[PATCH v2 2/2] rtc: efi: defer to the ACPI TAD if it can drive the RTC

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

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 is enabled, make RTC_DRV_EFI depend on 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)
---
 drivers/rtc/Kconfig   | 4 ++++
 drivers/rtc/rtc-efi.c | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 1e863f1d3..26f057f52 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -1262,10 +1262,14 @@ config RTC_DRV_DA9063
 config RTC_DRV_EFI
 	tristate "EFI RTC"
 	depends on EFI && !X86
+	depends on !ACPI || 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.
+
 	  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 a8c18ebc5..de4821c9b 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>
@@ -252,6 +253,9 @@ static int __init efi_rtc_probe(struct platform_device *dev)
 	efi_time_cap_t cap;
 	efi_bool_t enabled, pending;
 
+	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