mt76: usb: fix rx A-MSDU support

Commit f8f527b1 ("mt76: usb: use EP max packet aligned buffer sizes
for rx") breaks A-MSDU support. When A-MSDU is enable the device can
receive frames up to q->buf_size but they will be discarded in
mt76u_process_rx_entry since there is no enough room for
skb_shared_info. Fix the issue reallocating the skb and copying in the
linear area the first 128B of the received frames and in the frag_list
the remaining part

Fixes: f8f527b1 ("mt76: usb: use EP max packet aligned buffer sizes for rx")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76.h

@@ -30,6 +30,7 @@
 #define MT_TX_RING_SIZE     256
 #define MT_MCU_RING_SIZE    32
 #define MT_RX_BUF_SIZE      2048
+#define MT_SKB_HEAD_LEN     128
 
 struct mt76_dev;
 struct mt76_wcid;

drivers/net/wireless/mediatek/mt76/usb.c

@@ -429,6 +429,42 @@ static int mt76u_get_rx_entry_len(u8 *data, u32 data_len)
 	return dma_len;
 }
 
+static struct sk_buff *
+mt76u_build_rx_skb(void *data, int len, int buf_size)
+{
+	struct sk_buff *skb;
+
+	if (SKB_WITH_OVERHEAD(buf_size) < MT_DMA_HDR_LEN + len) {
+		struct page *page;
+
+		/* slow path, not enough space for data and
+		 * skb_shared_info
+		 */
+		skb = alloc_skb(MT_SKB_HEAD_LEN, GFP_ATOMIC);
+		if (!skb)
+			return NULL;
+
+		skb_put_data(skb, data + MT_DMA_HDR_LEN, MT_SKB_HEAD_LEN);
+		data += (MT_DMA_HDR_LEN + MT_SKB_HEAD_LEN);
+		page = virt_to_head_page(data);
+		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
+				page, data - page_address(page),
+				len - MT_SKB_HEAD_LEN, buf_size);
+
+		return skb;
+	}
+
+	/* fast path */
+	skb = build_skb(data, buf_size);
+	if (!skb)
+		return NULL;
+
+	skb_reserve(skb, MT_DMA_HDR_LEN);
+	__skb_put(skb, len);
+
+	return skb;
+}
+
 static int
 mt76u_process_rx_entry(struct mt76_dev *dev, struct urb *urb)
 {
@@ -446,19 +482,11 @@ mt76u_process_rx_entry(struct mt76_dev *dev, struct urb *urb)
 		return 0;
 
 	data_len = min_t(int, len, data_len - MT_DMA_HDR_LEN);
-	if (MT_DMA_HDR_LEN + data_len > SKB_WITH_OVERHEAD(q->buf_size)) {
-		dev_err_ratelimited(dev->dev, "rx data too big %d\n", data_len);
-		return 0;
-	}
-
-	skb = build_skb(data, q->buf_size);
+	skb = mt76u_build_rx_skb(data, data_len, q->buf_size);
 	if (!skb)
 		return 0;
 
-	skb_reserve(skb, MT_DMA_HDR_LEN);
-	__skb_put(skb, data_len);
 	len -= data_len;
-
 	while (len > 0 && nsgs < urb->num_sgs) {
 		data_len = min_t(int, len, urb->sg[nsgs].length);
 		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,