x86/init: Add option to skip using RTC

Hi,

We have a new Atom Airmont core based product which does not support RTC as persistent clock source.

Presently, platform ops get/set wallclock always use MC146818 RTC/CMOS device to read & set time. This causes boot failure on our SOC with no RTC. More specifically, it hangs in RTC driver's mach_get_cmos_time() when it polls RTC_FRQ_SELECT register and loops until Update-In-Progress (UIP) flag gets cleared i.e. below code snippet.

while ((CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP))

		cpu_relax();

After few rounds of review cycles/feedback, we concluded that we should control it from Motorola MC146818 compatible RTC devicetree node. Please see [1].

Make RTC read/write optional by detecting platforms which does not support RTC/CMOS device through the corresponding DT node status property. If status says disabled, then noop the get/set wallclock ops.

For non DT enabled platforms or for DT enabled platforms which does not define optional status property, proceed same as before.

Patch is baselined upon Linux 5.4-rc2 at below Git tree:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/core

[1] Documentation/devicetree/bindings/rtc/rtc-cmos.txt

v3:
* Rebase to latest 5.4-rc2 kernel.
* Fix a build warning reported by kbuild test robot.

v2:
* As per review feedback, do not hack RTC read/write functions directly. Instead, override get/set wallclock ops during setup_arch init sequence.

v1:
* Detect platforms with no RTC in RTC read/write functions and skip RTC read/write if not applicable.

Rahul Tanwar (1):
x86/init: Noop get/set wallclock when platform doesn't support RTC

 arch/x86/kernel/x86_init.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

2.11.0