Message-ID: <57fb0391-dd90-4a1c-b67f-b24fe544f6e6@amd.com>
Date:   Mon, 18 Sep 2023 08:04:16 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        Evan Quan <evan.quan@....com>, lenb@...nel.org,
        johannes@...solutions.net, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        alexander.deucher@....com, Lijo.Lazar@....com
Subject: Re: [V11 1/8] ACPI: Add support for AMD ACPI based Wifi band RFI
 mitigation feature

On 8/31/2023 01:20, Evan Quan wrote:
> Due to electrical and mechanical constraints in certain platform designs
> there may be likely interference of relatively high-powered harmonics of
> the (G-)DDR memory clocks with local radio module frequency bands used
> by Wifi 6/6e/7.
> 
> To mitigate this, AMD has introduced a mechanism that devices can use to
> notify active use of particular frequencies so that other devices can make
> relative internal adjustments as necessary to avoid this resonance.
> 
> Signed-off-by: Evan Quan <evan.quan@....com>
> --
> v10->v11:
>    - fix typo(Simon)

Rafael,

Friendly ping on reviewing patch 1 from v11.

Thank you,

> ---
>   drivers/acpi/Kconfig          |  17 ++
>   drivers/acpi/Makefile         |   2 +
>   drivers/acpi/amd_wbrf.c       | 414 ++++++++++++++++++++++++++++++++++
>   include/linux/acpi_amd_wbrf.h | 140 ++++++++++++
>   4 files changed, 573 insertions(+)
>   create mode 100644 drivers/acpi/amd_wbrf.c
>   create mode 100644 include/linux/acpi_amd_wbrf.h
> 
> diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
> index 00dd309b6682..a092ea72d152 100644
> --- a/drivers/acpi/Kconfig
> +++ b/drivers/acpi/Kconfig
> @@ -594,6 +594,23 @@ config ACPI_PRMT
>   	  substantially increase computational overhead related to the
>   	  initialization of some server systems.
>   
> +config WBRF_AMD_ACPI
> +	bool "ACPI based WBRF mechanism introduced by AMD"
> +	depends on ACPI
> +	default n
> +	help
> +	  Wifi band RFI mitigation mechanism allows multiple drivers from
> +	  different domains to notify the frequencies in use so that hardware
> +	  can be reconfigured to avoid harmonic conflicts.
> +
> +	  AMD has introduced an ACPI based mechanism to support WBRF for some
> +	  platforms with AMD dGPU and WLAN. This needs support from BIOS equipped
> +	  with necessary AML implementations and dGPU firmwares.
> +
> +	  Before enabling this ACPI based mechanism, it is suggested to confirm
> +	  with the hardware designer/provider first whether your platform
> +	  equipped with necessary BIOS and firmwares.
> +
>   endif	# ACPI
>   
>   config X86_PM_TIMER
> diff --git a/drivers/acpi/Makefile b/drivers/acpi/Makefile
> index eaa09bf52f17..a3d2f259d0a5 100644
> --- a/drivers/acpi/Makefile
> +++ b/drivers/acpi/Makefile
> @@ -132,3 +132,5 @@ obj-$(CONFIG_ARM64)		+= arm64/
>   obj-$(CONFIG_ACPI_VIOT)		+= viot.o
>   
>   obj-$(CONFIG_RISCV)		+= riscv/
> +
> +obj-$(CONFIG_WBRF_AMD_ACPI)	+= amd_wbrf.o
> diff --git a/drivers/acpi/amd_wbrf.c b/drivers/acpi/amd_wbrf.c
> new file mode 100644
> index 000000000000..8ee0e2977a30
> --- /dev/null
> +++ b/drivers/acpi/amd_wbrf.c
> @@ -0,0 +1,414 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Wifi Band Exclusion Interface (AMD ACPI Implementation)
> + * Copyright (C) 2023 Advanced Micro Devices
> + *
> + */
> +
> +#include <linux/acpi.h>
> +#include <linux/acpi_amd_wbrf.h>
> +
> +#define ACPI_AMD_WBRF_METHOD	"\\WBRF"
> +
> +/*
> + * Functions bit vector for WBRF method
> + *
> + * Bit 0: Supported for any functions other than function 0.
