linux-kernel - [PATCH v3 0/1] x86/init: Add option to skip using RTC

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Date:   Thu, 10 Oct 2019 17:28:55 +0800
From:   Rahul Tanwar <rahul.tanwar@...ux.intel.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        tony.luck@...el.com, x86@...nel.org
Cc:     andriy.shevchenko@...el.com, alan@...ux.intel.com,
        linux-kernel@...r.kernel.org, qi-ming.wu@...el.com,
        chuanhua.lei@...ux.intel.com, cheol.yong.kim@...el.com,
        Rahul Tanwar <rahul.tanwar@...ux.intel.com>
Subject: [PATCH v3 0/1] 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