[PATCH v3 1/2] ACPI: TAD: introduce acpi_has_rtc()

[PATCH v3 1/2] ACPI: TAD: introduce acpi_has_rtc()

[Hamza Mahfooz]

The same RTC may also be accessible by other means, like the EFI
GetTime() and SetTime() runtime services used by rtc-efi, which are best
avoided if there is a native ACPI interface to the RTC.

To allow other RTC drivers to stand down in that case, add an exported
helper, acpi_has_rtc(), that returns true if there is a TAD the driver
will register an RTC class device for.

The helper looks for a platform device matching the driver's ID table
whose _GCP indicates real time support, like acpi_tad_probe() checks,
instead of checking whether the driver has bound to such a device, so
that callers need not depend on the order in which drivers are probed.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
v1: https://lore.kernel.org/r/20261022211304.2285792-1-hamzamahfooz@linux.microsoft.com/
v2:
- Rename acpi_tad_rtc_available() to acpi_has_rtc(), move it to within
  acpi-tad (which allows us to simplify it quite a bit) and declare it
  in linux/acpi.h. (Ard)
- Add acpi_disabled check. (Ard)
---
 drivers/acpi/acpi_tad.c | 38 ++++++++++++++++++++++++++++++++++++++
 include/linux/acpi.h    |  6 ++++++
 2 files changed, 44 insertions(+)

diff --git a/drivers/acpi/acpi_tad.c b/drivers/acpi/acpi_tad.c
index 45459a4ca..eea4d2123 100644
--- a/drivers/acpi/acpi_tad.c
+++ b/drivers/acpi/acpi_tad.c
@@ -899,6 +899,44 @@ static const struct acpi_device_id acpi_tad_ids[] = {
 	{}
 };
 
+static int acpi_tad_rtc_match(struct device *dev, const void *data)
+{
+	unsigned long long caps;
+	acpi_status status;
+
+	if (!acpi_match_device(acpi_tad_ids, dev))
+		return 0;
+
+	status = acpi_evaluate_integer(ACPI_HANDLE(dev), "_GCP", NULL, &caps);
+
+	return ACPI_SUCCESS(status) && (caps & ACPI_TAD_RT);
+}
+
+/**
+ * acpi_has_rtc - Check if the RTC should be driven via the ACPI TAD
+ *
+ * Check if there is a TAD implementing the real time methods that this driver
+ * will register an RTC class device for.
+ *
+ * Return: %true if the RTC should be driven via the ACPI TAD, %false otherwise.
+ */
+bool acpi_has_rtc(void)
+{
+	struct device *dev;
+
+	if (acpi_disabled || !IS_ENABLED(CONFIG_RTC_CLASS))
+		return false;
+
+	dev = bus_find_device(&platform_bus_type, NULL, NULL,
+			      acpi_tad_rtc_match);
+	if (!dev)
+		return false;
+
+	put_device(dev);
+	return true;
+}
+EXPORT_SYMBOL_GPL(acpi_has_rtc);
+
 static struct platform_driver acpi_tad_driver = {
 	.driver = {
 		.name = "acpi-tad",
diff --git a/include/linux/acpi.h b/include/linux/acpi.h
index ddacac812..71524a821 100644
--- a/include/linux/acpi.h
+++ b/include/linux/acpi.h
@@ -1553,6 +1553,12 @@ extern bool acpi_has_watchdog(void);
 static inline bool acpi_has_watchdog(void) { return false; }
 #endif
 
+#if IS_REACHABLE(CONFIG_ACPI_TAD)
+bool acpi_has_rtc(void);
+#else
+static inline bool acpi_has_rtc(void) { return false; }
+#endif
+
 #ifdef CONFIG_ACPI_SPCR_TABLE
 extern bool qdf2400_e44_present;
 int acpi_parse_spcr(bool enable_earlycon, bool enable_console);
-- 
2.56.0

[PATCH v3 2/2] rtc: efi: defer to the ACPI TAD if it can drive the RTC

[Hamza Mahfooz]

EFI runtime services are best avoided where possible, and ACPI defines a
standard interface to the RTC in the form of the Time and Alarm Device
(TAD), which the acpi-tad driver has been exposing as an RTC class
device since commit 8a1e7f4b1764 ("ACPI: TAD: Add RTC class device
interface").

