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

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Date:	Fri, 10 Apr 2015 13:56:54 +0530
From:	Keerthy <a0393675@...com>
To:	Igor Grinberg <grinberg@...pulab.co.il>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<rtc-linux@...glegroups.com>
CC:	Keerthy <j-keerthy@...com>, <a.zummo@...ertech.it>,
	<linux-kernel@...r.kernel.org>, <linux-omap@...r.kernel.org>
Subject: Re: [rtc-linux] [PATCH] rtc: OMAP: Add external 32k clock feature

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@...com> 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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