[PATCH v4] platform/x86: Add driver for ACPI WMAA EC-based backlight control

[Daniel Dadap <ddadap@nvidia.com>]

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: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Daniel Dadap <ddadap@nvidia.com>
---

v2: Convert to WMI subsystem driver, suggested by Mario Limonciello
     <mario.limonciello@outlook.com>; various cleanups suggested by
     Barnabás Pőcze <pobrn@protonmail.com>
v3: Address assorted style nits raised by Andy Shevchenko
     <andy.shevchenko@gmail.com> in response to a related patch; remove
     additional behavior change to WMI subsystem from patch series as
     recommended by Hans de Goede <hdegoede@redhat.com> 
v4: Use MODULE_DEVICE_TABLE() (Thomas Weißschuh <thomas@t-8ch.de>)
    Fix scope of internal driver state; various style fixes (Barnabás
    PÅ‘cze, Andy Shevchenko)
    Use devm_backlight_device_register() (Barnabás Pőcze)
    Add kerneldoc comments for enums and structs (Andy Shevchenko)

 MAINTAINERS                               |   6 +
 drivers/platform/x86/Kconfig              |  16 ++
 drivers/platform/x86/Makefile             |   1 +
 drivers/platform/x86/wmaa-backlight-wmi.c | 185 ++++++++++++++++++++++
 4 files changed, 208 insertions(+)
 create mode 100644 drivers/platform/x86/wmaa-backlight-wmi.c

diff --git a/MAINTAINERS b/MAINTAINERS
index bbaecde94aa0..fd7362a86c6d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20008,6 +20008,12 @@ 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
diff --git a/drivers/platform/x86/Kconfig b/drivers/platform/x86/Kconfig
index d12db6c316ea..0df908ef8d7c 100644
--- a/drivers/platform/x86/Kconfig
+++ b/drivers/platform/x86/Kconfig
@@ -113,6 +113,22 @@ 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 system's
+	  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
diff --git a/drivers/platform/x86/Makefile b/drivers/platform/x86/Makefile
index 7ee369aab10d..109c1714237d 100644
--- a/drivers/platform/x86/Makefile
+++ b/drivers/platform/x86/Makefile
@@ -14,6 +14,7 @@ obj-$(CONFIG_INTEL_WMI_SBL_FW_UPDATE)	+= intel-wmi-sbl-fw-update.o
 obj-$(CONFIG_INTEL_WMI_THUNDERBOLT)	+= intel-wmi-thunderbolt.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
 
diff --git a/drivers/platform/x86/wmaa-backlight-wmi.c b/drivers/platform/x86/wmaa-backlight-wmi.c
new file mode 100644
index 000000000000..cb1a973803b1
--- /dev/null
+++ b/drivers/platform/x86/wmaa-backlight-wmi.c
@@ -0,0 +1,185 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
+ */
+
+#include <linux/backlight.h>
+#include <linux/module.h>
+#include <linux/wmi.h>
+
+/**
+ * enum wmaa_method - WMI method IDs for ACPI WMAA
+ *
+ * @WMAA_LEVEL:  Get or set the brightness level,
+ *               or get the maximum brightness level.
+ * @WMAA_SOURCE: Get the source for backlight control.
+ */
+enum wmaa_method {
+	WMAA_LEVEL = 1,
+	WMAA_SOURCE = 2,
+};
+
+/**
+ * enum wmaa_get_or_set - Operation mode for ACPI WMAA method
+ *
+ * @WMAA_GET:     Get the current brightness level or backlight source.
+ * @WMAA_SET:     Set the brightness level.
+ * @WMAA_GET_MAX: Get the maximum brightness level. This is only valid when the
+ *                WMI method is %WMAA_LEVEL.
+ */
+enum wmaa_get_or_set {
+	WMAA_GET = 0,
+	WMAA_SET = 1,
+	WMAA_GET_MAX = 2
+};
+
+/**
+ * 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
+};
+
+/**
+ * struct wmaa_args - arguments for the ACPI WMAA method
+ *
+ * @set:     Pass in an &enum wmaa_get_or_set value to select between getting or
+ *           setting a value.
+ * @val:     In parameter for value to set when operating in %WMAA_SET mode. Not
+ *           used in %WMAA_GET or %WMAA_GET_MAX mode.
+ * @ret:     Out parameter returning retrieved value when operating in %WMAA_GET
+ *           or %WMAA_GET_MAX mode. Not used in %WMAA_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_LEVEL, or an &enum wmaa_source value when
+ * the WMI method ID is %WMAA_SOURCE.
+ */
+struct wmaa_args {
+	u32 set;
+	u32 val;
+	u32 ret;
+	u32 ignored[3];
+};
+
+static int wmi_call_wmaa(struct wmi_device *w, enum wmaa_method method, enum wmaa_get_or_set set, u32 *val)
+{
+	struct wmaa_args args = {
+		.set = set,
+		.val = 0,
+		.ret = 0,
+	};
+	struct acpi_buffer buf = { (acpi_size)sizeof(args), &args };
+	acpi_status status;
+
+	if (set == WMAA_SET)
+		args.val = *val;
+
+	status = wmidev_evaluate_method(w, 0, method, &buf, &buf);
+	if (ACPI_FAILURE(status)) {
+		dev_err(&w->dev, "ACPI WMAA failed with %s\n",
+			acpi_format_exception(status));
+		return -EIO;
+	}
+
+	if (set != WMAA_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_LEVEL, WMAA_SET, &bd->props.brightness);
+}
+
+static int wmaa_backlight_get_brightness(struct backlight_device *bd)
+{
+	struct wmi_device *wdev = bl_get_data(bd);
+	u32 level = 0;
+	int ret;
+
+	ret = wmi_call_wmaa(wdev, WMAA_LEVEL, WMAA_GET, &level);
+
+	if (ret)
+		dev_err(&bd->dev, "ACPI WMAA failed to get backlight level.\n");
+
+	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 *w, const void *ctx)
+{
+	struct backlight_properties props = {};
+	struct backlight_device *backlight;
+	u32 source;
+	int ret;
+
+	ret = wmi_call_wmaa(w, WMAA_SOURCE, WMAA_GET, &source);
+	if (ret)
+		return ret;
+
+	if (source != WMAA_SOURCE_EC) {
+		/* This driver is only to be used when brightness control is
+		 * handled by the EC; otherwise, the GPU driver(s) should handle
+		 * brightness control. */
+		return -ENODEV;
+	}
+
+	/* Identify this backlight device as a platform device so that it can
+	 * be prioritized over any exposed GPU-driven raw device(s). */
+	props.type = BACKLIGHT_PLATFORM;
+
+	ret = wmi_call_wmaa(w, WMAA_LEVEL, WMAA_GET_MAX, &props.max_brightness);
+	if (ret)
+		return ret;
+
+	ret = wmi_call_wmaa(w, WMAA_LEVEL, WMAA_GET, &props.brightness);
+	if (ret)
+		return ret;
+
+	backlight = devm_backlight_device_register(
+		&w->dev, "wmaa_backlight",
+		&w->dev, w, &wmaa_backlight_ops, &props);
+	if (IS_ERR(backlight))
+		return PTR_ERR(backlight);
+
+	return 0;
+}
+
+#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("Aaron Plattner <aplattner@nvidia.com>");
+MODULE_AUTHOR("Daniel Dadap <ddadap@nvidia.com>");
+MODULE_DESCRIPTION("WMAA Backlight WMI driver");
+MODULE_LICENSE("GPL v2");
-- 
2.20.1