[PATCH] gve: DQO: avoid unused variable warnings

From: Arnd Bergmann <arnd@arndb.de>

The use of dma_unmap_addr()/dma_unmap_len() in the driver causes
multiple warnings when these macros are defined as empty:

drivers/net/ethernet/google/gve/gve_tx_dqo.c: In function 'gve_tx_add_skb_no_copy_dqo':
drivers/net/ethernet/google/gve/gve_tx_dqo.c:494:40: error: unused variable 'buf' [-Werror=unused-variable]
  494 |                 struct gve_tx_dma_buf *buf =

As it turns out, there are three copies of the same loop,
and one of them is already split out into a separate function.

Fix the warning in this one place, and change the other two
to call it instead of open-coding the same loop.

Fixes: a57e5de476be ("gve: DQO: Add TX path")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
The warning is present in both 5.14-rc2 and net-next as of today
---
 drivers/net/ethernet/google/gve/gve_tx_dqo.c | 92 ++++++++------------
 1 file changed, 35 insertions(+), 57 deletions(-)

diff --git a/drivers/net/ethernet/google/gve/gve_tx_dqo.c b/drivers/net/ethernet/google/gve/gve_tx_dqo.c
index 05ddb6a75c38..fffa882db493 100644
--- a/drivers/net/ethernet/google/gve/gve_tx_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_tx_dqo.c

@@ -73,6 +73,26 @@ gve_free_pending_packet(struct gve_tx_ring *tx,
 	}
 }
 
+static void gve_unmap_packet(struct device *dev,
+			     struct gve_tx_pending_packet_dqo *pending_packet)
+{
+	dma_addr_t addr;
+	size_t len;
+	int i;
+
+	/* SKB linear portion is guaranteed to be mapped */
+	addr = dma_unmap_addr(&pending_packet->bufs[0], dma);
+	len = dma_unmap_len(&pending_packet->bufs[0], len);
+	dma_unmap_single(dev, addr, len, DMA_TO_DEVICE);
+
+	for (i = 1; i < pending_packet->num_bufs; i++) {
+		addr = dma_unmap_addr(&pending_packet->bufs[i], dma);
+		len = dma_unmap_len(&pending_packet->bufs[i], len);
+		dma_unmap_page(dev, addr, len, DMA_TO_DEVICE);
+	}
+	pending_packet->num_bufs = 0;
+}
+
 /* gve_tx_free_desc - Cleans up all pending tx requests and buffers.
  */
 static void gve_tx_clean_pending_packets(struct gve_tx_ring *tx)

@@ -82,23 +102,8 @@ static void gve_tx_clean_pending_packets(struct gve_tx_ring *tx)
 	for (i = 0; i < tx->dqo.num_pending_packets; i++) {
 		struct gve_tx_pending_packet_dqo *cur_state =
 			&tx->dqo.pending_packets[i];
-		int j;
-
-		for (j = 0; j < cur_state->num_bufs; j++) {
-			struct gve_tx_dma_buf *buf = &cur_state->bufs[j];
-
-			if (j == 0) {
-				dma_unmap_single(tx->dev,
-						 dma_unmap_addr(buf, dma),
-						 dma_unmap_len(buf, len),
-						 DMA_TO_DEVICE);
-			} else {
-				dma_unmap_page(tx->dev,
-					       dma_unmap_addr(buf, dma),
-					       dma_unmap_len(buf, len),
-					       DMA_TO_DEVICE);
-			}
-		}
+
+		gve_unmap_packet(tx->dev, cur_state);
 		if (cur_state->skb) {
 			dev_consume_skb_any(cur_state->skb);
 			cur_state->skb = NULL;

@@ -445,6 +450,13 @@ gve_tx_fill_general_ctx_desc(struct gve_tx_general_context_desc_dqo *desc,
 	};
 }
 
+static inline void gve_tx_dma_buf_set(struct gve_tx_dma_buf *buf,
+				      dma_addr_t addr, size_t len)
+{
+	dma_unmap_len_set(buf, len, len);
+	dma_unmap_addr_set(buf, dma, addr);
+}
+
 /* Returns 0 on success, or < 0 on error.
  *
  * Before this function is called, the caller must ensure

@@ -459,6 +471,7 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 
 	struct gve_tx_pending_packet_dqo *pending_packet;
 	struct gve_tx_metadata_dqo metadata;
+	struct gve_tx_dma_buf *buf;
 	s16 completion_tag;
 	int i;
 

@@ -493,8 +506,6 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 
 	/* Map the linear portion of skb */
 	{
-		struct gve_tx_dma_buf *buf =
-			&pending_packet->bufs[pending_packet->num_bufs];
 		u32 len = skb_headlen(skb);
 		dma_addr_t addr;
 

