[PATCH v4 09/23] cxl: Add helper function to retrieve ACPI handle of CXL root device

[Dave Jiang <dave.jiang@xxxxxxxxx>]

Provide a helper to find the ACPI0017 device in order to issue the _DSM.
The helper will take the 'struct device' from a cxl_port and iterate until
the root device is reached. The ACPI handle will be returned from the root
device.

Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>

---
v2:
- Fix commenting style. (Jonathan)
- Fix var declaration aligning. (Jonathan)
---
 drivers/cxl/core/acpi.c |   34 ++++++++++++++++++++++++++++++++++
 drivers/cxl/cxl.h       |    1 +
 2 files changed, 35 insertions(+)

diff --git a/drivers/cxl/core/acpi.c b/drivers/cxl/core/acpi.c
index 6eda5cad8d59..191644d0ca6d 100644
--- a/drivers/cxl/core/acpi.c
+++ b/drivers/cxl/core/acpi.c
@@ -5,6 +5,7 @@
 #include <linux/kernel.h>
 #include <linux/acpi.h>
 #include <linux/pci.h>
+#include <linux/platform_device.h>
 #include <asm/div64.h>
 #include "cxlpci.h"
 #include "cxl.h"
@@ -13,6 +14,39 @@ const guid_t acpi_cxl_qtg_id_guid =
 	GUID_INIT(0xF365F9A6, 0xA7DE, 0x4071,
 		  0xA6, 0x6A, 0xB4, 0x0C, 0x0B, 0x4F, 0x8E, 0x52);
 
+/**
+ * cxl_acpi_get_rootdev_handle - get the ACPI handle of the CXL root device
+ * @dev: 'struct device' to start searching from. Should be from cxl_port->dev.
+ *
+ * Return: acpi_handle on success, errptr of errno on error.
+ *
+ * Looks for the ACPI0017 device and return the ACPI handle
+ **/
+acpi_handle cxl_acpi_get_rootdev_handle(struct device *dev)
+{
+	struct device *itr = dev;
+	struct device *root_dev;
+	acpi_handle handle;
+
+	if (!dev)
+		return ERR_PTR(-EINVAL);
+
+	while (itr->parent) {
+		root_dev = itr;
+		itr = itr->parent;
+	}
+
+	if (!dev_is_platform(root_dev))
+		return ERR_PTR(-ENODEV);
+
+	handle = ACPI_HANDLE(root_dev);
+	if (!handle)
+		return ERR_PTR(-ENODEV);
+
+	return handle;
+}
+EXPORT_SYMBOL_NS_GPL(cxl_acpi_get_rootdev_handle, CXL);
+
 /**
  * cxl_acpi_evaluate_qtg_dsm - Retrieve QTG ids via ACPI _DSM
  * @handle: ACPI handle
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 6426c4c22e28..d7c1410a437c 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -808,6 +808,7 @@ struct qtg_dsm_output {
 
 struct qtg_dsm_output *cxl_acpi_evaluate_qtg_dsm(acpi_handle handle,
 						 struct qtg_dsm_input *input);
+acpi_handle cxl_acpi_get_rootdev_handle(struct device *dev);
 
 /*
  * Unit test builds overrides this to __weak, find the 'strong' version