Subject: [PATCH v2] spi: spi-sun4i, spi-sun6i: Fix bit rate calculation

From: [email-masked] (Hermann Kraus)
Date: Thu, 06 Aug 2015 22:51:10 +0200

Use requested bit rate instead of maximum possible and correctly calculate
the divider.

There are two different maximum bitrates. "max_speed_hz" which is the
maximum for a given SPI device and "speed_hz" in "struct spi_transfer"
which is the rate used for this transfer. "speed_hz" must always be
used as the actual transfer rate.
The divider must be calculated by calling ilog2 only once, because this
function truncates the fractional part. Calling it twice increases
the error so much that certain bit rates which are available from the
hardware can't be reached by the kernel.
The result of this calculation must be checked to fit into the register
size instead of being truncated silently.

Signed-off-by: Hermann Kraus <[email-masked]>
---

Changes to v1: Unconditionally use tfr->speed_hz as requested by Mark  
Brown.

  drivers/spi/spi-sun4i.c | 15 +++++++++++----
  drivers/spi/spi-sun6i.c | 15 +++++++++++----
  2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
index fbb0a4d..729d70b 100644
--- a/drivers/spi/spi-sun4i.c
+++ b/drivers/spi/spi-sun4i.c
@@ -229,8 +229,8 @@ static int sun4i_spi_transfer_one(struct spi_master  
*master,

  	/* Ensure that we have a parent clock fast enough */
  	mclk_rate = clk_get_rate(sspi->mclk);
-	if (mclk_rate < (2 * spi->max_speed_hz)) {
-		clk_set_rate(sspi->mclk, 2 * spi->max_speed_hz);
+	if (mclk_rate < (2 * tfr->speed_hz)) {
+		clk_set_rate(sspi->mclk, 2 * tfr->speed_hz);
  		mclk_rate = clk_get_rate(sspi->mclk);
  	}

@@ -248,14 +248,21 @@ static int sun4i_spi_transfer_one(struct spi_master  
*master,
  	 * First try CDR2, and if we can't reach the expected
  	 * frequency, fall back to CDR1.
  	 */
-	div = mclk_rate / (2 * spi->max_speed_hz);
+	div = mclk_rate / (2 * tfr->speed_hz);
  	if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
  		if (div > 0)
  			div--;

  		reg = SUN4I_CLK_CTL_CDR2(div) | SUN4I_CLK_CTL_DRS;
  	} else {
-		div = ilog2(mclk_rate) - ilog2(spi->max_speed_hz);
+		/* ilog2 truncates the fractional part. Therefore we don't
+		 * subtract 1 from the result. */
+		div = ilog2(mclk_rate / tfr->speed_hz + 1);
+		if (div & ~SUN4I_CLK_CTL_CDR1_MASK) {
+			/* Can't reach low enough bit rate. */
+			ret = -EINVAL;
+			goto out;
+		}
  		reg = SUN4I_CLK_CTL_CDR1(div);
  	}

diff --git a/drivers/spi/spi-sun6i.c b/drivers/spi/spi-sun6i.c
index ac48f59..2d3dcda 100644
--- a/drivers/spi/spi-sun6i.c
+++ b/drivers/spi/spi-sun6i.c
@@ -217,8 +217,8 @@ static int sun6i_spi_transfer_one(struct spi_master  
*master,

  	/* Ensure that we have a parent clock fast enough */
  	mclk_rate = clk_get_rate(sspi->mclk);
-	if (mclk_rate < (2 * spi->max_speed_hz)) {
-		clk_set_rate(sspi->mclk, 2 * spi->max_speed_hz);
+	if (mclk_rate < (2 * tfr->speed_hz)) {
+		clk_set_rate(sspi->mclk, 2 * tfr->speed_hz);
  		mclk_rate = clk_get_rate(sspi->mclk);
  	}

@@ -236,14 +236,21 @@ static int sun6i_spi_transfer_one(struct spi_master  
*master,
  	 * First try CDR2, and if we can't reach the expected
  	 * frequency, fall back to CDR1.
  	 */
-	div = mclk_rate / (2 * spi->max_speed_hz);
+	div = mclk_rate / (2 * tfr->speed_hz);
  	if (div <= (SUN6I_CLK_CTL_CDR2_MASK + 1)) {
  		if (div > 0)
  			div--;

  		reg = SUN6I_CLK_CTL_CDR2(div) | SUN6I_CLK_CTL_DRS;
  	} else {
-		div = ilog2(mclk_rate) - ilog2(spi->max_speed_hz);
+		/* ilog2 truncates the fractional part. Therefore we don't
+		 * subtract 1 from the result. */
+		div = ilog2(mclk_rate / tfr->speed_hz + 1);
+		if (div & ~SUN6I_CLK_CTL_CDR1_MASK) {
+			/* Can't reach low enough bit rate. */
+			ret = -EINVAL;
+			goto out;
+		}
  		reg = SUN6I_CLK_CTL_CDR1(div);
  	}

-- 
2.1.4