can: c_can: Speed up rx_poll function

This patch speeds up the rx_poll function by reducing the number of
register reads.

Replace the 32bit register read by a 16bit register read. Currently
the 32bit register read is implemented by using 2 16bit reads. This is
inefficient as we only use the lower 16bit in rx_poll.

The for loop reads the pending interrupts in every iteration. This
leads up to 16 reads of pending interrupts. The patch introduces a new
outer loop to read the pending interrupts as long as 'quota' is above 0.
This reduces the total number of reads.

The third change is to replace the for-loop by a ffs loop.

Tested on AM335x. I removed all 'static' and 'inline' from c_can.c to
see the timings for all functions. I used the function tracer with
trace_stats.

125kbit:
  Function                               Hit    Time            Avg             s^2
  --------                               ---    ----            ---             ---
  c_can_do_rx_poll                     63960    10168178 us     158.977 us      1493056 us
With patch:
  c_can_do_rx_poll                     63941    3764057 us      58.867 us       776162.2 us

1Mbit:
  Function                               Hit    Time            Avg             s^2
  --------                               ---    ----            ---             ---
  c_can_do_rx_poll                     69489    30049498 us     432.435 us      9271851 us
With patch:
  c_can_do_rx_poll                    207109    24322185 us     117.436 us      171469047 us
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/c_can/c_can.c

@@ -808,17 +808,19 @@ static int c_can_do_rx_poll(struct net_device *dev, int quota)
 	u32 num_rx_pkts = 0;
 	unsigned int msg_obj, msg_ctrl_save;
 	struct c_can_priv *priv = netdev_priv(dev);
-	u32 val = c_can_read_reg32(priv, C_CAN_INTPND1_REG);
+	u16 val;
 
-	for (msg_obj = C_CAN_MSG_OBJ_RX_FIRST;
-			msg_obj <= C_CAN_MSG_OBJ_RX_LAST && quota > 0;
-			val = c_can_read_reg32(priv, C_CAN_INTPND1_REG),
-			msg_obj++) {
 	/*
-		 * as interrupt pending register's bit n-1 corresponds to
-		 * message object n, we need to handle the same properly.
+	 * It is faster to read only one 16bit register. This is only possible
+	 * for a maximum number of 16 objects.
 	 */
-		if (val & (1 << (msg_obj - 1))) {
+	BUILD_BUG_ON_MSG(C_CAN_MSG_OBJ_RX_LAST > 16,
+			"Implementation does not support more message objects than 16");
+
+	while (quota > 0 && (val = priv->read_reg(priv, C_CAN_INTPND1_REG))) {
+		while ((msg_obj = ffs(val)) && quota > 0) {
+			val &= ~BIT(msg_obj - 1);
+
 			c_can_object_get(dev, 0, msg_obj, IF_COMM_ALL &
 					~IF_COMM_TXRQST);
 			msg_ctrl_save = priv->read_reg(priv,