> + * Bit 1: Function 1 (Add / Remove frequency) is supported.
> + * Bit 2: Function 2 (Get frequency list) is supported.
> + */
> +#define WBRF_ENABLED				0x0
> +#define WBRF_RECORD				0x1
> +#define WBRF_RETRIEVE				0x2
> +
> +/* record actions */
> +#define WBRF_RECORD_ADD		0x0
> +#define WBRF_RECORD_REMOVE	0x1
> +
> +#define WBRF_REVISION		0x1
> +
> +/*
> + * The data structure used for WBRF_RETRIEVE is not naturally aligned.
> + * And unfortunately the design has been settled down.
> + */
> +struct amd_wbrf_ranges_out {
> +	u32			num_of_ranges;
> +	struct exclusion_range	band_list[MAX_NUM_OF_WBRF_RANGES];
> +} __packed;
> +
> +static const guid_t wifi_acpi_dsm_guid =
> +	GUID_INIT(0x7b7656cf, 0xdc3d, 0x4c1c,
> +		  0x83, 0xe9, 0x66, 0xe7, 0x21, 0xde, 0x30, 0x70);
> +
> +static BLOCKING_NOTIFIER_HEAD(wbrf_chain_head);
> +
> +static int wbrf_dsm(struct acpi_device *adev,
> +		    u8 fn,
> +		    union acpi_object *argv4)
> +{
> +	union acpi_object *obj;
> +	int rc;
> +
> +	obj = acpi_evaluate_dsm(adev->handle, &wifi_acpi_dsm_guid,
> +				WBRF_REVISION, fn, argv4);
> +	if (!obj)
> +		return -ENXIO;
> +
> +	switch (obj->type) {
> +	case ACPI_TYPE_INTEGER:
> +		rc = obj->integer.value ? -EINVAL : 0;
> +		break;
> +	default:
> +		rc = -EOPNOTSUPP;
> +	}
> +
> +	ACPI_FREE(obj);
> +
> +	return rc;
> +}
> +
> +static int wbrf_record(struct acpi_device *adev, uint8_t action,
> +		       struct wbrf_ranges_in_out *in)
> +{
> +	union acpi_object argv4;
> +	union acpi_object *tmp;
> +	u32 num_of_ranges = 0;
> +	u32 num_of_elements;
> +	u32 arg_idx = 0;
> +	u32 loop_idx;
> +	int ret;
> +
> +	if (!in)
> +		return -EINVAL;
> +
> +	for (loop_idx = 0; loop_idx < ARRAY_SIZE(in->band_list);
> +	     loop_idx++)
> +		if (in->band_list[loop_idx].start &&
> +		    in->band_list[loop_idx].end)
> +			num_of_ranges++;
> +
> +	/*
> +	 * The valid entry counter does not match with this told.
> +	 * Something must went wrong.
> +	 */
> +	if (num_of_ranges != in->num_of_ranges)
> +		return -EINVAL;
> +
> +	/*
> +	 * Every input frequency band comes with two end points(start/end)
> +	 * and each is accounted as an element. Meanwhile the range count
> +	 * and action type are accounted as an element each.
> +	 * So, the total element count = 2 * num_of_ranges + 1 + 1.
