platform/x86: Add driver for ACPI WMAA EC-based backlight control

A number of upcoming notebook computer designs drive the internal
display panel's backlight PWM through the Embedded Controller (EC).
This EC-based backlight control can be plumbed through to an ACPI
"WMAA" method interface, which in turn can be wrapped by WMI with
the GUID handle 603E9613-EF25-4338-A3D0-C46177516DB7.

Add a new driver, aliased to the WMAA WMI GUID, to expose a sysfs
backlight class driver to control backlight levels on systems with
EC-driven backlights.

Signed-off-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20210903003838.15797-1-ddadap@nvidia.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

MAINTAINERS

@@ -20246,6 +20246,12 @@ L:	linux-wireless@vger.kernel.org
 S:	Odd fixes
 F:	drivers/net/wireless/wl3501*
 
+WMAA BACKLIGHT DRIVER
+M:	Daniel Dadap <ddadap@nvidia.com>
+L:	platform-driver-x86@vger.kernel.org
+S:	Supported
+F:	drivers/platform/x86/wmaa-backlight-wmi.c
+
 WOLFSON MICROELECTRONICS DRIVERS
 L:	patches@opensource.cirrus.com
 S:	Supported

drivers/platform/x86/Kconfig

@@ -91,6 +91,22 @@ config PEAQ_WMI
 	help
 	 Say Y here if you want to support WMI-based hotkeys on PEAQ 2-in-1s.
 
+config WMAA_BACKLIGHT_WMI
+	tristate "ACPI WMAA Backlight Driver"
+	depends on ACPI_WMI
+	depends on BACKLIGHT_CLASS_DEVICE
+	help
+	  This driver provides a sysfs backlight interface for notebook
+	  systems which expose the WMAA ACPI method and an associated WMI
+	  wrapper to drive LCD backlight levels through the Embedded Controller
+	  (EC).
+
+	  Say Y or M here if you want to control the backlight on a notebook
+	  system with an EC-driven backlight using the ACPI WMAA method.
+
+	  If you choose to compile this driver as a module the module will be
+	  called wmaa-backlight-wmi.
+
 config XIAOMI_WMI
 	tristate "Xiaomi WMI key driver"
 	depends on ACPI_WMI

drivers/platform/x86/Makefile

@@ -12,6 +12,7 @@ obj-$(CONFIG_WMI_BMOF)		+= wmi-bmof.o
 obj-$(CONFIG_HUAWEI_WMI)		+= huawei-wmi.o
 obj-$(CONFIG_MXM_WMI)			+= mxm-wmi.o
 obj-$(CONFIG_PEAQ_WMI)			+= peaq-wmi.o
+obj-$(CONFIG_WMAA_BACKLIGHT_WMI)	+= wmaa-backlight-wmi.o
 obj-$(CONFIG_XIAOMI_WMI)		+= xiaomi-wmi.o
 obj-$(CONFIG_GIGABYTE_WMI)		+= gigabyte-wmi.o
 

