[PATCH (net-next.git) 13/18] stmmac: perf, remove modulo in stmmac_rx()

[Giuseppe Cavallaro]

The indexes into the ring buffer are always incremented, and
the entry is accessed via doing a modulo to find the "real" index.
Modulo is an expensive operation.

This patch replaces the modulo with a simple if clamp.
It helps improve stmmac RX path as it's being called inside RX loop.

Signed-off-by: Fabrice Gasnier <fabrice.gasn...@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavall...@st.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c |   28 ++++++++++++++++++---
 1 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c 
b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 0a39331..52ac648 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -211,6 +211,18 @@ static inline u32 stmmac_tx_avail(struct stmmac_priv *priv)
        return avail;
 }
 
+static inline u32 stmmac_rx_dirty(struct stmmac_priv *priv)
+{
+       unsigned dirty;
+
+       if (priv->dirty_rx <= priv->cur_rx)
+               dirty = priv->cur_rx - priv->dirty_rx;
+       else
+               dirty = priv->dma_rx_size - priv->dirty_rx + priv->cur_rx;
+
+       return dirty;
+}
+
 /**
  * stmmac_hw_fix_mac_speed - callback for speed selection
  * @priv: driver private structure
@@ -2165,9 +2177,10 @@ static inline void stmmac_rx_refill(struct stmmac_priv 
*priv)
 {
        unsigned int rxsize = priv->dma_rx_size;
        int bfsize = priv->dma_buf_sz;
+       unsigned int entry = priv->dirty_rx;
+       int dirty = stmmac_rx_dirty(priv);
 
-       for (; priv->cur_rx - priv->dirty_rx > 0; priv->dirty_rx++) {
-               unsigned int entry = priv->dirty_rx % rxsize;
+       while (dirty-- > 0) {
                struct dma_desc *p;
 
                if (priv->extend_desc)
@@ -2203,6 +2216,10 @@ static inline void stmmac_rx_refill(struct stmmac_priv 
*priv)
                wmb();
                priv->hw->desc->set_rx_owner(p);
                wmb();
+
+               if (unlikely(++priv->dirty_rx >= rxsize))
+                       priv->dirty_rx = 0;
+               entry = priv->dirty_rx;
        }
 }
 
@@ -2216,7 +2233,7 @@ static inline void stmmac_rx_refill(struct stmmac_priv 
*priv)
 static int stmmac_rx(struct stmmac_priv *priv, int limit)
 {
        unsigned int rxsize = priv->dma_rx_size;
-       unsigned int entry = priv->cur_rx % rxsize;
+       unsigned int entry = priv->cur_rx;
        unsigned int next_entry;
        unsigned int count = 0;
        int coe = priv->hw->rx_csum;
@@ -2246,7 +2263,10 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit)
 
                count++;
 
-               next_entry = (++priv->cur_rx) % rxsize;
+               if (unlikely(++priv->cur_rx >= rxsize))
+                       priv->cur_rx = 0;
+               next_entry = priv->cur_rx;
+
                if (priv->extend_desc)
                        prefetch(priv->dma_erx + next_entry);
                else
-- 
1.7.4.4

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