> +	 */
> +	num_of_elements = 2 * num_of_ranges + 1 + 1;
> +
> +	tmp = kcalloc(num_of_elements, sizeof(*tmp), GFP_KERNEL);
> +	if (!tmp)
> +		return -ENOMEM;
> +
> +	argv4.package.type = ACPI_TYPE_PACKAGE;
> +	argv4.package.count = num_of_elements;
> +	argv4.package.elements = tmp;
> +
> +	tmp[arg_idx].integer.type = ACPI_TYPE_INTEGER;
> +	tmp[arg_idx++].integer.value = num_of_ranges;
> +	tmp[arg_idx].integer.type = ACPI_TYPE_INTEGER;
> +	tmp[arg_idx++].integer.value = action;
> +
> +	for (loop_idx = 0; loop_idx < ARRAY_SIZE(in->band_list);
> +	     loop_idx++) {
> +		if (!in->band_list[loop_idx].start ||
> +		    !in->band_list[loop_idx].end)
> +			continue;
> +
> +		tmp[arg_idx].integer.type = ACPI_TYPE_INTEGER;
> +		tmp[arg_idx++].integer.value = in->band_list[loop_idx].start;
> +		tmp[arg_idx].integer.type = ACPI_TYPE_INTEGER;
> +		tmp[arg_idx++].integer.value = in->band_list[loop_idx].end;
> +	}
> +
> +	ret = wbrf_dsm(adev, WBRF_RECORD, &argv4);
> +
> +	kfree(tmp);
> +
> +	return ret;
> +}
> +
> +/**
> + * acpi_amd_wbrf_add_exclusion - broadcast the frequency band the device
> + *                               is using
> + *
> + * @dev: device pointer
> + * @in: input structure containing the frequency band the device is using
> + *
> + * Broadcast to other consumers the frequency band the device starts
> + * to use. Underneath the surface the information is cached into an
> + * internal buffer first. Then a notification is sent to all those
> + * registered consumers. So then they can retrieve that buffer to
> + * know the latest active frequency bands. The benefit with such design
> + * is for those consumers which have not been registered yet, they can
> + * still have a chance to retrieve such information later.
> + */
> +int acpi_amd_wbrf_add_exclusion(struct device *dev,
> +				struct wbrf_ranges_in_out *in)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(dev);
> +	int ret;
> +
> +	if (!adev)
> +		return -ENODEV;
> +
> +	ret = wbrf_record(adev, WBRF_RECORD_ADD, in);
> +	if (ret)
> +		return ret;
> +
> +	blocking_notifier_call_chain(&wbrf_chain_head,
> +				     WBRF_CHANGED,
> +				     NULL);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_add_exclusion);
> +
> +/**
> + * acpi_amd_wbrf_remove_exclusion - broadcast the frequency band the device
> + *                                  is no longer using
> + *
> + * @dev: device pointer
> + * @in: input structure containing the frequency band which is not used
> + *      by the device any more
> + *
> + * Broadcast to other consumers the frequency band the device stops
> + * to use. The stored information paired with this will be dropped
> + * from the internal buffer. And then a notification is sent to
> + * all registered consumers.
> + */
> +int acpi_amd_wbrf_remove_exclusion(struct device *dev,
> +				   struct wbrf_ranges_in_out *in)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(dev);
> +	int ret;
> +
> +	if (!adev)
> +		return -ENODEV;
> +
> +	ret = wbrf_record(adev, WBRF_RECORD_REMOVE, in);
> +	if (ret)
> +		return ret;
> +
> +	blocking_notifier_call_chain(&wbrf_chain_head,
> +				     WBRF_CHANGED,
> +				     NULL);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_remove_exclusion);
> +
> +static bool acpi_amd_wbrf_supported_system(void)
> +{
> +	acpi_status status;
> +	acpi_handle handle;
> +
> +	status = acpi_get_handle(NULL, ACPI_AMD_WBRF_METHOD, &handle);
> +
> +	return ACPI_SUCCESS(status);
> +}
> +
> +/**
> + * acpi_amd_wbrf_supported_producer - determine if the WBRF can be enabled
> + *                                    for the device as a producer
> + *
> + * @dev: device pointer
> + *
> + * Determine if the platform equipped with necessary implementations to
> + * support WBRF for the device as a producer.
