mfd: Add ACPI support

If an MFD device is backed by ACPI namespace, we should allow subdevice
drivers to access their corresponding ACPI companion devices through normal
means (e.g using ACPI_COMPANION()).

This patch adds such support to the MFD core. If the MFD parent device
does not specify any ACPI _HID/_CID for the child device, the child
device will share the parent ACPI companion device. Otherwise the child
device will be assigned with the corresponding ACPI companion, if found
in the namespace below the parent.

Signed-off-by: Mika Westerberg <[e-mail]>
Reviewed-by: Darren Hart <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/acpi/enumeration.txt

···
 
 There are also devm_* versions of these functions which release the
 descriptors once the device is released.
+
+MFD devices
+~~~~~~~~~~~
+The MFD devices register their children as platform devices. For the child
+devices there needs to be an ACPI handle that they can use to reference
+parts of the ACPI namespace that relate to them. In the Linux MFD subsystem
+we provide two ways:
+
+	o The children share the parent ACPI handle.
+	o The MFD cell can specify the ACPI id of the device.
+
+For the first case, the MFD drivers do not need to do anything. The
+resulting child platform device will have its ACPI_COMPANION() set to point
+to the parent device.
+
+If the ACPI namespace has a device that we can match using an ACPI id,
+the id should be set like:
+
+	static struct mfd_cell my_subdevice_cell = {
+		.name = "my_subdevice",
+		/* set the resources relative to the parent */
+		.acpi_pnpid = "XYZ0001",
+	};
+
+The ACPI id "XYZ0001" is then used to lookup an ACPI device directly under
+the MFD device and if found, that ACPI companion device is bound to the
+resulting child platform device.

drivers/mfd/mfd-core.c

···
 	return 0;
 }
 
+#if IS_ENABLED(CONFIG_ACPI)
+static void mfd_acpi_add_device(const struct mfd_cell *cell,
+				struct platform_device *pdev)
+{
+	struct acpi_device *parent_adev;
+	struct acpi_device *adev;
+
+	parent_adev = ACPI_COMPANION(pdev->dev.parent);
+	if (!parent_adev)
+		return;
+
+	/*
+	 * MFD child device gets its ACPI handle either from the ACPI
+	 * device directly under the parent that matches the acpi_pnpid or
+	 * it will use the parent handle if is no acpi_pnpid is given.
+	 */
+	adev = parent_adev;
+	if (cell->acpi_pnpid) {
+		struct acpi_device_id ids[2] = {};
+		struct acpi_device *child_adev;
+
+		strlcpy(ids[0].id, cell->acpi_pnpid, sizeof(ids[0].id));
+		list_for_each_entry(child_adev, &parent_adev->children, node)
+			if (acpi_match_device_ids(child_adev, ids)) {
+				adev = child_adev;
+				break;
+			}
+	}
+
+	ACPI_COMPANION_SET(&pdev->dev, adev);
+}
+#else
+static inline void mfd_acpi_add_device(const struct mfd_cell *cell,
+				       struct platform_device *pdev)
+{
+}
+#endif
+
 static int mfd_add_device(struct device *parent, int id,
 			  const struct mfd_cell *cell, atomic_t *usage_count,
 			  struct resource *mem_base,
···
 			}
 		}
 	}
+
+	mfd_acpi_add_device(cell, pdev);
 
 	if (cell->pdata_size) {
 		ret = platform_device_add_data(pdev,

include/linux/mfd/core.h

···
 	 */
 	const char		*of_compatible;
 
+	/* Matches ACPI PNP id, either _HID or _CID */
+	const char		*acpi_pnpid;
+
 	/*
 	 * These resources can be specified relative to the parent device.
 	 * For accessing hardware you should use resources from the platform dev