So, make rtc-efi decline to probe, before calling GetTime(), if
acpi_has_rtc() says that the RTC can be driven via the TAD.

Also, to ensure that acpi_has_rtc() is always reachable from rtc-efi
when ACPI_TAD is enabled, make RTC_DRV_EFI depend on
ACPI_TAD || !ACPI_TAD.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
v1: https://lore.kernel.org/all/20261022211304.2285792-2-hamzamahfooz@linux.microsoft.com/
v2:
- Make rtc-efi decline to probe instead of not registering the device in
  efisubsys_init(). (Ard)
- Make RTC_DRV_EFI depend on !ACPI || ACPI_TAD. (Ard)
v3: Make RTC_DRV_EFI depend on ACPI_TAD || !ACPI_TAD. (Sashiko)
---
 drivers/rtc/Kconfig   | 5 +++++
 drivers/rtc/rtc-efi.c | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 1e863f1d3..5b7e9d2a8 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -1262,10 +1262,15 @@ config RTC_DRV_DA9063
 config RTC_DRV_EFI
 	tristate "EFI RTC"
 	depends on EFI && !X86
+	depends on ACPI_TAD || !ACPI_TAD
 	help
 	  If you say yes here you will get support for the EFI
 	  Real Time Clock.
 
+	  On ACPI systems, the ACPI Time and Alarm Device (TAD) driver
+	  is used instead of this one if the TAD can drive the RTC (assuming
+	  it's enabled).
+
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-efi.
 
diff --git a/drivers/rtc/rtc-efi.c b/drivers/rtc/rtc-efi.c
index b4f44999e..545b21b17 100644
--- a/drivers/rtc/rtc-efi.c
+++ b/drivers/rtc/rtc-efi.c
@@ -10,6 +10,7 @@
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/acpi.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/stringify.h>
@@ -192,6 +193,9 @@ static int __init efi_rtc_probe(struct platform_device *dev)
 	efi_time_t eft;
 	efi_time_cap_t cap;
 
+	if (acpi_has_rtc())
+		return -ENODEV;
+
 	/* First check if the RTC is usable */
 	if (efi.get_time(&eft, &cap) != EFI_SUCCESS)
 		return -ENODEV;
-- 
2.56.0

Re: [PATCH v3 1/2] ACPI: TAD: introduce acpi_has_rtc()

[Ard Biesheuvel]

Hello Hamza,

On Mon, 26 Oct 2026, at 16:21, Hamza Mahfooz wrote:
> The same RTC may also be accessible by other means, like the EFI
> GetTime() and SetTime() runtime services used by rtc-efi, which are best
> avoided if there is a native ACPI interface to the RTC.
>
> To allow other RTC drivers to stand down in that case, add an exported
> helper, acpi_has_rtc(), that returns true if there is a TAD the driver
> will register an RTC class device for.
>
> The helper looks for a platform device matching the driver's ID table
> whose _GCP indicates real time support, like acpi_tad_probe() checks,
> instead of checking whether the driver has bound to such a device, so
> that callers need not depend on the order in which drivers are probed.
>
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
> ---
> v1: 
> https://lore.kernel.org/r/20261022211304.2285792-1-hamzamahfooz@linux.microsoft.com/
> v2:
> - Rename acpi_tad_rtc_available() to acpi_has_rtc(), move it to within
>   acpi-tad (which allows us to simplify it quite a bit) and declare it
>   in linux/acpi.h. (Ard)
> - Add acpi_disabled check. (Ard)

Next time, please use a cover latter when sending a series, so that you
can capture these changelogs in one place.

The series looks good to me:

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

I'll let Rafael and Alexandre decide how to queue this, once they are
happy with the patches as well.