> + */
> +bool acpi_amd_wbrf_supported_producer(struct device *dev)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(dev);
> +
> +	if (!acpi_amd_wbrf_supported_system())
> +		return false;
> +
> +	if (!adev)
> +		return false;
> +
> +	return acpi_check_dsm(adev->handle, &wifi_acpi_dsm_guid,
> +			      WBRF_REVISION,
> +			      BIT(WBRF_RECORD));
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_supported_producer);
> +
> +static union acpi_object *
> +acpi_evaluate_wbrf(acpi_handle handle, u64 rev, u64 func)
> +{
> +	acpi_status ret;
> +	struct acpi_buffer buf = {ACPI_ALLOCATE_BUFFER, NULL};
> +	union acpi_object params[4];
> +	struct acpi_object_list input = {
> +		.count = 4,
> +		.pointer = params,
> +	};
> +
> +	params[0].type = ACPI_TYPE_INTEGER;
> +	params[0].integer.value = rev;
> +	params[1].type = ACPI_TYPE_INTEGER;
> +	params[1].integer.value = func;
> +	params[2].type = ACPI_TYPE_PACKAGE;
> +	params[2].package.count = 0;
> +	params[2].package.elements = NULL;
> +	params[3].type = ACPI_TYPE_STRING;
> +	params[3].string.length = 0;
> +	params[3].string.pointer = NULL;
> +
> +	ret = acpi_evaluate_object(handle, "WBRF", &input, &buf);
> +	if (ACPI_FAILURE(ret))
> +		return NULL;
> +
> +	return buf.pointer;
> +}
> +
> +static bool check_acpi_wbrf(acpi_handle handle, u64 rev, u64 funcs)
> +{
> +	int i;
> +	u64 mask = 0;
> +	union acpi_object *obj;
> +
> +	if (funcs == 0)
> +		return false;
> +
> +	obj = acpi_evaluate_wbrf(handle, rev, 0);
> +	if (!obj)
> +		return false;
> +
> +	if (obj->type != ACPI_TYPE_BUFFER)
> +		return false;
> +
> +	/*
> +	 * Bit vector providing supported functions information.
> +	 * Each bit marks support for one specific function of the WBRF method.
> +	 */
> +	for (i = 0; i < obj->buffer.length && i < 8; i++)
> +		mask |= (u64)obj->buffer.pointer[i] << i * 8;
> +
> +	ACPI_FREE(obj);
> +
> +	return mask & BIT(WBRF_ENABLED) && (mask & funcs) == funcs;
> +}
> +
> +/**
> + * acpi_amd_wbrf_supported_consumer - determine if the WBRF can be enabled
> + *                                    for the device as a consumer
> + *
> + * @dev: device pointer
> + *
> + * Determine if the platform equipped with necessary implementations to
> + * support WBRF for the device as a consumer.
> + */
> +bool acpi_amd_wbrf_supported_consumer(struct device *dev)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(dev);
> +
> +	if (!acpi_amd_wbrf_supported_system())
> +		return false;
> +
> +	if (!adev)
> +		return false;
> +
> +	return check_acpi_wbrf(adev->handle,
> +			       WBRF_REVISION,
> +			       BIT(WBRF_RETRIEVE));
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_supported_consumer);
> +
> +/**
> + * acpi_amd_wbrf_retrieve_exclusions - retrieve current active frequency
> + *                                     bands
> + *
> + * @dev: device pointer
> + * @out: output structure containing all the active frequency bands
> + *
> + * Retrieve the current active frequency bands which were broadcasted
> + * by other producers. The consumer who calls this API should take
> + * proper actions if any of the frequency band may cause RFI with its
> + * own frequency band used.
> + */
> +int acpi_amd_wbrf_retrieve_exclusions(struct device *dev,
> +				      struct wbrf_ranges_in_out *out)
> +{
> +	struct acpi_device *adev = ACPI_COMPANION(dev);
> +	struct amd_wbrf_ranges_out acpi_out = {0};
> +	union acpi_object *obj;
> +	int ret = 0;
> +
> +	if (!adev)
> +		return -ENODEV;
> +
> +	obj = acpi_evaluate_wbrf(adev->handle,
> +				 WBRF_REVISION,
> +				 WBRF_RETRIEVE);
> +	if (!obj)
> +		return -EINVAL;
> +
> +	/*
> +	 * The return buffer is with variable length and the format below:
> +	 * number_of_entries(1 DWORD):       Number of entries
> +	 * start_freq of 1st entry(1 QWORD): Start frequency of the 1st entry
> +	 * end_freq of 1st entry(1 QWORD):   End frequency of the 1st entry
> +	 * ...
