spi: sunxi: improve SPI clock calculation

The current SPI clock divider calculation has two problems:
- We use a normal round-down division, which results in a divider
  typically being too small, resulting in a too high frequency on the bus.
- The calculaction for the power-of-two divider is very inaccurate, and
  again rounds down, which might lead to wild bus frequencies.

This wasn't a real problem so far, since most chips can handle slightly
higher bus frequencies just fine. Also the actual speed was mostly lost
anyway, due to release_bus() reseting the device. And the power-of-2
calculation was probably never used, because it only applies to
frequencies below 47 KHz.
However this will become a problem for the F1C100s support, due to its
much higher base frequency.

Calculate a safe divider correctly (using round-up), and re-use that
value when calculating the power-of-2 value. We also separate the
maximum frequency and the input clock on the way, since they will be
different for the F1C100s.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

diff --git a/drivers/spi/spi-sunxi.c b/drivers/spi/spi-sunxi.c
index 5d7ca5b..6282949 100644 (file)
--- a/drivers/spi/spi-sunxi.c
+++ b/drivers/spi/spi-sunxi.c
@@ -72,7 +72,9 @@ DECLARE_GLOBAL_DATA_PTR;
 #define SUN4I_XMIT_CNT(cnt)            ((cnt) & SUN4I_MAX_XFER_SIZE)
 #define SUN4I_FIFO_STA_RF_CNT_BITS     0
 
-#define SUN4I_SPI_MAX_RATE             24000000
+/* the SPI mod clock, defaulting to be 1/1 of the HOSC@24MHz */
+#define SUNXI_INPUT_CLOCK              24000000        /* 24 MHz */
+#define SUN4I_SPI_MAX_RATE             SUNXI_INPUT_CLOCK
 #define SUN4I_SPI_MIN_RATE             3000
 #define SUN4I_SPI_DEFAULT_RATE         1000000
 #define SUN4I_SPI_TIMEOUT_MS           1000
@@ -242,17 +244,18 @@ static void sun4i_spi_set_speed_mode(struct udevice *dev)
         * frequency, fall back to CDR1.
         */
 
-       div = SUN4I_SPI_MAX_RATE / (2 * priv->freq);
+       div = DIV_ROUND_UP(SUNXI_INPUT_CLOCK, priv->freq);
        reg = readl(SPI_REG(priv, SPI_CCR));
 
-       if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
+       if ((div / 2) <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
+               div /= 2;
                if (div > 0)
                        div--;
 
                reg &= ~(SUN4I_CLK_CTL_CDR2_MASK | SUN4I_CLK_CTL_DRS);
                reg |= SUN4I_CLK_CTL_CDR2(div) | SUN4I_CLK_CTL_DRS;
        } else {
-               div = __ilog2(SUN4I_SPI_MAX_RATE) - __ilog2(priv->freq);
+               div = fls(div - 1);
                reg &= ~((SUN4I_CLK_CTL_CDR1_MASK << 8) | SUN4I_CLK_CTL_DRS);
                reg |= SUN4I_CLK_CTL_CDR1(div);
        }