[U-Boot] [PATCH] spi: mxc_spi: Fix pre and post divider calculation

[Dirk Behme]

From: Dirk Behme <dirk.behme at gmail.com>

Fix two issues with the calculation of pre_div and post_div:

1. pre_div: While the calculation of pre_div looks correct, to set the
CONREG[15-12] bits pre_div needs to be decremented by 1:

The i.MX 6Dual/6Quad Applications Processor Reference Manual (IMX6DQRM
Rev. 0, 11/2012) states:

CONREG[15-12]: PRE_DIVIDER
0000 Divide by 1
0001 Divide by 2
0010 Divide by 3
...
1101 Divide by 14
1110 Divide by 15
1111 Divide by 16

I.e. if we want to divide by 2, we have to write 1 to CONREG[15-12].

2. In case the post divider becomes necessary, pre_div will be set to 15. To
calculate the post divider, divide by 15, too. And not 16.

Both issues above are tested using the following examples:

clk_src = 60000000 (60MHz, default i.MX6 ECSPI clock)

a) max_hz == 23000000 (23MHz, max i.MX6 ECSPI read clock)

-> pre_div =  3 (divide by 3 => CONREG[15-12] == 2)
-> post_div = 0 (divide by 1 => CONREG[11- 8] == 0)
               => 60MHz / 3 = 20MHz SPI clock

b) max_hz == 2000000 (2MHz)

-> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
-> post_div = 1  (divide by  2 => CONREG[11- 8] == 1)
               => 60MHz / 30 = 2MHz SPI clock

c) max_hz == 1000000 (1MHz)

-> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
-> post_div = 2  (divide by  4 => CONREG[11- 8] == 2)
               => 60MHz / 60 = 1MHz SPI clock

d) max_hz == 500000 (500kHz)

-> pre_div =  15 (divide by 15 => CONREG[15-12] == 14)
-> post_div = 3  (divide by  8 => CONREG[11- 8] == 3)
               => 60MHz / 120 = 500kHz SPI clock

Signed-off-by: Dirk Behme <dirk.behme at gmail.com>
---
 drivers/spi/mxc_spi.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
index 5bed858..8630bbd 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 = 0, post_div = 0, i, reg_ctrl, reg_config;
+	s32 pre_div = 1, post_div = 0, i, reg_ctrl, reg_config;
 	u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
 	struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
 
@@ -153,7 +153,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
 	if (clk_src > max_hz) {
 		pre_div = DIV_ROUND_UP(clk_src, max_hz);
 		if (pre_div > 16) {
-			post_div = pre_div / 16;
+			post_div = pre_div / 15;
 			pre_div = 15;
 		}
 		if (post_div != 0) {
@@ -174,7 +174,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave *mxcs, unsigned int cs,
 	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_SELCHAN(3)) |
 		MXC_CSPICTRL_SELCHAN(cs);
 	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_PREDIV(0x0F)) |
-		MXC_CSPICTRL_PREDIV(pre_div);
+		MXC_CSPICTRL_PREDIV(pre_div - 1);
 	reg_ctrl = (reg_ctrl & ~MXC_CSPICTRL_POSTDIV(0x0F)) |
 		MXC_CSPICTRL_POSTDIV(post_div);
 
-- 
1.8.2