[PATCH v2 2/2] spi: sunxi: fix clock divider calculation for max frequency setting

[Michael Walle <mwalle@kernel.org>]

If the maximum frequency is requested, we still fall into the CDR2
handling. But there the minimal divider is 2. For the sun6i and sun8i we
can do better with the CDR1 setting where the minimal divider is 1:
  SPI_CLK = MOD_CLK / 2 ^ cdr with cdr = 0

Thus, handle the div = 1 case specially.

While at it, correct the comment above the calculation.

Signed-off-by: Michael Walle <mwalle@kernel.org>
---
 drivers/spi/spi-sunxi.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi-sunxi.c b/drivers/spi/spi-sunxi.c
index f110a8b7658..81f1298adea 100644
--- a/drivers/spi/spi-sunxi.c
+++ b/drivers/spi/spi-sunxi.c
@@ -249,6 +249,8 @@ static void sun4i_spi_set_speed_mode(struct udevice *dev)
 	 * We have two choices there. Either we can use the clock
 	 * divide rate 1, which is calculated thanks to this formula:
 	 * SPI_CLK = MOD_CLK / (2 ^ (cdr + 1))
+	 * Or for sun6i/sun8i variants:
+	 * SPI_CLK = MOD_CLK / (2 ^ cdr)
 	 * Or we can use CDR2, which is calculated with the formula:
 	 * SPI_CLK = MOD_CLK / (2 * (cdr + 1))
 	 * Whether we use the former or the latter is set through the
@@ -256,13 +258,16 @@ static void sun4i_spi_set_speed_mode(struct udevice *dev)
 	 *
 	 * First try CDR2, and if we can't reach the expected
 	 * frequency, fall back to CDR1.
+	 * There is one exception if the requested clock is the input
+	 * clock. In that case we always use CDR1 because we'll get a
+	 * 1:1 ration for sun6i/sun8i variants.
 	 */
 
 	div = DIV_ROUND_UP(SUNXI_INPUT_CLOCK, priv->freq);
 	div_cdr2 = DIV_ROUND_UP(div, 2);
 	reg = readl(SPI_REG(priv, SPI_CCR));
 
-	if (div_cdr2 <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
+	if (div != 1 && (div_cdr2 <= (SUN4I_CLK_CTL_CDR2_MASK + 1))) {
 		reg &= ~(SUN4I_CLK_CTL_CDR2_MASK | SUN4I_CLK_CTL_DRS);
 		reg |= SUN4I_CLK_CTL_CDR2(div_cdr2 - 1) | SUN4I_CLK_CTL_DRS;
 	} else {
-- 
2.39.2