[PATCH 3/7] spi: sunxi: improve SPI clock calculation

Andre Przywara
2022-05-03T21:21:39Z

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 calculation 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>
---
drivers/spi/spi-sunxi.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-sunxi.c b/drivers/spi/spi-sunxi.c
index d6b2dd09514..d1e8692ede2 100644
--- 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_US 1000000
@@ -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);
}
--
2.35.3