arm, omap3: fix warm reset serial output on OMAP36xx/AM/DM37xx

In warm reset conditions on OMAP36xx/AM/DM37xx the rom code
incorrectly sets the DPLL4 clock input divider to /6.5 which
is an invalid value unless the input clock is 13MHz. When a JTAG
emulator is attached, a warm reset is necessary after the emulator
gains control of the process. This results in a loss of serial
output due to the invalid DPLL4 settings.

This patch fixes the issue by resetting the DPLL4 clock input
divider to /1 when the input clock is not 13MHz. AM/DM37x TRM
section 3.5.3.3.3.2.1 specifies that the /6.5 setting is only
used when the input clock is 13MHz.

Signed-off-by: Matt Porter <mporter@ti.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/arch/arm/cpu/armv7/omap3/clock.c b/arch/arm/cpu/armv7/omap3/clock.c
index 567817e..09c51f6 100644
--- a/arch/arm/cpu/armv7/omap3/clock.c
+++ b/arch/arm/cpu/armv7/omap3/clock.c
@@ -572,6 +572,22 @@ void prcm_init(void)
 	}
 
 	if (get_cpu_family() == CPU_OMAP36XX) {
+		/*
+		 * In warm reset conditions on OMAP36xx/AM/DM37xx
+		 * the rom code incorrectly sets the DPLL4 clock
+		 * input divider to /6.5. Section 3.5.3.3.3.2.1 of
+		 * the AM/DM37x TRM explains that the /6.5 divider
+		 * is used only when the input clock is 13MHz.
+		 *
+		 * If the part is in this cpu family *and* the input
+		 * clock *is not* 13 MHz, then reset the DPLL4 clock
+		 * input divider to /1 as it should never set to /6.5
+		 * in this case.
+		 */
+		if (sys_clkin_sel != 1) /* 13 MHz */
+			/* Bit 8: DPLL4_CLKINP_DIV */
+			sr32(&prm_base->clksrc_ctrl, 8, 1, 0);
+
 		/* Unlock MPU DPLL (slows things down, and needed later) */
 		sr32(&prcm_base->clken_pll_mpu, 0, 3, PLL_LOW_POWER_BYPASS);
 		wait_on_value(ST_MPU_CLK, 0, &prcm_base->idlest_pll_mpu,