linux-kernel - Re: [PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for AM572x errata i856.

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Date:	Wed, 17 Dec 2014 09:22:25 -0600
From:	Nishanth Menon <nm@...com>
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
CC:	Tero Kristo <t-kristo@...com>, Lokesh Vutla <lokeshvutla@...com>,
	<linux-omap@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Sekhar Nori <nsekhar@...com>
Subject: Re: [PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for
 AM572x errata i856.

On 09:55-20141217, Lennart Sorensen wrote:
> On Wed, Dec 17, 2014 at 03:21:27PM +0200, Tero Kristo wrote:
> > 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.
> 
> But it is only SYSCLK1 / 610 if the DRA7_CTRL_CORE_BOOTSTRAP register
> says it is.  Otherwise is is in fact 32768Hz.  I certainly would expect
> that if another revision of the chip is made (and even for the single
> core AM571x chips when they are done) will have this fixed and will work
> with the external 32.768KHz crystal, if it is present.
> 
> So how does one make the dtb reflect what the state of the CPU actually
> is?
> 
> The errata even says that if SYSCLK1 is 26MHz (not a supported option
> in the manual), then the 32.768KHz crystal does work and the counter
> frequency will in fact be 6.144MHz like it should be.
> 
> So just changing the dtb is not an option.

A clock mux might do the job?

value 1, 2 , 3 will imply sysclk1 / 610
value of 0 implies fixed 32768

soemthing like
sys_clk32_crystal {
               compatible = "fixed-clock";
               clock-frequency = <32768>;
}

sys_clk32_pseudo {
               compatible = "fixed-clock";
               compatible = "fixed-factor-clock";
               clocks = <&sys_clkin1>;
               clock-mult = <1>;
               clock-div = <610>;
}

sys_32k_ck: sys_32k_ck {
	compatible = "ti,mux-clock";
	clocks = <&sys_clk32_crystal>, <&sys_clk32_pseudo>, <&sys_clk32_pseudo>, <&sys_clk32_pseudo>;
};

I think... The only issue is that the BOOTSTRAP register is not around
the usual CM1,2 address region...

-- 
Regards,
Nishanth Menon
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