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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] rtc: checking time_is_valid only in rtc_hctosys() exposes invalid RTC time
--

commit 378dbc5f51408e7a59e18fa3e61590784e92a232
Author: Akashdeep Kaur <a-kaur@ti.com>

rtc: Add flag for dynamic hctosys control

This commit adds a time_is_valid flag to struct rtc_device and checks it in
rtc_hctosys() so drivers can prevent setting the system clock from an
uninitialized or power-lost hardware clock at boot.

> diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
> index 01ba04028f1f1..cec27c476545e 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,

[Severity: Medium]
When a driver sets rtc->time_is_valid to false instead of returning an error
from ops->read_time(), checking rtc->time_is_valid only in rtc_hctosys()
leaves rtc_read_time() in drivers/rtc/interface.c returning 0 with bogus
1970-01-01 time to all other callers:

drivers/rtc/interface.c:rtc_read_time() {
	err = mutex_lock_interruptible(&rtc->ops_lock);
	if (err)
		return err;

	err = __rtc_read_time(rtc, tm);
	mutex_unlock(&rtc->ops_lock);
	...
	return err;
}

drivers/rtc/dev.c:rtc_dev_ioctl() {
	case RTC_RD_TIME:
		mutex_unlock(&rtc->ops_lock);

		err = rtc_read_time(rtc, &tm);
		if (err < 0)
			return err;
}

On boot before rtc_set_time() is called, any userspace read via
rtc_dev_ioctl() -> rtc_read_time() (such as hwclock --hctosys, timedatectl,
or chronyd), sysfs reads in date_show(), time_show(), and since_epoch_show()
in drivers/rtc/sysfs.c, procfs reads in rtc_proc_show(), and alarm
initialization in __devm_rtc_register_device() -> __rtc_read_alarm() ->
rtc_initialize_alarm() -> rtc_read_time() will all succeed and consume the
uninitialized 1970-01-01 timestamp.

Additionally, rtc_set_time() in drivers/rtc/interface.c never updates
rtc->time_is_valid to true once a valid time is written.

Could drivers return -EINVAL from ops->read_time() until ops->set_time() is
called (which already causes rtc_hctosys() to skip setting system time via
rtc_read_time()), or should __rtc_read_time() and rtc_set_time() check and
update this flag so all callers see consistent validity reporting?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261021042435.3895543-1-a-kaur@ti.com?part=1

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

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261021042435.3895543-1-a-kaur@ti.com?part=2

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

[Akashdeep Kaur]

On 21/10/26 10:07, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] rtc: checking time_is_valid only in rtc_hctosys() exposes invalid RTC time
> --
> 
> commit 378dbc5f51408e7a59e18fa3e61590784e92a232
> Author: Akashdeep Kaur <a-kaur@ti.com>
> 
> rtc: Add flag for dynamic hctosys control
> 
> This commit adds a time_is_valid flag to struct rtc_device and checks it in
> rtc_hctosys() so drivers can prevent setting the system clock from an
> uninitialized or power-lost hardware clock at boot.
> 
>> diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
>> index 01ba04028f1f1..cec27c476545e 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,
> 
> [Severity: Medium]
> When a driver sets rtc->time_is_valid to false instead of returning an error
> from ops->read_time(), checking rtc->time_is_valid only in rtc_hctosys()
> leaves rtc_read_time() in drivers/rtc/interface.c returning 0 with bogus
> 1970-01-01 time to all other callers:
> 
> drivers/rtc/interface.c:rtc_read_time() {
> 	err = mutex_lock_interruptible(&rtc->ops_lock);
> 	if (err)
> 		return err;
> 
> 	err = __rtc_read_time(rtc, tm);
> 	mutex_unlock(&rtc->ops_lock);
> 	...
> 	return err;
> }
> 
> drivers/rtc/dev.c:rtc_dev_ioctl() {
> 	case RTC_RD_TIME:
> 		mutex_unlock(&rtc->ops_lock);
> 
> 		err = rtc_read_time(rtc, &tm);
> 		if (err < 0)
> 			return err;
> }
> 
> On boot before rtc_set_time() is called, any userspace read via
> rtc_dev_ioctl() -> rtc_read_time() (such as hwclock --hctosys, timedatectl,
> or chronyd), sysfs reads in date_show(), time_show(), and since_epoch_show()
> in drivers/rtc/sysfs.c, procfs reads in rtc_proc_show(), and alarm
> initialization in __devm_rtc_register_device() -> __rtc_read_alarm() ->
> rtc_initialize_alarm() -> rtc_read_time() will all succeed and consume the
> uninitialized 1970-01-01 timestamp.
> 
> Additionally, rtc_set_time() in drivers/rtc/interface.c never updates
> rtc->time_is_valid to true once a valid time is written.
> 
> Could drivers return -EINVAL from ops->read_time() until ops->set_time() is
> called (which already causes rtc_hctosys() to skip setting system time via
> rtc_read_time()), or should __rtc_read_time() and rtc_set_time() check and
> update this flag so all callers see consistent validity reporting?
> 
> [ ... ]
> 
Thank you for the review. The time_is_valid flag is specifically to 
control boot-time synchronization, not to block RTC reads.

The RTC hardware is readable - userspace can read it via hwclock/sysfs. 
Reading "1970-01-01" is acceptable as it reflects actual hardware state. 
The problem we're solving is specific: preventing boot-time sync from 
overwriting correct system time (from bootloader/NTP) with uninitialized 
RTC value.

Returning -EINVAL from __rtc_read_time() would block all reads, which is 
not the intent. The RTC is readable; we just shouldn't use it to set 
system time on this particular boot.

Regards,
Akashdeep Kaur

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