i2c: at91: Read all available bytes at once

With FIFO enabled it is possible to read multiple bytes
at once in the interrupt handler as long as RXRDY is
set. This may also reduce the number of interrupts.

This patch polls RXRDY and reads all available bytes at
once.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[wsa: reformatted comment]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-at91.c

@@ -518,8 +518,16 @@ static irqreturn_t atmel_twi_interrupt(int irq, void *dev_id)
 	 * the RXRDY interrupt first in order to not keep garbage data in the
 	 * Receive Holding Register for the next transfer.
 	 */
-	if (irqstatus & AT91_TWI_RXRDY)
-		at91_twi_read_next_byte(dev);
+	if (irqstatus & AT91_TWI_RXRDY) {
+		/*
+		 * Read all available bytes at once by polling RXRDY usable w/
+		 * and w/o FIFO. With FIFO enabled we could also read RXFL and
+		 * avoid polling RXRDY.
+		 */
+		do {
+			at91_twi_read_next_byte(dev);
+		} while (at91_twi_read(dev, AT91_TWI_SR) & AT91_TWI_RXRDY);
+	}
 
 	/*
 	 * When a NACK condition is detected, the I2C controller sets the NACK,