@@ -502,8 +513,8 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 		if (unlikely(dma_mapping_error(tx->dev, addr)))
 			goto err;
 
-		dma_unmap_len_set(buf, len, len);
-		dma_unmap_addr_set(buf, dma, addr);
+		buf = &pending_packet->bufs[pending_packet->num_bufs];
+		gve_tx_dma_buf_set(buf, addr, len);
 		++pending_packet->num_bufs;
 
 		gve_tx_fill_pkt_desc_dqo(tx, &desc_idx, skb, len, addr,

@@ -512,8 +523,6 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 	}
 
 	for (i = 0; i < shinfo->nr_frags; i++) {
-		struct gve_tx_dma_buf *buf =
-			&pending_packet->bufs[pending_packet->num_bufs];
 		const skb_frag_t *frag = &shinfo->frags[i];
 		bool is_eop = i == (shinfo->nr_frags - 1);
 		u32 len = skb_frag_size(frag);

@@ -523,8 +532,8 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 		if (unlikely(dma_mapping_error(tx->dev, addr)))
 			goto err;
 
-		dma_unmap_len_set(buf, len, len);
-		dma_unmap_addr_set(buf, dma, addr);
+		buf = &pending_packet->bufs[pending_packet->num_bufs];
+		gve_tx_dma_buf_set(buf, addr, len);
 		++pending_packet->num_bufs;
 
 		gve_tx_fill_pkt_desc_dqo(tx, &desc_idx, skb, len, addr,

@@ -552,21 +561,8 @@ static int gve_tx_add_skb_no_copy_dqo(struct gve_tx_ring *tx,
 	return 0;
 
 err:
-	for (i = 0; i < pending_packet->num_bufs; i++) {
-		struct gve_tx_dma_buf *buf = &pending_packet->bufs[i];
-
-		if (i == 0) {
-			dma_unmap_single(tx->dev, dma_unmap_addr(buf, dma),
-					 dma_unmap_len(buf, len),
-					 DMA_TO_DEVICE);
-		} else {
-			dma_unmap_page(tx->dev, dma_unmap_addr(buf, dma),
-				       dma_unmap_len(buf, len), DMA_TO_DEVICE);
-		}
-	}
-
+	gve_unmap_packet(tx->dev, pending_packet);
 	pending_packet->skb = NULL;
-	pending_packet->num_bufs = 0;
 	gve_free_pending_packet(tx, pending_packet);
 
 	return -1;

@@ -746,24 +742,6 @@ static void remove_from_list(struct gve_tx_ring *tx,
 	}
 }
 
-static void gve_unmap_packet(struct device *dev,
-			     struct gve_tx_pending_packet_dqo *pending_packet)
-{
-	struct gve_tx_dma_buf *buf;
-	int i;
-
-	/* SKB linear portion is guaranteed to be mapped */
-	buf = &pending_packet->bufs[0];
-	dma_unmap_single(dev, dma_unmap_addr(buf, dma),
-			 dma_unmap_len(buf, len), DMA_TO_DEVICE);
-	for (i = 1; i < pending_packet->num_bufs; i++) {
-		buf = &pending_packet->bufs[i];
-		dma_unmap_page(dev, dma_unmap_addr(buf, dma),
-			       dma_unmap_len(buf, len), DMA_TO_DEVICE);
-	}
-	pending_packet->num_bufs = 0;
-}
-
 /* Completion types and expected behavior:
  * No Miss compl + Packet compl = Packet completed normally.
  * Miss compl + Re-inject compl = Packet completed normally.

-- 
2.29.2

Thanks for the patch!

On Wed, Jul 21, 2021 at 8:11 AM Arnd Bergmann [off-list ref] wrote:

"SKB linear portion is guaranteed to be mapped" is only true if
gve_tx_add_skb_no_copy_dqo completed successfully.

This optimization is important for the success path because otherwise
there would be a per-packet branch misprediction, which I found to
have a large performance impact.

A solution which should address this would be something like:

+static void gve_unmap_packet(struct device *dev,
+     struct gve_tx_pending_packet_dqo *pending_packet
+     bool always_unmap_first)
+{
+ dma_addr_t addr;
+ size_t len;
+ int i;
+
+ if (always_unmap_first || pending_packet->num_bufs > 0) {
+  addr = dma_unmap_addr(&pending_packet->bufs[0], dma);
+  len = dma_unmap_len(&pending_packet->bufs[0], len);
+  dma_unmap_single(dev, addr, len, DMA_TO_DEVICE);
+ }
+
+ for (i = 1; i < pending_packet->num_bufs; i++) {
+  addr = dma_unmap_addr(&pending_packet->bufs[i], dma);
+  len = dma_unmap_len(&pending_packet->bufs[i], len);
+  dma_unmap_page(dev, addr, len, DMA_TO_DEVICE);
+ }
+ pending_packet->num_bufs = 0;
+}

(Sorry my email client keeps turning tabs into spaces...)