Re: [PATCH v3 1/2] ACPI: TAD: introduce acpi_has_rtc()

[Rafael J. Wysocki (Intel)]

On Mon, Oct 26, 2026 at 4:21 PM Hamza Mahfooz
<hamzamahfooz@linux.microsoft.com> wrote:
>
> The same RTC may also be accessible by other means, like the EFI
> GetTime() and SetTime() runtime services used by rtc-efi, which are best
> avoided if there is a native ACPI interface to the RTC.
>
> To allow other RTC drivers to stand down in that case, add an exported
> helper, acpi_has_rtc(), that returns true if there is a TAD the driver
> will register an RTC class device for.
>
> The helper looks for a platform device matching the driver's ID table
> whose _GCP indicates real time support, like acpi_tad_probe() checks,
> instead of checking whether the driver has bound to such a device, so
> that callers need not depend on the order in which drivers are probed.
>
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
> ---
> v1: https://lore.kernel.org/r/20261022211304.2285792-1-hamzamahfooz@linux.microsoft.com/
> v2:
> - Rename acpi_tad_rtc_available() to acpi_has_rtc(), move it to within
>   acpi-tad (which allows us to simplify it quite a bit) and declare it
>   in linux/acpi.h. (Ard)
> - Add acpi_disabled check. (Ard)
> ---
>  drivers/acpi/acpi_tad.c | 38 ++++++++++++++++++++++++++++++++++++++
>  include/linux/acpi.h    |  6 ++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/drivers/acpi/acpi_tad.c b/drivers/acpi/acpi_tad.c
> index 45459a4ca..eea4d2123 100644
> --- a/drivers/acpi/acpi_tad.c
> +++ b/drivers/acpi/acpi_tad.c
> @@ -899,6 +899,44 @@ static const struct acpi_device_id acpi_tad_ids[] = {
>         {}
>  };
>
> +static int acpi_tad_rtc_match(struct device *dev, const void *data)
> +{
> +       unsigned long long caps;
> +       acpi_status status;
> +
> +       if (!acpi_match_device(acpi_tad_ids, dev))
> +               return 0;
> +
> +       status = acpi_evaluate_integer(ACPI_HANDLE(dev), "_GCP", NULL, &caps);
> +
> +       return ACPI_SUCCESS(status) && (caps & ACPI_TAD_RT);
> +}
> +
> +/**
> + * acpi_has_rtc - Check if the RTC should be driven via the ACPI TAD
> + *
> + * Check if there is a TAD implementing the real time methods that this driver
> + * will register an RTC class device for.
> + *
> + * Return: %true if the RTC should be driven via the ACPI TAD, %false otherwise.
> + */
> +bool acpi_has_rtc(void)
> +{
> +       struct device *dev;
> +
> +       if (acpi_disabled || !IS_ENABLED(CONFIG_RTC_CLASS))
> +               return false;

You may as well put these new functions under the existing
CONFIG_RTC_CLASS #ifdef so they don't get compiled unnecessarily, and
add an empty stub for acpi_has_rtc() in the !CONFIG_RTC_CLASS case.

The IS_ENABLED(CONFIG_RTC_CLASS) check above will not be necessary any
more then.

Apart from the above, the patch looks fine to me.

> +
> +       dev = bus_find_device(&platform_bus_type, NULL, NULL,
> +                             acpi_tad_rtc_match);
> +       if (!dev)
> +               return false;
> +
> +       put_device(dev);
> +       return true;
> +}
> +EXPORT_SYMBOL_GPL(acpi_has_rtc);
> +
>  static struct platform_driver acpi_tad_driver = {
>         .driver = {
>                 .name = "acpi-tad",
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index ddacac812..71524a821 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -1553,6 +1553,12 @@ extern bool acpi_has_watchdog(void);
>  static inline bool acpi_has_watchdog(void) { return false; }
>  #endif
>
> +#if IS_REACHABLE(CONFIG_ACPI_TAD)
> +bool acpi_has_rtc(void);
> +#else
> +static inline bool acpi_has_rtc(void) { return false; }
> +#endif
> +
>  #ifdef CONFIG_ACPI_SPCR_TABLE
>  extern bool qdf2400_e44_present;
>  int acpi_parse_spcr(bool enable_earlycon, bool enable_console);
> --
> 2.56.0
>