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Subject[PATCH 3/3] spi: bcm2835: Micro-optimise FIFO loops
Date
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>
---
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
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