[PATCH 0/2] rtc: Prevent hctosys from using invalid RTC time

[PATCH 0/2] rtc: Prevent hctosys from using invalid RTC time

[Akashdeep Kaur]

The kernel's boot-time RTC synchronization blindly trusts the RTC 
value when setting system time, but RTCs may have invalid time 
depending on the boot path:

- On first cold boot: RTC may have lost power, time is invalid
- After low-power mode (RTC-only) cold boot: RTC retained power, time 
  is valid

On normal cold boot where bootloader or NTP has already established 
correct time, boot-time sync (triggered when RTC driver loads as 
module or probes at boot) can overwrite it with invalid RTC data, 
causing time rollback to 1970. However, on boot from RTC-only power 
mode, the RTC has valid time and boot-time sync should work to 
restore system time from RTC.

This series adds a flag that drivers set dynamically per-boot to 
indicate whether RTC has valid time. Drivers check at probe if RTC 
retained power through this specific boot. Boot-time sync checks this 
flag before setting system time. This enables correct behavior for 
both boot paths while keeping PM suspend/resume compensation 
unaffected.

---

Akashdeep Kaur (2):
  rtc: Add flag for dynamic hctosys control
  rtc: ti-k3: Mark RTC time as invalid without battery backup

 drivers/rtc/class.c     | 15 +++++++++++++++
 drivers/rtc/rtc-ti-k3.c |  7 +++++++
 include/linux/rtc.h     |  7 +++++++
 3 files changed, 29 insertions(+)


base-commit: 8f7df005a7564bd7ffa75da9c3085d5b996f1e16
-- 
2.34.1

[PATCH 1/2] rtc: Add flag for dynamic hctosys control

[Akashdeep Kaur]

Add flag to indicate whether RTC has valid absolute time on current
boot. This allows drivers to dynamically communicate RTC validity
per-boot to prevent hctosys from setting system time from invalid
RTC values.
The flag defaults to true for backward compatibility with existing
drivers.

Signed-off-by: Akashdeep Kaur <a-kaur@ti.com>
---
 drivers/rtc/class.c | 15 +++++++++++++++
 include/linux/rtc.h |  7 +++++++
 2 files changed, 22 insertions(+)

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 01ba04028f1f..cec27c476545 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -63,6 +63,18 @@ static void rtc_hctosys(struct rtc_device *rtc)
 		.tv_nsec = NSEC_PER_SEC >> 1,
 	};
 
+	/*
+	 * If the RTC does not have valid time, do not set system time at boot.
+	 * This can happen when RTC loses power or has never been initialized.
+	 * PM resume compensation will still work via rtc_resume().
+	 */
+	if (!rtc->time_is_valid) {
+		dev_info(rtc->dev.parent,
+			 "hctosys: RTC time not valid, skipping\n");
+		err = -EINVAL;
+		goto err_read;
+	}
+
 	err = rtc_read_time(rtc, &tm);
 	if (err) {
 		dev_err(rtc->dev.parent,
@@ -243,6 +255,9 @@ static struct rtc_device *rtc_allocate_device(void)
 	hrtimer_setup(&rtc->pie_timer, rtc_pie_update_irq, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	rtc->pie_enabled = 0;
 
+	/* Assume RTC has valid time by default. Drivers can override. */
+	rtc->time_is_valid = true;
+
 	set_bit(RTC_FEATURE_ALARM, rtc->features);
 	set_bit(RTC_FEATURE_UPDATE_INTERRUPT, rtc->features);
 
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 0f43b5027235..f6f982610938 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -152,6 +152,13 @@ struct rtc_device {
 	time64_t offset_secs;
 	bool set_start_time;
 
+	/*
+	 * Set by drivers to indicate RTC has valid absolute time.
+	 * Used to prevent hctosys from using RTC when time is invalid
+	 * (e.g., lost power, never initialized). PM compensation is unaffected.
+	 */
+	bool time_is_valid;
+
 #ifdef CONFIG_RTC_INTF_DEV_UIE_EMUL
 	struct work_struct uie_task;
 	struct timer_list uie_timer;
-- 
2.34.1

[PATCH 2/2] rtc: ti-k3: Mark RTC time as invalid without battery backup

[Akashdeep Kaur]

TI K3 RTC typically has no battery backup. Prevent HCTOSYS from
overwriting valid system time (established by bootloader or early
NTP sync) with invalid RTC value.

Signed-off-by: Akashdeep Kaur <a-kaur@ti.com>
---
 drivers/rtc/rtc-ti-k3.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/rtc/rtc-ti-k3.c b/drivers/rtc/rtc-ti-k3.c
index 8df00319cb6b..99fdf1192675 100644
--- a/drivers/rtc/rtc-ti-k3.c
+++ b/drivers/rtc/rtc-ti-k3.c
@@ -613,6 +613,13 @@ static int ti_k3_rtc_probe(struct platform_device *pdev)
 	else
 		device_set_wakeup_capable(dev, true);
 
+	/*
+	 * TI K3 RTC has no battery backup in typical configurations.
+	 * Mark time as invalid to prevent hctosys from overwriting
+	 * correct system time (from bootloader/NTP) with invalid RTC value.
+	 */
+	priv->rtc_dev->time_is_valid = false;
+
 	ret = devm_rtc_register_device(priv->rtc_dev);
 	if (ret)
 		return ret;
-- 
2.34.1

Re: [PATCH 0/2] rtc: Prevent hctosys from using invalid RTC time

[Alexandre Belloni]

Hello,

On 21/10/2026 09:54:33+0530, Akashdeep Kaur wrote:
> The kernel's boot-time RTC synchronization blindly trusts the RTC 
> value when setting system time, but RTCs may have invalid time 
> depending on the boot path:
> 
> - On first cold boot: RTC may have lost power, time is invalid
> - After low-power mode (RTC-only) cold boot: RTC retained power, time 
>   is valid
> 
> On normal cold boot where bootloader or NTP has already established 
> correct time, boot-time sync (triggered when RTC driver loads as 
> module or probes at boot) can overwrite it with invalid RTC data, 
> causing time rollback to 1970. However, on boot from RTC-only power 
> mode, the RTC has valid time and boot-time sync should work to 
> restore system time from RTC.
> 
> This series adds a flag that drivers set dynamically per-boot to 
> indicate whether RTC has valid time. Drivers check at probe if RTC 
> retained power through this specific boot. Boot-time sync checks this 
> flag before setting system time. This enables correct behavior for 
> both boot paths while keeping PM suspend/resume compensation 
> unaffected.

The proper way to do this is to return -EINVAL in read_time until you
know the date is correct. There is no need for an extra flag for the core.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com