linux-kernel - [PATCH 3/3] spi: bcm2835: Micro-optimise FIFO loops

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Date:   Tue, 16 Jun 2020 01:09:29 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     broonie@...nel.org, nsaenzjulienne@...e.de, f.fainelli@...il.com,
        rjui@...adcom.com, sbranden@...adcom.com
Cc:     bcm-kernel-feedback-list@...adcom.com, linux-spi@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, kernel@...tin.sperl.org,
        lukas@...ner.de, linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] 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@....com>
---
 drivers/spi/spi-bcm2835.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index aec70ac8911e..e9a91adf03eb 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/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)
-- 
2.23.0.dirty