Subject: [PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for AM572x errata i856.

From: [email-masked] (Tero Kristo)
Date: Wed, 17 Dec 2014 15:21:27 +0200

On 12/16/2014 04:58 PM, Nishanth Menon wrote:
> On 17:05-20141216, Lokesh Vutla wrote:
> [...]
>>
>> @@ -545,6 +546,16 @@ static void __init realtime_counter_init(void)
>>   		break;
>>   	}
>>
>> +	if (soc_is_dra7xx()) {
>> +		reg = omap_ctrl_readl(DRA7_CTRL_CORE_BOOTSTRAP);
>> +		reg = reg & DRA7_SPEEDSELECT_MASK;
>> +
>> +		if (reg) {
>> +			num = 75;
>> +			den = 244;
>> +		}
>> +	}
>> +
>>   	/* Program numerator and denumerator registers */
>>   	reg = readl_relaxed(base + INCREMENTER_NUMERATOR_OFFSET) &
>>   			NUMERATOR_DENUMERATOR_MASK;
>
> A) So, it does look like the 32k node is actually wrong then -> Tero:
> any comments? should'nt this now be modeled based on
> CTRL_CORE_BOOTSTRAP::SPEEDSELECT considering that clock nodes do have
> clk mux options based on the 32k..
> sys_32k_ck: sys_32k_ck {
> 	#clock-cells = <0>;
> 	compatible = "fixed-clock";
> 	clock-frequency = <32768>;
> };

Yea I think the 32k clock node should be fixed based on this. Something 
like this:

--- a/arch/arm/boot/dts/dra7xx-clocks.dtsi
+++ b/arch/arm/boot/dts/dra7xx-clocks.dtsi
@@ -100,8 +100,10 @@

         sys_32k_ck: sys_32k_ck {
                 #clock-cells = <0>;
-               compatible = "fixed-clock";
-               clock-frequency = <32768>;
+               compatible = "fixed-factor-clock";
+               clocks = <&sys_clkin1>;
+               clock-mult = <1>;
+               clock-div = <610>;
         };

         virt_12000000_ck: virt_12000000_ck {

It might be better then just query the actual clock rate from the timer 
code.

-Tero

>
> B) Since rate switch is no longer needed, how about something like the
> following:
> diff --git a/arch/arm/mach-omap2/control.h b/arch/arm/mach-omap2/control.h
> index a3c0133..315d6d1 100644
> --- a/arch/arm/mach-omap2/control.h
> +++ b/arch/arm/mach-omap2/control.h
> @@ -286,6 +286,11 @@
>   #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 4f61148..783d3c3 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"
>
> @@ -511,6 +512,35 @@ 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 in 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
> +		 * course 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 sysclk_based;
> +		}
> +	}
> +
>   	/* Numerator/denumerator values refer TRM Realtime Counter section */
>   	switch (rate) {
>   	case 1200000:
> @@ -545,6 +575,7 @@ static void __init realtime_counter_init(void)
>   		break;
>   	}
>
> +sysclk_based:
>   	/* Program numerator and denumerator registers */
>   	reg = readl_relaxed(base + INCREMENTER_NUMERATOR_OFFSET) &
>   			NUMERATOR_DENUMERATOR_MASK;
>