ethoc: remove division from loops

Calculating the BD entry using a modulus operation isn't optimal, especially
inside the loop.  This patch removes the modulus operations in favour of:

i)  simply checking for wrapping in the case of cur_rx
ii) forcing num_tx to be a power of two and using it to mask out the
    entry from cur_tx

The also prevents possible issues related overflow of the cur_rx and cur_tx
counters.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 4 deletions

diff --git a/drivers/net/ethoc.c b/drivers/net/ethoc.c
index 93b50d674b1e..b79d7e1555d5 100644
--- a/drivers/net/ethoc.c
+++ b/drivers/net/ethoc.c
@@ -412,7 +412,7 @@ static int ethoc_rx(struct net_device *dev, int limit)
                 unsigned int entry;
                 struct ethoc_bd bd;
 
-                entry = priv->num_tx + (priv->cur_rx % priv->num_rx);
+                entry = priv->num_tx + priv->cur_rx;
                 ethoc_read_bd(priv, entry, &bd);
                 if (bd.stat & RX_BD_EMPTY) {
                         ethoc_ack_irq(priv, INT_MASK_RX);
@@ -456,7 +456,8 @@ static int ethoc_rx(struct net_device *dev, int limit)
                 bd.stat &= ~RX_BD_STATS;
                 bd.stat |=  RX_BD_EMPTY;
                 ethoc_write_bd(priv, entry, &bd);
-                priv->cur_rx++;
+                if (++priv->cur_rx == priv->num_rx)
+                        priv->cur_rx = 0;
         }
 
         return count;
@@ -503,7 +504,7 @@ static int ethoc_tx(struct net_device *dev, int limit)
         for (count = 0; count < limit; ++count) {
                 unsigned int entry;
 
-                entry = priv->dty_tx % priv->num_tx;
+                entry = priv->dty_tx & (priv->num_tx-1);
 
                 ethoc_read_bd(priv, entry, &bd);
 
@@ -1000,9 +1001,17 @@ static int __devinit ethoc_probe(struct platform_device *pdev)
         /* calculate the number of TX/RX buffers, maximum 128 supported */
         num_bd = min_t(unsigned int,
                 128, (netdev->mem_end - netdev->mem_start + 1) / ETHOC_BUFSIZ);
-        priv->num_tx = max(2, num_bd / 4);
+        if (num_bd < 4) {
+                ret = -ENODEV;
+                goto error;
+        }
+        /* num_tx must be a power of two */
+        priv->num_tx = rounddown_pow_of_two(num_bd >> 1);
         priv->num_rx = num_bd - priv->num_tx;
 
+        dev_dbg(&pdev->dev, "ethoc: num_tx: %d num_rx: %d\n",
+                priv->num_tx, priv->num_rx);
+
         priv->vma = devm_kzalloc(&pdev->dev, num_bd*sizeof(void*), GFP_KERNEL);
         if (!priv->vma) {
                 ret = -ENOMEM;