doc: Add Xtensa HiFi documentation

Adds some documentation about the Xtensa HiFi AudioEngine DSP and
how to enable its use with Zephyr.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

doc/hardware/arch/index.rst

@@ -11,3 +11,4 @@ Architecture-related Guides
    risc-v.rst
    semihost.rst
    x86.rst
+   xtensa.rst

doc/hardware/arch/xtensa.rst

@@ -0,0 +1,73 @@
+.. _xtensa_developer_guide:
+
+Xtensa Developer Guide
+######################
+
+Overview
+********
+
+This page contains information on certain aspects when developing for
+Xtensa-based platforms.
+
+HiFi Audio Engine DSP
+*********************
+
+The kernel allows threads to use the HiFi Audio Engine DSP registers on boards
+that support these registers. The kernel only supports the use of the HiFi
+registers by threads and not ISRs.
+
+.. note::
+    Presently, only the Intel ADSP ACE hardware platforms are configured for
+    HiFi support by default.
+
+Concepts
+========
+
+The kernel can be configured for an application to leverage the services
+provided by the Xtensa HiFi Audio Engine DSP. Three modes of operation are
+supported, which are described below.
+
+No HiFi registers mode
+----------------------
+
+This mode is used when the application has no threads that use the HiFi
+registers. It is the kernel's default HiFi services mode.
+
+Unshared HiFi registers mode
+----------------------------
+
+This mode is used when the application has only a single thread that uses the
+HiFi registers. The HiFi registers are left unchanged whenever a context
+switch occurs.
+
+.. note::
+    The behavior is undefined, if two or more threads attempt to use
+    the HiFi registers, as the kernel does not attempt to detect
+    (nor prevent) multiple threads from using these registers.
+
+Shared HiFi registers mode
+--------------------------
+
+This mode is used when the application has two or more threads that use HiFi
+registers. When enabled, the kernel automatically allows all threads to use the
+HiFi registers. During each thread context switch, the kernel saves the outgoing
+thread's HiFi registers and loads the incoming thread's HiFi registers,
+regardless of whether the thread utilizes them or not.
+
+Additional stack space may be required for each thread to account for the extra
+registers that must be saved.
+
+Configuration Options
+=====================
+
+The unshared HiFi registers mode is selected when configuration option
+:kconfig:option:`CONFIG_XTENSA_HIFI_SHARING` is disabled but configuration
+options :kconfig:option:`CONFIG_XTENSA_HIFI3` and/or
+:kconfig:option:`CONFIG_XTENSA_HIFI4` are enabled.
+
+The shared HiFi registers mode is selected when the configuration option
+:kconfig:option:`CONFIG_XTENSA_HIFI_SHARING` is enabled in addition to
+configuration options :kconfig:option:`CONFIG_XTENSA_HIFI3` and/or
+:kconfig:option:`CONFIG_XTENSA_HIFI4`. Threads must have sufficient
+stack space for saving the HiFi register values during context switches
+as described above.