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List:       git-commits-head
Subject:    spi: davinci: Set prescale value based on register value
From:       "Linux Kernel Mailing List" <linux-kernel () vger ! kernel ! org>
Date:       2015-08-31 23:02:10
Message-ID: 20150831230210.13DF9661216 () gitolite ! kernel ! org
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Web:        https://git.kernel.org/torvalds/c/bba732d86694f7217a41e83a2c1deb42ed9aa7fd
Commit:     bba732d86694f7217a41e83a2c1deb42ed9aa7fd
Parent:     bc0195aad0daa2ad5b0d76cce22b167bc3435590
Refname:    refs/heads/master
Author:     Franklin S Cooper Jr <fcooper@ti.com>
AuthorDate: Wed Jul 22 07:32:21 2015 -0500
Committer:  Mark Brown <broonie@kernel.org>
CommitDate: Fri Jul 24 17:35:48 2015 +0100

    spi: davinci: Set prescale value based on register value
    
    Within davinci_spi_get_prescale() the prescale has two meanings. First one
    being the calculated prescale value and then at the end translates it to the
    prescale value that will be written to the SPI register.
    
    At first glance this can be confusing especially when comparing the minimum
    prescale value against what is seen in the TRM.
    
    To simplify things make it clear that the calculated prescale value will always
    be based on the value that will be written into the SPI register.
    
    Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
    Signed-off-by: Mark Brown <broonie@kernel.org>
---
 drivers/spi/spi-davinci.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-davinci.c b/drivers/spi/spi-davinci.c
index 987afeb..b4605c4 100644
--- a/drivers/spi/spi-davinci.c
+++ b/drivers/spi/spi-davinci.c
@@ -255,7 +255,7 @@ static void davinci_spi_chipselect(struct spi_device *spi, int value)
  * This function calculates the prescale value that generates a clock rate
  * less than or equal to the specified maximum.
  *
- * Returns: calculated prescale - 1 for easy programming into SPI registers
+ * Returns: calculated prescale value for easy programming into SPI registers
  * or negative error number if valid prescalar cannot be updated.
  */
 static inline int davinci_spi_get_prescale(struct davinci_spi *dspi,
@@ -263,12 +263,13 @@ static inline int davinci_spi_get_prescale(struct davinci_spi *dspi,
 {
 	int ret;
 
-	ret = DIV_ROUND_UP(clk_get_rate(dspi->clk), max_speed_hz);
+	/* Subtract 1 to match what will be programmed into SPI register. */
+	ret = DIV_ROUND_UP(clk_get_rate(dspi->clk), max_speed_hz) - 1;
 
-	if (ret < 1 || ret > 256)
+	if (ret < 0 || ret > 255)
 		return -EINVAL;
 
-	return ret - 1;
+	return ret;
 }
 
 /**
--
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