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

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

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