applications: nrf_desktop: Document introducing custom HID providers

The documentation simplifies using custom HID providers. Change also
adds a link to the newly introduced section from the HID configuration
documentation.

Jira: NCSDK-35005

Signed-off-by: Marek Pieta <[email protected]>
Signed-off-by: Pekka Niskanen <[email protected]>

Author: MarekPieta

applications/nrf_desktop/application_kconfig.rst

@@ -52,6 +52,11 @@ You can enable the following HID reports:
 * HID system control report (:ref:`CONFIG_DESKTOP_HID_REPORT_SYSTEM_CTRL_SUPPORT <config_desktop_app_options>`)
 * HID consumer control report (:ref:`CONFIG_DESKTOP_HID_REPORT_CONSUMER_CTRL_SUPPORT <config_desktop_app_options>`)
 
+.. note::
+   nRF Desktop application allows you to modify the used HID input reports or introduce support for a new HID input report.
+   This can be achieved by implementing a custom HID report provider that can be used together with the :ref:`nrf_desktop_hid_state`.
+   For details, see the :ref:`nrf_desktop_hid_state_providing_hid_input_reports` documentation section.
+
 Apart from this, you can specify the supported HID boot protocol interface as one of the following:
 
 * mouse (:ref:`CONFIG_DESKTOP_HID_BOOT_INTERFACE_MOUSE <config_desktop_app_options>`)

applications/nrf_desktop/doc/hid_state.rst

@@ -175,6 +175,38 @@ The following application modules are used as default implementations of HID rep
 The respective HID report provider is automatically enabled if support for a given HID input report is enabled in the :ref:`nrf_desktop_hid_configuration`.
 See the documentation page of a HID report provider for detailed information about the provider.
 
+Custom HID providers
+--------------------
+
+You can implement your own HID report provider as part of the application.
+The HID report provider can perform one of the following two actions:
+
+* Handle a HID input report that is already supported by the application instead of a default HID report provider (substitute the default HID report provider).
+  Make sure to disable the default HID report provider while implementing the custom provider.
+* Support a new HID input report.
+
+HID report map update
+~~~~~~~~~~~~~~~~~~~~~
+
+If your HID report provider implementation uses a different HID input report format or you add a new HID input report, you need to align the HID report configuration (including the HID report map).
+If the default HID report descriptor is used (:ref:`CONFIG_DESKTOP_USE_DEFAULT_REPORT_DESCR <config_desktop_app_options>`), the configuration is defined by the following files:
+
+* :file:`configuration/common/hid_report_desc.h`
+* :file:`configuration/common/hid_report_desc.c`
+
+.. note::
+   nRF Desktop dongles share a common HID report format with the nRF Desktop peripherals.
+   The aligned HID report configuration is required for the dongle to forward HID input reports from the peripherals.
+
+HID transport update
+~~~~~~~~~~~~~~~~~~~~
+
+If you add a new HID input report, you might also need to update the modules that act as HID transports (for example :ref:`nrf_desktop_hids` or :ref:`nrf_desktop_usb_state`).
+This is needed to fulfill the following requirements:
+
+* Proper configuration of the module and libraries used by the module.
+* Support for the newly added HID input report.
+
 HID output reports
 ==================