spi: bcm2835: Avoid 64-bit arithmetic in xfer len calc

Avoid 64 bit aritmetics when deciding if we need to use polling or not
This replicates: commit d704afffe65c
("spi: bcm2835aux: Avoid 64-bit arithmetic in xfer len calc")
from spi-bcm2835aux

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Changelog:
  V1 -> V2: applied feedback by Stefan Wahren
            reorganized patchset
	    added extra rational, descriptions
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 6c73e69..b7bb07c 100644 (file)
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -749,7 +749,6 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
        struct bcm2835_spi *bs = spi_master_get_devdata(master);
        unsigned long spi_hz, clk_hz, cdiv;
        unsigned long spi_used_hz;
-       unsigned long long xfer_time_us;
        u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
 
        /* set clock */
@@ -790,14 +789,15 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
        bs->tx_len = tfr->len;
        bs->rx_len = tfr->len;
 
-       /* calculate the estimated time in us the transfer runs */
-       xfer_time_us = (unsigned long long)tfr->len
-               * 9 /* clocks/byte - SPI-HW waits 1 clock after each byte */
-               * 1000000;
-       do_div(xfer_time_us, spi_used_hz);
-
-       /* for short requests run polling*/
-       if (xfer_time_us <= BCM2835_SPI_POLLING_LIMIT_US)
+       /* Calculate the estimated time in us the transfer runs.  Note that
+        * there is 1 idle clocks cycles after each byte getting transferred
+        * so we have 9 cycles/byte.  This is used to find the number of Hz
+        * per byte per polling limit.  E.g., we can transfer 1 byte in 30 us
+        * per 300,000 Hz of bus clock.
+        */
+#define HZ_PER_BYTE ((9 * 1000000) / BCM2835_SPI_POLLING_LIMIT_US)
+       /* run in polling mode for short transfers */
+       if (tfr->len < spi_used_hz / HZ_PER_BYTE)
                return bcm2835_spi_transfer_one_poll(master, spi, tfr, cs);
 
        /* run in dma mode if conditions are right */