Add option to skip using RTC

Hi,

There is a new product which does not support RTC as persistent clock source.

Platform ops get/set wallclock are used to get/set timespec through kernel timekeeping read/update_persistent_clock64() routines. Presently, get/set wallclock ops always use MC146818A RTC/CMOS device to read & set time. This causes boot failure on our new SOC with no RTC.

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

These patches are baselined upon Linux 5.3-rc6 at below Git tree:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/core

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

Use wallclock_init() op to detect platforms which does not support RTC and noop get/set wallclock ops for such platforms.

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

index 1bef687faf22..88c120710d5d 100644
@@ -31,6 +31,30 @@ static int __init iommu_init_noop(void) { return 0; }
 static void iommu_shutdown_noop(void) { }
 bool __init bool_x86_init_noop(void) { return false; }
 void x86_op_int_noop(int cpu) { }
+static int set_rtc_noop(const struct timespec64 *now) { return -EINVAL; }
+static void get_rtc_noop(struct timespec64 *now) { }
+
+static const struct of_device_id of_cmos_match[] = {
+	{ .compatible = "motorola,mc146818" },
+	{}
+};
+
+void x86_wallclock_init(void)
+{
+	struct device_node *node;
+
+	node = of_find_matching_node(NULL, of_cmos_match);
+	if (node && !of_device_is_available(node)) {
+		/*
+		 * Some products do not support RTC as persistent clock source. This can be
+		 * optionally indicated by having status property as disabled in the
+		 * corresponding DT node. Override get/set wallclock routines to noops for
+		 * such products.
+		 */
+		x86_platform.get_wallclock = get_rtc_noop;
+		x86_platform.set_wallclock = set_rtc_noop;
+	}
+}
 
 /*
  * The platform setup functions are preset with the default functions

@@ -73,7 +97,7 @@ struct x86_init_ops x86_init __initdata = {
 	.timers = {
 		.setup_percpu_clockev	= setup_boot_APIC_clock,
 		.timer_init		= hpet_time_init,
-		.wallclock_init		= x86_init_noop,
+		.wallclock_init		= x86_wallclock_init,
 	},
 
 	.iommu = {

2.11.0