[PATCH] rtc: class: support hctosys from modular RTC drivers

[Steve Muckle <smuckle@google.com>]

Due to distribution constraints it may not be possible to statically
compile the required RTC driver into the kernel.

Expand RTC_HCTOSYS support to cover all RTC devices (statically compiled
or not) by checking at the end of RTC device registration whether the
time should be synced.

Signed-off-by: Steve Muckle <smuckle@google.com>
---
 drivers/rtc/Kconfig   | 18 ++++++-----
 drivers/rtc/Makefile  |  1 -
 drivers/rtc/class.c   | 63 +++++++++++++++++++++++++++++++++++++++
 drivers/rtc/hctosys.c | 69 -------------------------------------------
 4 files changed, 73 insertions(+), 78 deletions(-)
 delete mode 100644 drivers/rtc/hctosys.c

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 1adf9f815652..f663d77deb41 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -35,14 +35,16 @@ config RTC_HCTOSYS_DEVICE
 	depends on RTC_HCTOSYS
 	default "rtc0"
 	help
-	  The RTC device that will be used to (re)initialize the system
-	  clock, usually rtc0. Initialization is done when the system
-	  starts up, and when it resumes from a low power state. This
-	  device should record time in UTC, since the kernel won't do
-	  timezone correction.
-
-	  The driver for this RTC device must be loaded before late_initcall
-	  functions run, so it must usually be statically linked.
+	  The RTC device that will be used to (re)initialize the system clock,
+	  usually rtc0. Initialization is done when the driver for the RTC
+	  device registers, and when it resumes from a low power state. This
+	  device should record time in UTC, since the kernel won't do timezone
+	  correction.
+
+	  During startup it is useful to initialize the system clock from the
+	  RTC as early as possible. For this reason the driver for this RTC
+	  device should be statically linked, or alternately, the kernel module
+	  for the RTC device driver loaded immediately.
 
 	  This clock should be battery-backed, so that it reads the correct
 	  time when the system boots from a power-off state. Otherwise, your
diff --git a/drivers/rtc/Makefile b/drivers/rtc/Makefile
index 4ac8f19fb631..33f932d08262 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -6,7 +6,6 @@
 ccflags-$(CONFIG_RTC_DEBUG)	:= -DDEBUG
 
 obj-$(CONFIG_RTC_LIB)		+= lib.o
-obj-$(CONFIG_RTC_HCTOSYS)	+= hctosys.o
 obj-$(CONFIG_RTC_SYSTOHC)	+= systohc.o
 obj-$(CONFIG_RTC_CLASS)		+= rtc-core.o
 obj-$(CONFIG_RTC_MC146818_LIB)	+= rtc-mc146818-lib.o
diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 9458e6d6686a..b1ed5f3be223 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -34,6 +34,64 @@ static void rtc_device_release(struct device *dev)
 #ifdef CONFIG_RTC_HCTOSYS_DEVICE
 /* Result of the last RTC to system clock attempt. */
 int rtc_hctosys_ret = -ENODEV;
+
+/* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
+ * whether it stores the most close value or the value with partial
+ * seconds truncated. However, it is important that we use it to store
+ * the truncated value. This is because otherwise it is necessary,
+ * in an rtc sync function, to read both xtime.tv_sec and
+ * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
+ * of >32bits is not possible. So storing the most close value would
+ * slow down the sync API. So here we have the truncated value and
+ * the best guess is to add 0.5s.
+ */
+
+static int rtc_hctosys(void)
+{
+	int err = -ENODEV;
+	struct rtc_time tm;
+	struct timespec64 tv64 = {
+		.tv_nsec = NSEC_PER_SEC >> 1,
+	};
+	struct rtc_device *rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
+
+	if (!rtc) {
+		pr_info("unable to open rtc device (%s)\n",
+			CONFIG_RTC_HCTOSYS_DEVICE);
+		goto err_open;
+	}
+
+	err = rtc_read_time(rtc, &tm);
+	if (err) {
+		dev_err(rtc->dev.parent,
+			"hctosys: unable to read the hardware clock\n");
+		goto err_read;
+	}
+
+	tv64.tv_sec = rtc_tm_to_time64(&tm);
+
+#if BITS_PER_LONG == 32
+	if (tv64.tv_sec > INT_MAX) {
+		err = -ERANGE;
+		goto err_read;
+	}
+#endif
+
+	err = do_settimeofday64(&tv64);
+
+	dev_info(rtc->dev.parent, "setting system clock to %ptR UTC (%lld)\n",
+		 &tm, (long long)tv64.tv_sec);
+
+err_read:
+	rtc_class_close(rtc);
+
+err_open:
+	rtc_hctosys_ret = err;
+
+	return err;
+}
+
+
 #endif
 
 #if defined(CONFIG_PM_SLEEP) && defined(CONFIG_RTC_HCTOSYS_DEVICE)
@@ -375,6 +433,11 @@ int __rtc_register_device(struct module *owner, struct rtc_device *rtc)
 	dev_info(rtc->dev.parent, "registered as %s\n",
 		 dev_name(&rtc->dev));
 
+#ifdef CONFIG_RTC_HCTOSYS_DEVICE
+	if (!strcmp(dev_name(&rtc->dev), CONFIG_RTC_HCTOSYS_DEVICE))
+		rtc_hctosys();
+#endif
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__rtc_register_device);
diff --git a/drivers/rtc/hctosys.c b/drivers/rtc/hctosys.c
deleted file mode 100644
index a74d0d890600..000000000000
--- a/drivers/rtc/hctosys.c
+++ /dev/null
@@ -1,69 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-/*
- * RTC subsystem, initialize system time on startup
- *
- * Copyright (C) 2005 Tower Technologies
- * Author: Alessandro Zummo <a.zummo@towertech.it>
- */
-
-#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
-
-#include <linux/rtc.h>
-
-/* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
- * whether it stores the most close value or the value with partial
- * seconds truncated. However, it is important that we use it to store
- * the truncated value. This is because otherwise it is necessary,
- * in an rtc sync function, to read both xtime.tv_sec and
- * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
- * of >32bits is not possible. So storing the most close value would
- * slow down the sync API. So here we have the truncated value and
- * the best guess is to add 0.5s.
- */
-
-static int __init rtc_hctosys(void)
-{
-	int err = -ENODEV;
-	struct rtc_time tm;
-	struct timespec64 tv64 = {
-		.tv_nsec = NSEC_PER_SEC >> 1,
-	};
-	struct rtc_device *rtc = rtc_class_open(CONFIG_RTC_HCTOSYS_DEVICE);
-
-	if (!rtc) {
-		pr_info("unable to open rtc device (%s)\n",
-			CONFIG_RTC_HCTOSYS_DEVICE);
-		goto err_open;
-	}
-
-	err = rtc_read_time(rtc, &tm);
-	if (err) {
-		dev_err(rtc->dev.parent,
-			"hctosys: unable to read the hardware clock\n");
-		goto err_read;
-	}
-
-	tv64.tv_sec = rtc_tm_to_time64(&tm);
-
-#if BITS_PER_LONG == 32
-	if (tv64.tv_sec > INT_MAX) {
-		err = -ERANGE;
-		goto err_read;
-	}
-#endif
-
-	err = do_settimeofday64(&tv64);
-
-	dev_info(rtc->dev.parent, "setting system clock to %ptR UTC (%lld)\n",
-		 &tm, (long long)tv64.tv_sec);
-
-err_read:
-	rtc_class_close(rtc);
-
-err_open:
-	rtc_hctosys_ret = err;
-
-	return err;
-}
-
-late_initcall(rtc_hctosys);
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog