[TG3]: Avoid an expensive divide.

During an oprofile session of linux-2.6.20 on a dual opteron system, I noticed
an expensive divide was done in tg3_poll().

I am using gcc-4.1.1, so the following comment from drivers/net/tg3.c seems
over-optimistic :

/* Do not place this n-ring entries value into the tp struct itself,
  * we really want to expose these constants to GCC so that modulo et
  * al.  operations are done with shifts and masks instead of with
  * hw multiply/modulo instructions.  Another solution would be to
  * replace things like '% foo' with '& (foo - 1)'.
  */
#define TG3_RX_RCB_RING_SIZE(tp)        \
         ((tp->tg3_flags2 & TG3_FLG2_5705_PLUS) ?  512 : 1024)

Assembly code before patch :
(oprofile results included)
   6434  0.0088 :ffffffff803684b9:       mov    0x6f0(%r15),%eax
    587 8.0e-04 :ffffffff803684c0:       and    $0x40000,%eax
   2170  0.0030 :ffffffff803684c5:       cmp    $0x1,%eax
                :ffffffff803684c8:       lea    0x1(%r13),%eax
                :ffffffff803684cc:       sbb    %ecx,%ecx
   2051  0.0028 :ffffffff803684ce:       xor    %edx,%edx
                :ffffffff803684d0:       and    $0x200,%ecx
     20 2.7e-05 :ffffffff803684d6:       add    $0x200,%ecx
   1986  0.0027 :ffffffff803684dc:       div    %ecx
103427  0.1410 :ffffffff803684de:       cmp    %edx,0xffffffffffffff7c(%rbp)

Assembly code after the suggested patch :

ffffffff803684b9:           mov    0x6f0(%r15),%eax
ffffffff803684c0:           and    $0x40000,%eax
ffffffff803684c5:           cmp    $0x1,%eax
ffffffff803684c8:           sbb    %eax,%eax
ffffffff803684ca:           inc    %r13d
ffffffff803684cd:           and    $0x200,%eax
ffffffff803684d2:           add    $0x1ff,%eax
ffffffff803684d7:           and    %eax,%r13d
ffffffff803684da:           cmp    %r13d,0xffffffffffffff7c(%rbp)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tg3.c

@@ -3380,7 +3380,7 @@ static int tg3_rx(struct tg3 *tp, int budget)
 		}
 next_pkt_nopost:
 		sw_idx++;
-		sw_idx %= TG3_RX_RCB_RING_SIZE(tp);
+		sw_idx &= (TG3_RX_RCB_RING_SIZE(tp) - 1);
 
 		/* Refresh hw_idx to see if there is new work */
 		if (sw_idx == hw_idx) {