> +	 * ...
> +	 * start_freq of the last entry(1 QWORD)
> +	 * end_freq of the last entry(1 QWORD)
> +	 *
> +	 * Thus the buffer length is determined by the number of entries.
> +	 * - For zero entry scenario, the buffer length will be 4 bytes.
> +	 * - For one entry scenario, the buffer length will be 20 bytes.
> +	 */
> +	if (obj->buffer.length > sizeof(acpi_out) ||
> +	    obj->buffer.length < 4) {
> +		dev_err(dev, "Wrong sized WBRT information");
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +	memcpy(&acpi_out, obj->buffer.pointer, obj->buffer.length);
> +
> +	out->num_of_ranges = acpi_out.num_of_ranges;
> +	memcpy(out->band_list, acpi_out.band_list, sizeof(acpi_out.band_list));
> +
> +out:
> +	ACPI_FREE(obj);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_retrieve_exclusions);
> +
> +/**
> + * acpi_amd_wbrf_register_notifier - register for notifications of frequency
> + *                                   band update
> + *
> + * @nb: driver notifier block
> + *
> + * The consumer should register itself via this API. So that it can get
> + * notified timely on the frequency band updates from other producers.
> + */
> +int acpi_amd_wbrf_register_notifier(struct notifier_block *nb)
> +{
> +	return blocking_notifier_chain_register(&wbrf_chain_head, nb);
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_register_notifier);
> +
> +/**
> + * acpi_amd_wbrf_unregister_notifier - unregister for notifications of
> + *                                     frequency band update
> + *
> + * @nb: driver notifier block
> + *
> + * The consumer should call this API when it is longer interested with
> + * the frequency band updates from other producers. Usually, this should
> + * be performed during driver cleanup.
> + */
> +int acpi_amd_wbrf_unregister_notifier(struct notifier_block *nb)
> +{
> +	return blocking_notifier_chain_unregister(&wbrf_chain_head, nb);
> +}
> +EXPORT_SYMBOL_GPL(acpi_amd_wbrf_unregister_notifier);
> diff --git a/include/linux/acpi_amd_wbrf.h b/include/linux/acpi_amd_wbrf.h
> new file mode 100644
> index 000000000000..c2363d664641
> --- /dev/null
> +++ b/include/linux/acpi_amd_wbrf.h
> @@ -0,0 +1,140 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Wifi Band Exclusion Interface (AMD ACPI Implementation)
> + * Copyright (C) 2023 Advanced Micro Devices
> + *
> + * Due to electrical and mechanical constraints in certain platform designs
> + * there may be likely interference of relatively high-powered harmonics of
> + * the (G-)DDR memory clocks with local radio module frequency bands used
> + * by Wifi 6/6e/7.
> + *
> + * To mitigate this, AMD has introduced an ACPI based mechanism to support
> + * WBRF(Wifi Band RFI mitigation Feature) for platforms with AMD dGPU + WLAN.
> + * This needs support from BIOS equipped with necessary AML implementations
> + * and dGPU firmwares.
> + *
> + * Some general terms:
> + * Producer: such component who can produce high-powered radio frequency
> + * Consumer: such component who can adjust its in-use frequency in
> + *           response to the radio frequencies of other components to
> + *           mitigate the possible RFI.
> + *
> + * To make the mechanism function, those producers should notify active use
> + * of their particular frequencies so that other consumers can make relative
> + * internal adjustments as necessary to avoid this resonance.
> + */
> +
> +#ifndef _ACPI_AMD_WBRF_H
> +#define _ACPI_AMD_WBRF_H
> +
> +#include <linux/device.h>
> +#include <linux/notifier.h>
> +
> +/*
> + * A wbrf range is defined as a frequency band with start and end
> + * frequency point specified(in Hz). And a vaild range should have
> + * its start and end frequency point filled with non-zero values.
> + * Meanwhile, the maximum number of wbrf ranges is limited as
> + * `MAX_NUM_OF_WBRF_RANGES`.
