[v3] powerpc/configs: enable RTC class support

[Vaibhav]

Hi Behn,

Need your help in understanding the historical context for having two
rtc interfaces in the kernel (CONFIG_GEN_RTC & CONFIG_RTC_CLASS) and
which one should be enabled for pseries_defconfig. I have posted a patch
(http://patchwork.ozlabs.org/patch/478899/) to enable CONFIG_RTC_CLASS
in the defconfig so that opal rtc driver (/drivers/rtc/opal-rtc.c) is
enabled.

Afaik CONFIG_RTC_CLASS is more generic and can emulate the older
interface provided by gen_rtc. So imho it would make sense to enable
rtc-class with pseries_defconfig as we already have an opal-rtc-driver.

My limited testing with gen_rtc on a tulleta reveals issues with hwclock
as it reports invalid rtc-time error which I suspect is due to a genrtc
driver trying to read date time values from an rtc port that is not
available on the tulleta.

With RTC_CLASS enabled however hwclock seems to work fine.

Requesting your inputs,

Thanks,
~Vaibhav

On Wed, 2015-06-17 at 15:43 +1000, Michael Ellerman wrote:
> On Mon, 2015-01-06 at 10:48:55 UTC, Vaibhav Jain wrote:
> > A working rtc kernel driver is needed so that hwclock can synchronize
> > system clock to rtc during shutdown/boot. We already have a rtc platform
> > driver for power arch located at drivers/rtc/rtc-opal.c However it
> > depends on CONFIG_RTC_CLASS which is disabled by default. So this driver
> > is not compiled with pseries defconfig as rtc class support is missing
> > from the kernel.
> > 
> > We fix this by enabling rtc class support in pseries defconfig so that
> > this driver gets enabled and is compiled into the pseries kernel.
> 
> So that seems fair enough.
> 
> But, it disables CONFIG_GEN_RTC. Which *I think* means we won't get an RTC
> driver at all on pseries machines.

> There's some history between those two options but I don't know it off the top
> of my head. Ben should know?
> 
> cheers
>