Subject: [PATCH V2 2/2] ARM: dra7xx: Fix counter frequency drift for AM572x errata i856.

From: [email-masked] (Lokesh Vutla)
Date: Wed, 17 Dec 2014 14:06:38 +0530

Hi Len,
On Wednesday 17 December 2014 02:38 AM, Lennart Sorensen wrote:
> Errata i856 for the AM572x (DRA7xx) points out that the 32.768KHz external
> crystal is not enabled at power up.  Instead the CPU falls back to using
> an emulation for the 32KHz clock which is SYSCLK1/610.  SYSCLK1 is usually
> 20MHz on boards so far (which gives an emulated frequency of 32.786KHz),
> but can also be 19.2 or 27MHz which result in much larger drift.
> 
> Since this is used to drive the master counter at 32.768KHz * 375 /
> 2 = 6.144MHz, the emulated speed for 20MHz is of by 570ppm, or about 43
> seconds per day, and more than the 500ppm NTP is able to tolerate.
> 
> Checking the CTRL_CORE_BOOTSTRAP register can determine if the CPU
> is using the real 32.768KHz crystal or the emulated SYSCLK1/610, and
> by known that the real counter frequency can be determined and used.
> The real speed is then SYSCLK1 / 610 * 375 / 2 or SYSCLK1 * 75 / 244.
Looks good to me.
Tested this on DRA7 evm.

Tested-by: Lokesh Vutla <[email-masked]>

Thanks and regards,
Lokesh

> 
> Signed-off-by: Len Sorensen <[email-masked]>
> ---
>  arch/arm/mach-omap2/control.h |    4 ++++
>  arch/arm/mach-omap2/timer.c   |   36 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/mach-omap2/control.h b/arch/arm/mach-omap2/control.h
> index a3c0133..a80ac2d 100644
> --- a/arch/arm/mach-omap2/control.h
> +++ b/arch/arm/mach-omap2/control.h
> @@ -286,6 +286,10 @@
>  #define OMAP5XXX_CONTROL_STATUS                0x134
>  #define OMAP5_DEVICETYPE_MASK          (0x7 << 6)
>  
> +/* DRA7XX CONTROL CORE BOOTSTRAP */
> +#define DRA7_CTRL_CORE_BOOTSTRAP	0x6c4
> +#define DRA7_SPEEDSELECT_MASK		(0x3 << 8)
> +
>  /*
>   * REVISIT: This list of registers is not comprehensive - there are more
>   * that should be added.
> diff --git a/arch/arm/mach-omap2/timer.c b/arch/arm/mach-omap2/timer.c
> index fb0cb2b..7d45c84 100644
> --- a/arch/arm/mach-omap2/timer.c
> +++ b/arch/arm/mach-omap2/timer.c
> @@ -54,6 +54,7 @@
>  
>  #include "soc.h"
>  #include "common.h"
> +#include "control.h"
>  #include "powerdomain.h"
>  #include "omap-secure.h"
>  
> @@ -496,7 +497,8 @@ static void __init realtime_counter_init(void)
>  	void __iomem *base;
>  	static struct clk *sys_clk;
>  	unsigned long rate;
> -	unsigned int reg, num, den;
> +	unsigned int reg;
> +	unsigned long long num, den;
>  
>  	base = ioremap(REALTIME_COUNTER_BASE, SZ_32);
>  	if (!base) {
> @@ -511,6 +513,35 @@ static void __init realtime_counter_init(void)
>  	}
>  
>  	rate = clk_get_rate(sys_clk);
> +
> +	if (soc_is_dra7xx()) {
> +		/*
> +		 * Errata i856 says the 32.768KHz crystal does not start at
> +		 * power on, so the CPU falls back to an emulated 32KHz clock
> +		 * based on sysclk / 610 instead. This causes the master counter
> +		 * frequency to not be 6.144MHz but at sysclk / 610 * 375 / 2
> +		 * (OR sysclk * 75 / 244)
> +		 *
> +		 * This affects at least the DRA7/AM572x 1.0, 1.1 revisions.
> +		 * Of course any board built without a populated 32.768KHz
> +		 * crystal would also need this fix even if the CPU is fixed
> +		 * later.
> +		 *
> +		 * Either case can be detected by using the two speedselect bits
> +		 * If they are not 0, then the 32.768KHz clock driving the
> +		 * coarse counter that corrects the fine counter every time it
> +		 * ticks is actually rate/610 rather than 32.768KHz and we
> +		 * should compensate to avoid the 570ppm (at 20MHz, much worse
> +		 * at other rates) too fast system time.
> +		 */
> +		reg = omap_ctrl_readl(DRA7_CTRL_CORE_BOOTSTRAP);
> +		if (reg & DRA7_SPEEDSELECT_MASK) {
> +			num = 75;
> +			den = 244;
> +			goto sysclk1_based;
> +		}
> +	}
> +
>  	/* Numerator/denumerator values refer TRM Realtime Counter section */
>  	switch (rate) {
>  	case 12000000:
> @@ -545,6 +576,7 @@ static void __init realtime_counter_init(void)
>  		break;
>  	}
>  
> +sysclk1_based:
>  	/* Program numerator and denumerator registers */
>  	reg = readl_relaxed(base + INCREMENTER_NUMERATOR_OFFSET) &
>  			NUMERATOR_DENUMERATOR_MASK;
> @@ -556,7 +588,7 @@ static void __init realtime_counter_init(void)
>  	reg |= den;
>  	writel_relaxed(reg, base + INCREMENTER_DENUMERATOR_RELOAD_OFFSET);
>  
> -	arch_timer_freq = (rate / den) * num;
> +	arch_timer_freq = DIV_ROUND_UP_ULL(rate * num, den);
>  	set_cntfreq();
>  
>  	iounmap(base);
>