[PATCH v15 1/9] Documentation/driver-api: Add document about WBRF mechanism

[Ma Jun <Jun.Ma2@amd.com>]

Add documentation about AMD's Wifi band RFI mitigation (WBRF) mechanism
explaining the theory and how it is used.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
---
 Documentation/driver-api/wbrf.rst | 78 +++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 Documentation/driver-api/wbrf.rst

diff --git a/Documentation/driver-api/wbrf.rst b/Documentation/driver-api/wbrf.rst
new file mode 100644
index 000000000000..12793bb1abce
--- /dev/null
+++ b/Documentation/driver-api/wbrf.rst
@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+=================================
+WBRF - Wifi Band RFI Mitigations
+=================================
+
+Due to electrical and mechanical constraints in certain platform designs
+there may be likely interference of relatively high-powered harmonics of
+the GPU memory clocks with local radio module frequency bands used by
+certain Wifi bands.
+
+To mitigate possible RFI interference producers can advertise the
+frequencies in use and consumers can use this information to avoid using
+these frequencies for sensitive features.
+
+When a platform is known to have this issue with any contained devices,
+the platform designer will advertise the availability of this feature via
+ACPI devices with a device specific method (_DSM).
+* Producers with this _DSM will be able to advertise the frequencies in use.
+* Consumers with this _DSM will be able to register for notifications of
+frequencies in use.
+
+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.
+
+ACPI interface
+==============
+
+Although initially used by for wifi + dGPU use cases, the ACPI interface
+can be scaled to any type of device that a platform designer discovers
+can cause interference.
+
+The GUID used for the _DSM is 7B7656CF-DC3D-4C1C-83E9-66E721DE3070.
+
+3 functions are available in this _DSM:
+
+* 0: discover # of functions available
+* 1: record RF bands in use
+* 2: retrieve RF bands in use
+
+Driver programming interface
+============================
+
+.. kernel-doc:: drivers/platform/x86/amd/wbrf.c
+
+Sample Usage
+=============
+
+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_remove` with 'add'
+   param to get other consumers properly notified.
+3. Or on stopping using some frequency band, call
+   `acpi_amd_wbrf_add_remove` with 'remove' param to get other consumers notified.
+
+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 `amd_wbrf_register_notifier` to register for notification
+   of frequency band change(add or remove) from other producers.
+3. Call the `amd_wbrf_retrieve_freq_band` initally 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
+   `amd_wbrf_retrieve_freq_band` again to retrieve the latest
+   active frequency bands.
+5. During driver cleanup, call `amd_wbrf_unregister_notifier` to
+   unregister the notifier.
-- 
2.34.1