[U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm

[Dirk Behme]

The spi clock divisor is of the form x * (2**y),  or  x  << y, where x is
1 to 16, and y is 0 to 15. Note the similarity with floating point numbers.
Convert the desired divisor to the smallest number which is >= desired divisor,
and can be represented in this form. The previous algorithm chose a divisor
which could be almost twice as large as needed.

Signed-off-by: Troy Kisky <troy.kisky at boundarydevices.com>
Signed-off-by: Dirk Behme <dirk.behme at gmail.com>
---
 drivers/spi/mxc_spi.c |   27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
index 3e903b3..66c2ad8 100644
--- a/drivers/spi/mxc_spi.c
+++ b/drivers/spi/mxc_spi.c
@@ -128,7 +128,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
 		unsigned int max_hz, unsigned int mode)
 {
 	u32 clk_src = mxc_get_clock(MXC_CSPI_CLK);
-	s32 pre_div = 1, post_div = 0, i, reg_ctrl, reg_config;
+	s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config;
 	u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
 	struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
 
@@ -147,27 +147,24 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
 	reg_ctrl |=  MXC_CSPICTRL_EN;
 	reg_write(&regs->ctrl, reg_ctrl);
 
-	/*
-	 * The following computation is taken directly from Freescale's code.
-	 */
 	if (clk_src > max_hz) {
 		pre_div = DIV_ROUND_UP(clk_src, max_hz);
-		if (pre_div > 16) {
-			post_div = pre_div / 16;
-			pre_div = 16;
-		}
-		if (post_div != 0) {
-			for (i = 0; i < 16; i++) {
-				if ((1 << i) >= post_div)
-					break;
-			}
-			if (i == 16) {
+		/* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
+		post_div = fls(pre_div - 1);
+		if (post_div > 4) {
+			post_div -= 4;
+
+			if (post_div >= 16) {
 				printf("Error: no divider for the freq: %d\n",
 					max_hz);
 				return -1;
 			}
-			post_div = i;
+			pre_div = (pre_div + (1 << post_div) - 1) >> post_div;
+
+		} else {
+			post_div = 0;
 		}
+
 	}
 
 	debug("pre_div = %d, post_div=%d\n", pre_div, post_div);
-- 
1.7.10.4