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

[Nishanth Menon]

On 14:06-20141217, Lokesh Vutla wrote:
> 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 <lokeshvutla at ti.com>
> 
> Thanks and regards,
> Lokesh
> 
> > 
> > Signed-off-by: Len Sorensen <lsorense at csclub.uwaterloo.ca>
> > ---
> >  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);
> > 
> 
If tony is ok with the arch_timer_freq change in this patch, I am ok
with it as well.
-- 
Regards,
Nishanth Menon