> + */
> +#define MAX_NUM_OF_WBRF_RANGES		11
> +
> +struct exclusion_range {
> +	u64		start;
> +	u64		end;
> +};
> +
> +struct wbrf_ranges_in_out {
> +	u64			num_of_ranges;
> +	struct exclusion_range	band_list[MAX_NUM_OF_WBRF_RANGES];
> +};
> +
> +/*
> + * The notification types for the consumers are defined as below.
> + * The consumers may need to take different actions in response to
> + * different notifications.
> + * WBRF_CHANGED: there was some frequency band updates. The consumers
> + *               should retrieve the latest active frequency bands.
> + */
> +enum wbrf_notifier_actions {
> +	WBRF_CHANGED,
> +};
> +
> +#if IS_ENABLED(CONFIG_WBRF_AMD_ACPI)
> +/*
> + * The expected flow for the producers:
> + * 1) During probe, call `acpi_amd_wbrf_supported_producer` to check
> + *    if WBRF can be enabled for the device.
> + * 2) On using some frequency band, call `acpi_amd_wbrf_add_exclusion`
> + *    to get other consumers properly notified.
> + * 3) Or on stopping using some frequency band, call
> + *    `acpi_amd_wbrf_remove_exclusion` to get other consumers notified.
> + */
> +bool acpi_amd_wbrf_supported_producer(struct device *dev);
> +int acpi_amd_wbrf_remove_exclusion(struct device *dev,
> +				   struct wbrf_ranges_in_out *in);
> +int acpi_amd_wbrf_add_exclusion(struct device *dev,
> +				struct wbrf_ranges_in_out *in);
> +
> +/*
> + * The expected flow for the consumers:
> + * 1) During probe, call `acpi_amd_wbrf_supported_consumer` to check if WBRF
> + *    can be enabled for the device.
> + * 2) Call `acpi_amd_wbrf_register_notifier` to register for notification
> + *    of frequency band change(add or remove) from other producers.
> + * 3) Call the `acpi_amd_wbrf_retrieve_exclusions` intentionally to retrieve
> + *    current active frequency bands considering some producers may broadcast
> + *    such information before the consumer is up.
> + * 4) On receiving a notification for frequency band change, run
> + *    `acpi_amd_wbrf_retrieve_exclusions` again to retrieve the latest
> + *    active frequency bands.
> + * 5) During driver cleanup, call `acpi_amd_wbrf_unregister_notifier` to
> + *    unregister the notifier.
> + */
> +bool acpi_amd_wbrf_supported_consumer(struct device *dev);
> +int acpi_amd_wbrf_retrieve_exclusions(struct device *dev,
> +				      struct wbrf_ranges_in_out *out);
> +int acpi_amd_wbrf_register_notifier(struct notifier_block *nb);
> +int acpi_amd_wbrf_unregister_notifier(struct notifier_block *nb);
> +#else
> +static inline
> +bool acpi_amd_wbrf_supported_consumer(struct device *dev)
> +{
> +	return false;
> +}
> +static inline
> +int acpi_amd_wbrf_remove_exclusion(struct device *dev,
> +				   struct wbrf_ranges_in_out *in)
> +{
> +	return -ENODEV;
> +}
> +static inline
> +int acpi_amd_wbrf_add_exclusion(struct device *dev,
> +				struct wbrf_ranges_in_out *in)
> +{
> +	return -ENODEV;
> +}
> +static inline
> +bool acpi_amd_wbrf_supported_producer(struct device *dev)
> +{
> +	return false;
> +}
> +static inline
> +int acpi_amd_wbrf_retrieve_exclusions(struct device *dev,
> +				      struct wbrf_ranges_in_out *out)
> +{
> +	return -ENODEV;
> +}
> +static inline
> +int acpi_amd_wbrf_register_notifier(struct notifier_block *nb)
> +{
> +	return -ENODEV;
> +}
> +static inline
> +int acpi_amd_wbrf_unregister_notifier(struct notifier_block *nb)
> +{
> +	return -ENODEV;
> +}
> +#endif
> +
> +#endif /* _ACPI_AMD_WBRF_H */

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