By doing this, we can rely on the compiler to optimize away the extra
branch in cases we know the first buffer will be mapped.

+static inline void gve_tx_dma_buf_set(struct gve_tx_dma_buf *buf,
+                                     dma_addr_t addr, size_t len)
+{
+       dma_unmap_len_set(buf, len, len);
+       dma_unmap_addr_set(buf, dma, addr);
+}

checkpatch.pl will complain about `inline` in a C file.

However, I would prefer to just not introduce this helper because it
introduces indirection for the reader and the risk of passing the
arguments in the wrong order. Don't have a strong opinion here though.

dma_unmap_len_set(pending_packet->bufs[pending_packet->num_bufs], len, len);
On Wed, Jul 21, 2021 at 5:36 PM Bailey Forrest [off-list ref] wrote:

On Wed, Jul 21, 2021 at 8:11 AM Arnd Bergmann [off-list ref] wrote:

quoted

+static void gve_unmap_packet(struct device *dev,
+                            struct gve_tx_pending_packet_dqo *pending_packet)
+{
+       dma_addr_t addr;
+       size_t len;
+       int i;
+
+       /* SKB linear portion is guaranteed to be mapped */
+       addr = dma_unmap_addr(&pending_packet->bufs[0], dma);
+       len = dma_unmap_len(&pending_packet->bufs[0], len);
+       dma_unmap_single(dev, addr, len, DMA_TO_DEVICE);

"SKB linear portion is guaranteed to be mapped" is only true if
gve_tx_add_skb_no_copy_dqo completed successfully.

This optimization is important for the success path because otherwise
there would be a per-packet branch misprediction, which I found to
have a large performance impact.

A solution which should address this would be something like:

+static void gve_unmap_packet(struct device *dev,
+     struct gve_tx_pending_packet_dqo *pending_packet
+     bool always_unmap_first)
+{
+ dma_addr_t addr;
+ size_t len;
+ int i;
+
+ if (always_unmap_first || pending_packet->num_bufs > 0) {
+  addr = dma_unmap_addr(&pending_packet->bufs[0], dma);
+  len = dma_unmap_len(&pending_packet->bufs[0], len);
+  dma_unmap_single(dev, addr, len, DMA_TO_DEVICE);
+ }
+
+ for (i = 1; i < pending_packet->num_bufs; i++) {
+  addr = dma_unmap_addr(&pending_packet->bufs[i], dma);
+  len = dma_unmap_len(&pending_packet->bufs[i], len);
+  dma_unmap_page(dev, addr, len, DMA_TO_DEVICE);
+ }
+ pending_packet->num_bufs = 0;
+}

(Sorry my email client keeps turning tabs into spaces...)

By doing this, we can rely on the compiler to optimize away the extra
branch in cases we know the first buffer will be mapped.

I didn't really change it here, I just moved the function up and changed
the dma_unmap_addr/dma_unmap_len calls to avoid the warning.

quoted

+static inline void gve_tx_dma_buf_set(struct gve_tx_dma_buf *buf,
+                                     dma_addr_t addr, size_t len)
+{
+       dma_unmap_len_set(buf, len, len);
+       dma_unmap_addr_set(buf, dma, addr);
+}

checkpatch.pl will complain about `inline` in a C file.

However, I would prefer to just not introduce this helper because it
introduces indirection for the reader and the risk of passing the
arguments in the wrong order. Don't have a strong opinion here
though.

Sure, feel free to just treat my patch as a bug report and send a different
fix if you prefer to not have an inline function. This is usually the easiest
way to get around the macro ignoring its arguments since the compiler
does not warn for unused function arguments.

Open-codiung the call as

dma_unmap_len_set(pending_packet->bufs[pending_packet->num_bufs], len, len);
dma_unmap_len_addr(pending_packet->bufs[pending_packet->num_bufs], addr, dma);

works as well, I just found the inline function more readable.

     Arnd

Sure, feel free to just treat my patch as a bug report and send a different
fix if you prefer to not have an inline function. This is usually the easiest
way to get around the macro ignoring its arguments since the compiler
does not warn for unused function arguments.

Okay, I will propose my own fix and send the patch hopefully today or tomorrow.