Re: [rtc-linux] [PATCH] rtc: OMAP: Add external 32k clock feature

[Keerthy]

Hi

On Tuesday 07 April 2015 12:36 PM, Igor Grinberg wrote:
> Hi,
>
> On 04/07/15 06:29, Keerthy wrote:
> > Hi Andrew,
> >
> > Apologies for replying late.
> >
> > On Wednesday 25 March 2015 04:29 AM, Andrew Morton wrote:
> > > On Tue, 3 Mar 2015 15:12:02 +0530 Keerthy <j-keerthy@xxxxxx> wrote:
> > >
> > > > Add external 32k clock feature. The internal clock will be gated during suspend.
> > > > Hence make use of the external 32k clock so that rtc is functional accross
> > > > suspend/resume.
> > > >
> > > > ...
> > > >
> > > > @@ -446,6 +449,7 @@ static const struct omap_rtc_device_type omap_rtc_default_type = {
> > > >
> > > >    static const struct omap_rtc_device_type omap_rtc_am3352_type = {
> > > >        .has_32kclk_en    = true,
> > > > +    .has_osc_ext_32k = true,
> > > >        .has_kicker    = true,
> > > >        .has_irqwakeen    = true,
> > > >        .has_pmic_mode    = true,
> > > > @@ -543,7 +547,16 @@ static int __init omap_rtc_probe(struct platform_device *pdev)
> > > >        if (rtc->type->has_32kclk_en) {
> > > >            reg = rtc_read(rtc, OMAP_RTC_OSC_REG);
> > > >            rtc_writel(rtc, OMAP_RTC_OSC_REG,
> > > > -                reg | OMAP_RTC_OSC_32KCLK_EN);
> > > > +               reg | OMAP_RTC_OSC_32KCLK_EN);
> > > > +    }
> > > > +
> > > > +    /* Enable External clock as the source */
> > > > +
> > > > +    if (rtc->type->has_osc_ext_32k) {
> > > > +        rtc_writel(rtc, OMAP_RTC_OSC_REG,
> > > > +               (OMAP_RTC_OSC_EXT_32K |
> > > > +               rtc_read(rtc, OMAP_RTC_OSC_REG)) &
> > > > +               (~OMAP_RTC_OSC_OSC32K_GZ));
> > > >        }
> > >
> > > How do we know that all systems have this external clock and that it
> > > works OK?
> >
> > AM335 and AM43X have the external clock feature which we choose using
> > RTC_OSC_REG. I verified it works OK by seeing the RTC seconds ticking
> > even after switching the source to the external 32k Clock.
>
> AFAIU, this is related to the external (outside of SoC) oscillator, right?
> If so, there is a possibility to not assemble it on the board (at least
> on AM335) and use the internal clock source instead.

Yes. The register 'OMAP_RTC_OSC_REG' is part of the RTC IP register set
so i assumed that it would be with the RTCs of those particular SoCs.

> There are dozens of boards out there that do not have the external
> oscillator assembled.
>
> Am I missing something?

Regards,
Keerthy
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