drivers/platform/x86/wmaa-backlight-wmi.c

@@ -0,0 +1,205 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+ */
+
+#include <linux/acpi.h>
+#include <linux/backlight.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/wmi.h>
+
+/**
+ * enum wmaa_method - WMI method IDs for ACPI WMAA
+ * @WMAA_METHOD_LEVEL:  Get or set the brightness level,
+ *                      or get the maximum brightness level.
+ * @WMAA_METHOD_SOURCE: Get the source for backlight control.
+ */
+enum wmaa_method {
+	WMAA_METHOD_LEVEL = 1,
+	WMAA_METHOD_SOURCE = 2,
+	WMAA_METHOD_MAX
+};
+
+/**
+ * enum wmaa_mode - Operation mode for ACPI WMAA method
+ * @WMAA_MODE_GET:           Get the current brightness level or source.
+ * @WMAA_MODE_SET:           Set the brightness level.
+ * @WMAA_MODE_GET_MAX_LEVEL: Get the maximum brightness level. This is only
+ *                           valid when the WMI method is %WMAA_METHOD_LEVEL.
+ */
+enum wmaa_mode {
+	WMAA_MODE_GET = 0,
+	WMAA_MODE_SET = 1,
+	WMAA_MODE_GET_MAX_LEVEL = 2,
+	WMAA_MODE_MAX
+};
+
+/**
+ * enum wmaa_source - Backlight brightness control source identification
+ * @WMAA_SOURCE_GPU:   Backlight brightness is controlled by the GPU.
+ * @WMAA_SOURCE_EC:    Backlight brightness is controlled by the system's
+ *                     Embedded Controller (EC).
+ * @WMAA_SOURCE_AUX:   Backlight brightness is controlled over the DisplayPort
+ *                     AUX channel.
+ */
+enum wmaa_source {
+	WMAA_SOURCE_GPU = 1,
+	WMAA_SOURCE_EC = 2,
+	WMAA_SOURCE_AUX = 3,
+	WMAA_SOURCE_MAX
+};
+
+/**
+ * struct wmaa_args - arguments for the ACPI WMAA method
+ * @mode:    Pass in an &enum wmaa_mode value to select between getting or
+ *           setting a value.
+ * @val:     In parameter for value to set when operating in %WMAA_MODE_SET
+ *           mode. Not used in %WMAA_MODE_GET or %WMAA_MODE_GET_MAX_LEVEL mode.
+ * @ret:     Out parameter returning retrieved value when operating in
+ *           %WMAA_MODE_GET or %WMAA_MODE_GET_MAX_LEVEL mode. Not used in
+ *           %WMAA_MODE_SET mode.
+ * @ignored: Padding; not used. The ACPI method expects a 24 byte params struct.
+ *
+ * This is the parameters structure for the ACPI WMAA method as wrapped by WMI.
+ * The value passed in to @val or returned by @ret will be a brightness value
+ * when the WMI method ID is %WMAA_METHOD_LEVEL, or an &enum wmaa_source value
+ * when the WMI method ID is %WMAA_METHOD_SOURCE.
+ */
+struct wmaa_args {
+	u32 mode;
+	u32 val;
+	u32 ret;
+	u32 ignored[3];
+};
+
+/**
+ * wmi_call_wmaa() - helper function for calling ACPI WMAA via its WMI wrapper
+ * @w:    Pointer to the struct wmi_device identified by %WMAA_WMI_GUID
+ * @id:   The method ID for the ACPI WMAA method (e.g. %WMAA_METHOD_LEVEL or
+ *        %WMAA_METHOD_SOURCE)
+ * @mode: The operation to perform on the ACPI WMAA method (e.g. %WMAA_MODE_SET
+ *        or %WMAA_MODE_GET)
+ * @val:  Pointer to a value passed in by the caller when @mode is
+ *        %WMAA_MODE_SET, or a value passed out to the caller when @mode is
+ *        %WMAA_MODE_GET or %WMAA_MODE_GET_MAX_LEVEL.
+ *
+ * Returns 0 on success, or a negative error number on failure.
+ */
+static int wmi_call_wmaa(struct wmi_device *w, enum wmaa_method id, enum wmaa_mode mode, u32 *val)
+{
+	struct wmaa_args args = {
+		.mode = mode,
+		.val = 0,
+		.ret = 0,
+	};
+	struct acpi_buffer buf = { (acpi_size)sizeof(args), &args };
+	acpi_status status;
+
+	if (id < WMAA_METHOD_LEVEL || id >= WMAA_METHOD_MAX ||
+	    mode < WMAA_MODE_GET || mode >= WMAA_MODE_MAX)
+		return -EINVAL;
+
+	if (mode == WMAA_MODE_SET)
+		args.val = *val;
+
+	status = wmidev_evaluate_method(w, 0, id, &buf, &buf);
+	if (ACPI_FAILURE(status)) {
+		dev_err(&w->dev, "ACPI WMAA failed: %s\n",
+			acpi_format_exception(status));
+		return -EIO;
+	}
+
+	if (mode != WMAA_MODE_SET)
+		*val = args.ret;
+
+	return 0;
+}
+
+static int wmaa_backlight_update_status(struct backlight_device *bd)
+{
+	struct wmi_device *wdev = bl_get_data(bd);
+
+	return wmi_call_wmaa(wdev, WMAA_METHOD_LEVEL, WMAA_MODE_SET,
+			     &bd->props.brightness);
+}
+
+static int wmaa_backlight_get_brightness(struct backlight_device *bd)
+{
+	struct wmi_device *wdev = bl_get_data(bd);
+	u32 level;
+	int ret;
+
+	ret = wmi_call_wmaa(wdev, WMAA_METHOD_LEVEL, WMAA_MODE_GET, &level);
+	if (ret < 0)
+		return ret;
+
+	return level;
+}
+
+static const struct backlight_ops wmaa_backlight_ops = {
+	.update_status = wmaa_backlight_update_status,
+	.get_brightness = wmaa_backlight_get_brightness,
+};
+
+static int wmaa_backlight_wmi_probe(struct wmi_device *wdev, const void *ctx)
+{
+	struct backlight_properties props = {};
+	struct backlight_device *bdev;
+	u32 source;
+	int ret;
+
+	ret = wmi_call_wmaa(wdev, WMAA_METHOD_SOURCE, WMAA_MODE_GET, &source);
+	if (ret)
+		return ret;
+
+	/*
+	 * This driver is only to be used when brightness control is handled
+	 * by the EC; otherwise, the GPU driver(s) should control brightness.
+	 */
+	if (source != WMAA_SOURCE_EC)
+		return -ENODEV;
+
+	/*
+	 * Identify this backlight device as a firmware device so that it can
+	 * be prioritized over any exposed GPU-driven raw device(s).
+	 */
+	props.type = BACKLIGHT_FIRMWARE;
+
+	ret = wmi_call_wmaa(wdev, WMAA_METHOD_LEVEL, WMAA_MODE_GET_MAX_LEVEL,
+			    &props.max_brightness);
+	if (ret)
+		return ret;
+
+	ret = wmi_call_wmaa(wdev, WMAA_METHOD_LEVEL, WMAA_MODE_GET,
+			    &props.brightness);
+	if (ret)
+		return ret;
+
+	bdev = devm_backlight_device_register(&wdev->dev, "wmaa_backlight",
+					      &wdev->dev, wdev,
+					      &wmaa_backlight_ops, &props);
+	return PTR_ERR_OR_ZERO(bdev);
+}
+
+#define WMAA_WMI_GUID "603E9613-EF25-4338-A3D0-C46177516DB7"
+
+static const struct wmi_device_id wmaa_backlight_wmi_id_table[] = {
+	{ .guid_string = WMAA_WMI_GUID },
+	{ }
+};
+MODULE_DEVICE_TABLE(wmi, wmaa_backlight_wmi_id_table);
+
+static struct wmi_driver wmaa_backlight_wmi_driver = {
+	.driver = {
+		.name = "wmaa-backlight",
+	},
+	.probe = wmaa_backlight_wmi_probe,
+	.id_table = wmaa_backlight_wmi_id_table,
+};
+module_wmi_driver(wmaa_backlight_wmi_driver);
+
+MODULE_AUTHOR("Daniel Dadap <ddadap@nvidia.com>");
+MODULE_DESCRIPTION("WMAA Backlight WMI driver");
+MODULE_LICENSE("GPL");