Hi,
Gadiyar, Anand a écrit :
> Alan Stern wrote:
>> On Tue, 15 Dec 2009, Matthieu CASTET wrote:
>>
>>
>
> For g_ether, the buffers are allocated using kmalloc(), but the
> networking code seems to move the data in the buffer out by 2
> bytes, and passes this unaligned address down to the gadget driver.
Yes it does that to avoid unaligned access in network stack.
>
> The MUSB controller in the beagleboard could handle these unaligned
> DMA address, but looks like newer MUSB controllers don't seem to
> like this.
There is other controllers that can't handle this. For example s3c-hsotg
[1] and a controller that we are using (not present in upstream).
What we should decide is where the modification should be done : in
controller driver or function ?
Fixing it in function driver seems better because the function driver
has knowledge of the data in the buffer. Also even if the data are on a
32bit aligned addresses, this is not enough, we want to be aligned on a
cache boundary.
Matthieu
PS : Also note that wifi stack does similar stuff on rx side [2].
[1]
/**
* using_dma - return the DMA status of the driver.
* @hsotg: The driver state.
*
* Return true if we're using DMA.
*
* Currently, we have the DMA support code worked into everywhere
* that needs it, but the AMBA DMA implementation in the hardware can
* only DMA from 32bit aligned addresses. This means that gadgets such
* as the CDC Ethernet cannot work as they often pass packets which are
* not 32bit aligned.
*
* Unfortunately the choice to use DMA or not is global to the controller
* and seems to be only settable when the controller is being put through
* a core reset. This means we either need to fix the gadgets to take
* account of DMA alignment, or add bounce buffers (yuerk).
*
* Until this issue is sorted out, we always return 'false'.
[2]
#if defined(CONFIG_MAC80211_DEBUG_PACKET_ALIGNMENT) ||
!defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
/*
* 'align' will only take the values 0 or 2 here
* since all frames are required to be aligned
* to 2-byte boundaries when being passed to
* mac80211. That also explains the __skb_push()
* below.
*/
align = ((unsigned long)(skb->data + sizeof(struct ethhdr))) & 3;
if (align) {
if (WARN_ON(skb_headroom(skb) < 3)) {
dev_kfree_skb(skb);
skb = NULL;
} else {
u8 *data = skb->data;
size_t len = skb_headlen(skb);
skb->data -= align;
memmove(skb->data, data, len);
skb_set_tail_pointer(skb, len);
}
}
#endif
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
