Re: [PATCHv3 4/5] rtc: s3c: Add support for RTC of Exynos3250 SoC

[Doug Anderson]

Hi,

On Tue, Sep 16, 2014 at 8:20 AM, Javier Martinez Canillas
<javier@xxxxxxxxxxxx> wrote:
> Hello Daniel,
>
> On Tue, Sep 16, 2014 at 5:03 PM, Daniel Drake <drake@xxxxxxxxxxxx> wrote:
>> On Tue, Sep 16, 2014 at 7:48 AM, Javier Martinez Canillas
>> <javier@xxxxxxxxxxxx> wrote:
>>>> Clock list for s3c-rtc device:
>>>> - rtc : CLK_RTC of CLK_GATE_IP_PERIR is gate clock for RTC.
>>>> - rtc_src : XrtcXTI is 32.768.kHz source clock for RTC.
>>>
>>> Is this RTC source clock needed for all Exynos SoCs?
>>
>> It is at least needed on Exynos4412, which has the XrtcXTI thing
>> exactly as you describe. However the very standard setup there is to
>> hook it up to the CP clock output of the MAX76686 PMIC. This CP clock
>> is on by default, so you can potentially live without that detail
>> being present in the DT.
>>
>
> Thanks for confirming for Exynos4412, I just answered my own email
> saying that I found to be needed on Exynos5420 as well and as you
> said, it was just working because the Maxim clocks were left on by
> default.
>
>> However... one small issue with this setup is that when you enable
>> CONFIG_COMMON_CLK_MAX77686, the CP clock gets exposed in Linux's
>> common clock framework, and Linux then turns it off because it
>> believes it is unused. Then the RTC stops ticking.
>>
>
> Indeed, this is an issue about relying on default state. We had a
> quite long discussion a couple of weeks ago about simplefb relying on
> clocks and regulators left enabled by the bootloader but once these
> were know to the kernel, the frameworks disable them because were
> unused making simplefb to fail.
>
>> So the rtc_src idea would also be good for Exynos4412. Maybe it would
>> make sense to drop the needs_src_clk flag, and simply require/enable
>> the src clock whenever it is present in the DT.
>>
>
> That sounds more sensible to me as well. I wonder what should happen
> in this case with DT backward compatibility though. But as you said,
> the external clock is required and the kernel will disable this clock
> once is know to the CCF since is not used so maybe will be hard to
> maintain DT backward compatibility in this case.

I think you can turn off CONFIG_COMMON_CLK_MAX77686 and then this
clock will be left at whatever the bootloader set it to, right?  Then
there will be no auto-disabling by the CCF and the RTC will work.
That's one argument for making the clock "optional".

NOTE: I don't think that the builtin RTC is terribly important for any
exynos-based Chromebooks that I'm aware of.  We rely on the RTC that's
part of the Maxim PMIC itself and pretty much ignore the one built-in
to the exynos.  I think there are some cases it was used (as a
fallback wakeup source in certain test scripts), but nothing very
important.

-Doug
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