rtc: pm8xxx: mitigate flash wear

On many Qualcomm platforms the PMIC RTC control and time registers are
read-only so that the RTC time can not be updated. Instead an offset
needs be stored in some machine-specific non-volatile memory, which the
driver can take into account.

On machines like the Lenovo ThinkPad X13s the PMIC RTC drifts about one
second every 3.5 hours, something which leads to repeated updates of the
offset when NTP synchronisation is enabled.

Reduce wear of the underlying flash storage (used for UEFI variables) by
deferring writes until shutdown in case they appear to be due to clock
drift.

As an example, deferring writes when the new offset differs up to 30 s
from the previous one reduces the number of writes on the X13s during a
ten day session with the machine not suspending for more than four days
in a row from up to 68 writes (every 3.5 h) to at most two (boot and
shutdown).

Tested-by: Jens Glathe <jens.glathe@oldschoolsolutions.biz>
Tested-by: Steev Klimaszewski <steev@kali.org>
Tested-by: Joel Stanley <joel@jms.id.au>
Tested-by: Sebastian Reichel <sre@kernel.org> # Lenovo T14s Gen6
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20250219134118.31017-4-johan+linaro@kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: jhovold

drivers/rtc/rtc-pm8xxx.c

@@ -65,6 +65,7 @@ struct qcom_uefi_rtc_info {
  * @rtc_info:		qcom uefi rtc-info structure
  * @nvmem_cell:		nvmem cell for offset
  * @offset:		offset from epoch in seconds
+ * @offset_dirty:	offset needs to be stored on shutdown
  */
 struct pm8xxx_rtc {
 	struct rtc_device *rtc;
@@ -77,6 +78,7 @@ struct pm8xxx_rtc {
 	struct qcom_uefi_rtc_info rtc_info;
 	struct nvmem_cell *nvmem_cell;
 	u32 offset;
+	bool offset_dirty;
 };
 
 #ifdef CONFIG_EFI
@@ -256,6 +258,15 @@ static int pm8xxx_rtc_update_offset(struct pm8xxx_rtc *rtc_dd, u32 secs)
 	if (offset == rtc_dd->offset)
 		return 0;
 
+	/*
+	 * Reduce flash wear by deferring updates due to clock drift until
+	 * shutdown.
+	 */
+	if (abs_diff(offset, rtc_dd->offset) < 30) {
+		rtc_dd->offset_dirty = true;
+		goto out;
+	}
+
 	if (rtc_dd->nvmem_cell)
 		rc = pm8xxx_rtc_write_nvmem_offset(rtc_dd, offset);
 	else
@@ -264,6 +275,8 @@ static int pm8xxx_rtc_update_offset(struct pm8xxx_rtc *rtc_dd, u32 secs)
 	if (rc)
 		return rc;
 
+	rtc_dd->offset_dirty = false;
+out:
 	rtc_dd->offset = offset;
 
 	return 0;
@@ -634,8 +647,21 @@ static int pm8xxx_rtc_probe(struct platform_device *pdev)
 	return devm_rtc_register_device(rtc_dd->rtc);
 }
 
+static void pm8xxx_shutdown(struct platform_device *pdev)
+{
+	struct pm8xxx_rtc *rtc_dd = platform_get_drvdata(pdev);
+
+	if (rtc_dd->offset_dirty) {
+		if (rtc_dd->nvmem_cell)
+			pm8xxx_rtc_write_nvmem_offset(rtc_dd, rtc_dd->offset);
+		else
+			pm8xxx_rtc_write_uefi_offset(rtc_dd, rtc_dd->offset);
+	}
+}
+
 static struct platform_driver pm8xxx_rtc_driver = {
 	.probe		= pm8xxx_rtc_probe,
+	.shutdown	= pm8xxx_shutdown,
 	.driver	= {
 		.name		= "rtc-pm8xxx",
 		.of_match_table	= pm8xxx_id_table,