From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

If the ACPI_FADT_FIXED_RTC flag is unset, the platform is declaring that
it supports the ACPI RTC fixed event which should be used instead of a
dedicated CMOS RTC IRQ.  However, the driver only enables it when
is_hpet_enabled() returns true, which is questionable because there is
no clear connection between enabled HPET and signaling wakeup via the
ACPI RTC fixed event (for instance, the latter can be expected to work
on systems that don't include a functional HPET).

Moreover, since use_hpet_alarm() returns false if use_acpi_alarm is set,
the ACPI RTC fixed event is effectively used instead of the HPET alarm
if the latter is functional, but there is no particular reason why it
could not be used otherwise.

Accordingly, on x86 systems with ACPI, set use_acpi_alarm if
ACPI_FADT_FIXED_RTC is unset without looking at whether or not HPET is
enabled.

Also, do the ACPI FADT check in use_acpi_alarm_quirks() before the DMI
BIOS year checks which are more expensive and it's better to skip them
if ACPI_FADT_FIXED_RTC is set.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/rtc/rtc-cmos.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -817,6 +817,9 @@ static void rtc_wake_off(struct device *
 #ifdef CONFIG_X86
 static void use_acpi_alarm_quirks(void)
 {
+	if (acpi_gbl_FADT.flags & ACPI_FADT_FIXED_RTC)
+		return;
+
 	switch (boot_cpu_data.x86_vendor) {
 	case X86_VENDOR_INTEL:
 		if (dmi_get_bios_year() < 2015)
@@ -830,8 +833,6 @@ static void use_acpi_alarm_quirks(void)
 	default:
 		return;
 	}
-	if (!is_hpet_enabled())
-		return;
 
 	use_acpi_alarm = true;
 }

On 3/14/2026 7:11 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the ACPI_FADT_FIXED_RTC flag is unset, the platform is declaring that
> it supports the ACPI RTC fixed event which should be used instead of a
> dedicated CMOS RTC IRQ.  However, the driver only enables it when
> is_hpet_enabled() returns true, which is questionable because there is
> no clear connection between enabled HPET and signaling wakeup via the
> ACPI RTC fixed event (for instance, the latter can be expected to work
> on systems that don't include a functional HPET).
> 
> Moreover, since use_hpet_alarm() returns false if use_acpi_alarm is set,
> the ACPI RTC fixed event is effectively used instead of the HPET alarm
> if the latter is functional, but there is no particular reason why it
> could not be used otherwise.
> 
> Accordingly, on x86 systems with ACPI, set use_acpi_alarm if
> ACPI_FADT_FIXED_RTC is unset without looking at whether or not HPET is
> enabled.
> 
> Also, do the ACPI FADT check in use_acpi_alarm_quirks() before the DMI
> BIOS year checks which are more expensive and it's better to skip them
> if ACPI_FADT_FIXED_RTC is set.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

This makes sense to me.

Sorry for my delay, I kept on meaning to check a few machines and then 
more things came up and it fell lower.

I did check 3 generations of AMD machines and I don't expect any 
problems from this change.

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>

> ---
>   drivers/rtc/rtc-cmos.c |    5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- a/drivers/rtc/rtc-cmos.c
> +++ b/drivers/rtc/rtc-cmos.c
> @@ -817,6 +817,9 @@ static void rtc_wake_off(struct device *
>   #ifdef CONFIG_X86
>   static void use_acpi_alarm_quirks(void)
>   {
> +	if (acpi_gbl_FADT.flags & ACPI_FADT_FIXED_RTC)
> +		return;
> +
>   	switch (boot_cpu_data.x86_vendor) {
>   	case X86_VENDOR_INTEL:
>   		if (dmi_get_bios_year() < 2015)
> @@ -830,8 +833,6 @@ static void use_acpi_alarm_quirks(void)
>   	default:
>   		return;
>   	}
> -	if (!is_hpet_enabled())
> -		return;
>   
>   	use_acpi_alarm = true;
>   }
> 
> 
>

On 14/03/2026 13:11:20+0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If the ACPI_FADT_FIXED_RTC flag is unset, the platform is declaring that
> it supports the ACPI RTC fixed event which should be used instead of a
> dedicated CMOS RTC IRQ.  However, the driver only enables it when
> is_hpet_enabled() returns true, which is questionable because there is
> no clear connection between enabled HPET and signaling wakeup via the
> ACPI RTC fixed event (for instance, the latter can be expected to work
> on systems that don't include a functional HPET).
> 
> Moreover, since use_hpet_alarm() returns false if use_acpi_alarm is set,
> the ACPI RTC fixed event is effectively used instead of the HPET alarm
> if the latter is functional, but there is no particular reason why it
> could not be used otherwise.
> 
> Accordingly, on x86 systems with ACPI, set use_acpi_alarm if
> ACPI_FADT_FIXED_RTC is unset without looking at whether or not HPET is
> enabled.
> 
> Also, do the ACPI FADT check in use_acpi_alarm_quirks() before the DMI
> BIOS year checks which are more expensive and it's better to skip them
> if ACPI_FADT_FIXED_RTC is set.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

> ---
>  drivers/rtc/rtc-cmos.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- a/drivers/rtc/rtc-cmos.c
> +++ b/drivers/rtc/rtc-cmos.c
> @@ -817,6 +817,9 @@ static void rtc_wake_off(struct device *
>  #ifdef CONFIG_X86
>  static void use_acpi_alarm_quirks(void)
>  {
> +	if (acpi_gbl_FADT.flags & ACPI_FADT_FIXED_RTC)
> +		return;
> +
>  	switch (boot_cpu_data.x86_vendor) {
>  	case X86_VENDOR_INTEL:
>  		if (dmi_get_bios_year() < 2015)
> @@ -830,8 +833,6 @@ static void use_acpi_alarm_quirks(void)
>  	default:
>  		return;
>  	}
> -	if (!is_hpet_enabled())
> -		return;
>  
>  	use_acpi_alarm = true;
>  }
> 
> 
> 

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