sfc: The RX page_ring is optional

The RX page_ring is an optional feature that improves
performance. When allocation fails the driver can still
function, but possibly with a lower bandwidth.
Guard against dereferencing a NULL page_ring.

Fixes: 2768935a46 ("sfc: reuse pages to avoid DMA mapping/unmapping costs")
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Reported-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/164111288276.5798.10330502993729113868.stgit@palantir17.mph.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/falcon/rx.c

@@ -110,6 +110,8 @@ static struct page *ef4_reuse_page(struct ef4_rx_queue *rx_queue)
 	struct ef4_rx_page_state *state;
 	unsigned index;
 
+	if (unlikely(!rx_queue->page_ring))
+		return NULL;
 	index = rx_queue->page_remove & rx_queue->page_ptr_mask;
 	page = rx_queue->page_ring[index];
 	if (page == NULL)
@@ -293,6 +295,9 @@ static void ef4_recycle_rx_pages(struct ef4_channel *channel,
 {
 	struct ef4_rx_queue *rx_queue = ef4_channel_get_rx_queue(channel);
 
+	if (unlikely(!rx_queue->page_ring))
+		return;
+
 	do {
 		ef4_recycle_rx_page(channel, rx_buf);
 		rx_buf = ef4_rx_buf_next(rx_queue, rx_buf);

drivers/net/ethernet/sfc/rx_common.c

@@ -45,6 +45,8 @@ static struct page *efx_reuse_page(struct efx_rx_queue *rx_queue)
 	unsigned int index;
 	struct page *page;
 
+	if (unlikely(!rx_queue->page_ring))
+		return NULL;
 	index = rx_queue->page_remove & rx_queue->page_ptr_mask;
 	page = rx_queue->page_ring[index];
 	if (page == NULL)
@@ -114,6 +116,9 @@ void efx_recycle_rx_pages(struct efx_channel *channel,
 {
 	struct efx_rx_queue *rx_queue = efx_channel_get_rx_queue(channel);
 
+	if (unlikely(!rx_queue->page_ring))
+		return;
+
 	do {
 		efx_recycle_rx_page(channel, rx_buf);
 		rx_buf = efx_rx_buf_next(rx_queue, rx_buf);