usb: gadget: u_ether: add a flag to avoid skb_reserve() calling

This patch adds a flag "no_skb_reserve" in struct eth_dev.
So, if a peripheral driver sets the quirk_avoids_skb_reserve flag,
upper network gadget drivers (e.g. f_ncm.c) can avoid skb_reserve()
calling using the flag as well.
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_ether.c

@@ -82,6 +82,7 @@ struct eth_dev {
 #define	WORK_RX_MEMORY		0
 
 	bool			zlp;
+	bool			no_skb_reserve;
 	u8			host_mac[ETH_ALEN];
 	u8			dev_mac[ETH_ALEN];
 };
@@ -233,7 +234,8 @@ rx_submit(struct eth_dev *dev, struct usb_request *req, gfp_t gfp_flags)
 	 * but on at least one, checksumming fails otherwise.  Note:
 	 * RNDIS headers involve variable numbers of LE32 values.
 	 */
-	skb_reserve(skb, NET_IP_ALIGN);
+	if (likely(!dev->no_skb_reserve))
+		skb_reserve(skb, NET_IP_ALIGN);
 
 	req->buf = skb->data;
 	req->length = size;
@@ -1063,6 +1065,7 @@ struct net_device *gether_connect(struct gether *link)
 
 	if (result == 0) {
 		dev->zlp = link->is_zlp_ok;
+		dev->no_skb_reserve = link->no_skb_reserve;
 		DBG(dev, "qlen %d\n", qlen(dev->gadget, dev->qmult));
 
 		dev->header_len = link->header_len;

drivers/usb/gadget/function/u_ether.h

@@ -64,6 +64,7 @@ struct gether {
 	struct usb_ep			*out_ep;
 
 	bool				is_zlp_ok;
+	bool				no_skb_reserve;
 
 	u16				cdc_filter;