linux-kernel - Re: [PATCH v2 2/3] ARM: DRA7x: dts: Update the OSC_32K

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Date:	Tue, 3 May 2016 09:43:23 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	"J.D. Schroeder" <Linux.HWI@...min.com>
Cc:	Tero Kristo <t-kristo@...com>, linux-kernel@...r.kernel.org,
	bcousson@...libre.com, robh+dt@...nel.org, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, linux@....linux.org.uk,
	linux-omap@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, jay.schroeder@...min.com,
	Matthijs van Duin <matthijsvanduin@...il.com>
Subject: Re: [PATCH v2 2/3] ARM: DRA7x: dts: Update the OSC_32K_CLK frequency

* J.D. Schroeder <Linux.HWI@...min.com> [160503 06:32]:
> On 05/03/2016 03:16 AM, Tero Kristo wrote:
> > On 02/05/16 20:12, J.D. Schroeder wrote:
> >> From: "J.D. Schroeder" <jay.schroeder@...min.com>
> >>
> >> This commit updates the OSC_32K_CLK (secure_32k_clk_src_ck) frequency
> >> from the precise 32kHz frequency (i.e., 32.768 kHz) to a more
> >> accurate frequency of ~34.6 kHz. Actual measured frequencies of the
> >> clock vary from processor to processor anywhere from 34.4 kHz up to
> >> 34.8 kHz. Note that the ~34 kHz frequency clock is generated
> >> internally by the processor, not an input to the processor. This
> >> change makes it more clear that the consumer of the
> >> secure_32k_clk_src_ck will not get a precise 32.768 kHz frequency
> >> output.
> >>
> >> Signed-off-by: J.D. Schroeder <jay.schroeder@...min.com>
> >> Reviewed-by: Trenton Andres <trenton.andres@...min.com>
> >> ---
> >>   arch/arm/boot/dts/dra7xx-clocks.dtsi | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/boot/dts/dra7xx-clocks.dtsi
> >> b/arch/arm/boot/dts/dra7xx-clocks.dtsi
> >> index 3f0c61d..f7ec976 100644
> >> --- a/arch/arm/boot/dts/dra7xx-clocks.dtsi
> >> +++ b/arch/arm/boot/dts/dra7xx-clocks.dtsi
> >> @@ -95,7 +95,7 @@
> >>       secure_32k_clk_src_ck: secure_32k_clk_src_ck {
> >>           #clock-cells = <0>;
> >>           compatible = "fixed-clock";
> >> -        clock-frequency = <32768>;
> >> +        clock-frequency = <34600>; /* approximate frequency */
> >>       };
> >>
> >>       sys_clk32_crystal_ck: sys_clk32_crystal_ck {
> >>
> > 
> > I still don't agree with this patch. The actual frequency can drift much more,
> > you are just seeing this number at your setup.
> 
> Yes, it can drift significantly from processor to processor. Do you agree that
> this frequency is closer to what can be expected than 32768 Hz?
> 
> Like I said, I would have renamed the clock also but I opted to go the less
> obtrusive route while still helping others that might think they can
> reasonably use this clock in their design as a 32768 Hz clock source. Perhaps
> my comment and selection of the approximate frequency is not the best (I'm
> open for suggestions). However, I do think this change is an improvement and
> clarifying change to what is currently present in the clock description.

Does a fixed divider calculation of input * (32768 / 0x27e6) make sense
here too as pointed out earlier by Matthijs for the ti81xx?

Regards,

Tony