enc28j60: reduce the number of spi transfers in enc28j60_set_bank()

A major source of overhead in the enc28j60 driver is the SPI transfers. Each
SPI transfer entails two kernel thread context switches. One major source of
SPI transfers is the enc28j60_set_bank() functions which runs before every
register access. This patch reduces the number of SPI transfers that
enc28j60_set_bank() performs in two ways:

  1. removes unnecessary bank switch for the registers that are present in all
	 banks

  2. when switching from banks 0 or 3 to banks 1 or 2 (i.e. only one bit
	 changes) enc28j60_set_bank() does only one SPI transfer instead of two

According to my tests these changes reduce the number of SPI transfers in
about 25%.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 23 insertions, 7 deletions

diff --git a/drivers/net/enc28j60.c b/drivers/net/enc28j60.c
index 535e14094971..b0ef46c51a9d 100644
--- a/drivers/net/enc28j60.c
+++ b/drivers/net/enc28j60.c
@@ -196,16 +196,32 @@ static void enc28j60_soft_reset(struct enc28j60_net *priv)
  */
 static void enc28j60_set_bank(struct enc28j60_net *priv, u8 addr)
 {
-        if ((addr & BANK_MASK) != priv->bank) {
-                u8 b = (addr & BANK_MASK) >> 5;
+        u8 b = (addr & BANK_MASK) >> 5;
 
-                if (b != (ECON1_BSEL1 | ECON1_BSEL0))
+        /* These registers (EIE, EIR, ESTAT, ECON2, ECON1)
+         * are present in all banks, no need to switch bank
+         */
+        if (addr >= EIE && addr <= ECON1)
+                return;
+
+        /* Clear or set each bank selection bit as needed */
+        if ((b & ECON1_BSEL0) != (priv->bank & ECON1_BSEL0)) {
+                if (b & ECON1_BSEL0)
+                        spi_write_op(priv, ENC28J60_BIT_FIELD_SET, ECON1,
+                                        ECON1_BSEL0);
+                else
+                        spi_write_op(priv, ENC28J60_BIT_FIELD_CLR, ECON1,
+                                        ECON1_BSEL0);
+        }
+        if ((b & ECON1_BSEL1) != (priv->bank & ECON1_BSEL1)) {
+                if (b & ECON1_BSEL1)
+                        spi_write_op(priv, ENC28J60_BIT_FIELD_SET, ECON1,
+                                        ECON1_BSEL1);
+                else
                         spi_write_op(priv, ENC28J60_BIT_FIELD_CLR, ECON1,
-                                     ECON1_BSEL1 | ECON1_BSEL0);
-                if (b != 0)
-                        spi_write_op(priv, ENC28J60_BIT_FIELD_SET, ECON1, b);
-                priv->bank = (addr & BANK_MASK);
+                                        ECON1_BSEL1);
         }
+        priv->bank = b;
 }
 
 /*