[PATCH 3/4] i2c: nomadik: fix BRCR computation

["Théo Lebrun" <theo.lebrun@bootlin.com>]

Current BRCR computation is:

    brcr = floor(i2cclk / (clkfreq * div))

With brcr: "baud rate counter", an internal clock divider,
 and i2cclk: input clock rate (24MHz, 38.4MHz or 48MHz),
 and clkfreq: desired bus rate,
 and div: speed-mode dependent divider (2 for standard, 3 otherwise).

Assume i2cclk=48MHz, clkfreq=3.4MHz, div=3,
  then brcr = floor(48MHz / (3.4MHz * 3)) = 4
   and resulting bus rate = 48MHz / (4 * 3) = 4MHz

Assume i2cclk=38.4MHz, clkfreq=1.0MHz, div=3,
  then brcr = floor(38.4MHz / (1.0MHz * 3)) = 12
   and resulting bus rate = 38.4MHz / (12 * 3) = 1066kHz

The current computation means we always pick the smallest divider that
gives a bus rate above target. We should instead pick the largest
divider that gives a bus rate below target, using:

    brcr = floor(i2cclk / (clkfreq * div)) + 1

If we redo the above examples:

Assume i2cclk=48MHz, clkfreq=3.4MHz, div=3,
  then brcr = floor(48MHz / (3.4MHz * 3)) + 1 = 5
   and resulting bus rate = 48MHz / (5 * 3) = 3.2MHz

Assume i2cclk=38.4MHz, clkfreq=1.0MHz, div=3,
  then brcr = floor(38.4MHz / (1.0MHz * 3)) + 1 = 13
   and resulting bus rate = 38.4MHz / (13 * 3) = 985kHz

This is much less of an issue with slower bus rates (ie those currently
supported), because the gap from one divider to the next is much
smaller. It however keeps us from always using bus rates superior to
the target.

This fix is required for later on supporting faster bus rates:
I2C_FREQ_MODE_FAST_PLUS (1MHz) and I2C_FREQ_MODE_HIGH_SPEED (3.4MHz).

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/i2c/busses/i2c-nomadik.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-nomadik.c b/drivers/i2c/busses/i2c-nomadik.c
index ea511d3a58073eaedb63850026e05b59427a69c6..68ce39352d67477fa22424e2dc0f8d1741498cd1 100644
--- a/drivers/i2c/busses/i2c-nomadik.c
+++ b/drivers/i2c/busses/i2c-nomadik.c
@@ -453,9 +453,12 @@ static void setup_i2c_controller(struct nmk_i2c_dev *priv)
 	 * operation, and the other is for std, fast mode, fast mode
 	 * plus operation. Currently we do not supprt high speed mode
 	 * so set brcr1 to 0.
+	 *
+	 * BRCR is a clock divider amount. Pick highest value that
+	 * leads to rate strictly below target.
 	 */
 	brcr1 = FIELD_PREP(I2C_BRCR_BRCNT1, 0);
-	brcr2 = FIELD_PREP(I2C_BRCR_BRCNT2, i2c_clk / (priv->clk_freq * div));
+	brcr2 = FIELD_PREP(I2C_BRCR_BRCNT2, i2c_clk / (priv->clk_freq * div) + 1);
 
 	/* set the baud rate counter register */
 	writel((brcr1 | brcr2), priv->virtbase + I2C_BRCR);

-- 
2.46.2