

Regards,
	Chr