gve: Make gve_rx_slot_page_info.page_offset an absolute offset

Using `page_offset` like a boolean means a page may only be split into
two sections. With page sizes larger than 4k, this can be very wasteful.
Future commits in this patchset use `struct gve_rx_slot_page_info` in a
way which supports a fixed buffer size and a variable page size.
Signed-off-by: Bailey Forrest <bcf@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/google/gve/gve.h

 /* SPDX-License-Identifier: (GPL-2.0 OR MIT)
  * Google virtual Ethernet (gve) driver
  *
- * Copyright (C) 2015-2019 Google, Inc.
+ * Copyright (C) 2015-2021 Google, Inc.
  */
 
 #ifndef _GVE_H_
@@ -51,7 +51,7 @@ struct gve_rx_desc_queue {
 struct gve_rx_slot_page_info {
 	struct page *page;
 	void *page_address;
-	u8 page_offset; /* flipped to second half? */
+	u32 page_offset; /* offset to write to in page */
 	u8 can_flip;
 };
 

drivers/net/ethernet/google/gve/gve_rx.c

@@ -272,7 +272,7 @@ static struct sk_buff *gve_rx_add_frags(struct napi_struct *napi,
 		return NULL;
 
 	skb_add_rx_frag(skb, 0, page_info->page,
-			(page_info->page_offset ? PAGE_SIZE / 2 : 0) +
+			page_info->page_offset +
 			GVE_RX_PAD, len, PAGE_SIZE / 2);
 
 	return skb;
@@ -283,7 +283,7 @@ static void gve_rx_flip_buff(struct gve_rx_slot_page_info *page_info, __be64 *sl
 	const __be64 offset = cpu_to_be64(PAGE_SIZE / 2);
 
 	/* "flip" to other packet buffer on this page */
-	page_info->page_offset ^= 0x1;
+	page_info->page_offset ^= PAGE_SIZE / 2;
 	*(slot_addr) ^= offset;
 }
 

drivers/net/ethernet/google/gve/gve_utils.c

@@ -50,7 +50,7 @@ struct sk_buff *gve_rx_copy(struct net_device *dev, struct napi_struct *napi,
 {
 	struct sk_buff *skb = napi_alloc_skb(napi, len);
 	void *va = page_info->page_address + pad +
-		   (page_info->page_offset ? PAGE_SIZE / 2 : 0);
+		   page_info->page_offset;
 
 	if (unlikely(!skb))
 		return NULL;