spi: bcm2835: Micro-optimise FIFO loops

The blind and counted loops are always called with nonzero count, so
convert them to do-while loops that lead to slightly more efficient
code generation. With GCC 8.3 this shaves off 1-2 instructions per
iteration in each case.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/9242863077acf9a64e4b3720e479855b88d19e82.1592261248.git.robin.murphy@arm.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -245,13 +245,13 @@ static inline void bcm2835_rd_fifo_count(struct bcm2835_spi *bs, int count)
 
 	bs->rx_len -= count;
 
-	while (count > 0) {
+	do {
 		val = bcm2835_rd(bs, BCM2835_SPI_FIFO);
 		len = min(count, 4);
 		memcpy(bs->rx_buf, &val, len);
 		bs->rx_buf += len;
 		count -= 4;
-	}
+	} while (count > 0);
 }
 
 /**
@@ -271,7 +271,7 @@ static inline void bcm2835_wr_fifo_count(struct bcm2835_spi *bs, int count)
 
 	bs->tx_len -= count;
 
-	while (count > 0) {
+	do {
 		if (bs->tx_buf) {
 			len = min(count, 4);
 			memcpy(&val, bs->tx_buf, len);
@@ -281,7 +281,7 @@ static inline void bcm2835_wr_fifo_count(struct bcm2835_spi *bs, int count)
 		}
 		bcm2835_wr(bs, BCM2835_SPI_FIFO, val);
 		count -= 4;
-	}
+	} while (count > 0);
 }
 
 /**
@@ -310,12 +310,11 @@ static inline void bcm2835_rd_fifo_blind(struct bcm2835_spi *bs, int count)
 	count = min(count, bs->rx_len);
 	bs->rx_len -= count;
 
-	while (count) {
+	do {
 		val = bcm2835_rd(bs, BCM2835_SPI_FIFO);
 		if (bs->rx_buf)
 			*bs->rx_buf++ = val;
-		count--;
-	}
+	} while (--count);
 }
 
 /**
@@ -330,11 +329,10 @@ static inline void bcm2835_wr_fifo_blind(struct bcm2835_spi *bs, int count)
 	count = min(count, bs->tx_len);
 	bs->tx_len -= count;
 
-	while (count) {
+	do {
 		val = bs->tx_buf ? *bs->tx_buf++ : 0;
 		bcm2835_wr(bs, BCM2835_SPI_FIFO, val);
-		count--;
-	}
+	} while (--count);
 }
 
 static void bcm2835_spi_reset_hw(struct bcm2835_spi *bs)