linux-kernel - RE: [PATCH V6 2/2] rtc: zynqmp: Add calibration set and get support

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Date:   Fri, 10 Jun 2022 10:02:47 +0000
From:   "Neeli, Srinivas" <srinivas.neeli@....com>
To:     Peter Korsgaard <peter@...sgaard.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
CC:     Srinivas Neeli <srinivas.neeli@...inx.com>,
        "a.zummo@...ertech.it" <a.zummo@...ertech.it>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        "sgoud@...inx.com" <sgoud@...inx.com>,
        "shubhraj@...inx.com" <shubhraj@...inx.com>,
        "neelisrinivas18@...il.com" <neelisrinivas18@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "git@...inx.com" <git@...inx.com>
Subject: RE: [PATCH V6 2/2] rtc: zynqmp: Add calibration set and get support

[AMD Official Use Only - General]

Hi,

> -----Original Message-----
> From: Peter Korsgaard <peter@...sgaard.com>
> Sent: Thursday, June 9, 2022 8:30 PM
> To: Alexandre Belloni <alexandre.belloni@...tlin.com>
> Cc: Srinivas Neeli <srinivas.neeli@...inx.com>; a.zummo@...ertech.it;
> robh+dt@...nel.org; krzysztof.kozlowski+dt@...aro.org;
> michal.simek@...inx.com; sgoud@...inx.com; shubhraj@...inx.com; Neeli,
> Srinivas <srinivas.neeli@....com>; neelisrinivas18@...il.com;
> devicetree@...r.kernel.org; linux-rtc@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; git@...inx.com
> Subject: Re: [PATCH V6 2/2] rtc: zynqmp: Add calibration set and get support
>
> >>>>> "Alexandre" == Alexandre Belloni <alexandre.belloni@...tlin.com>
> writes:
>
>  > On 09/06/2022 16:50:03+0200, Peter Korsgaard wrote:
>  >> >>>>> "Srinivas" == Srinivas Neeli <srinivas.neeli@...inx.com> writes:
>  >>
>  >> > Zynqmp RTC controller has a calibration feature to compensate  >> >
> time deviation due to input clock inaccuracy.
>  >> > Set and get calibration API's are used for setting and getting  >> >
> calibration value from the controller calibration register.
>  >> > As per RTC spec default calibration value is 0x7FFF.
>  >>
>  >> Where do you see that? ug1085 rev2.2 still lists:
>  >>
>  >> Programming Sequences
>  >> init rtc
>  >> 1. Write the value 0019_8231h into the calibration register, CALIB_WRITE.
>  >>
>
>  > This is a very weird value. Last time I had a look, I couldn't find an  >
> explanation for it.
>
> Agreed. I am not arguing that it is sensible, I just wondered where this new
> value comes from?
>
This information is missing in our TRM, as per the internal RTC IP design specification document, the calibration value should be crystal clock frequency minus one.
For a crystal oscillator of 32.768KHz(0x8000) frequency, the calibration value will be 0x7FFF.

Will update TRM with the required information.

> --
> Bye, Peter Korsgaard