Documentation/mhi/mhi.rst

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.. SPDX-License-Identifier: GPL-2.0

==========================
MHI (Modem Host Interface)
==========================

This document provides information about the MHI protocol.

Overview
========

MHI is a protocol developed by Qualcomm Innovation Center, Inc. It is used
by the host processors to control and communicate with modem devices over high
speed peripheral buses or shared memory. Even though MHI can be easily adapted
to any peripheral buses, it is primarily used with PCIe based devices. MHI
provides logical channels over the physical buses and allows transporting the
modem protocols, such as IP data packets, modem control messages, and
diagnostics over at least one of those logical channels. Also, the MHI
protocol provides data acknowledgment feature and manages the power state of the
modems via one or more logical channels.

MHI Internals
=============

MMIO
----

MMIO (Memory mapped IO) consists of a set of registers in the device hardware,
which are mapped to the host memory space by the peripheral buses like PCIe.
Following are the major components of MMIO register space:

MHI control registers: Access to MHI configurations registers

MHI BHI registers: BHI (Boot Host Interface) registers are used by the host
for downloading the firmware to the device before MHI initialization.

Channel Doorbell array: Channel Doorbell (DB) registers used by the host to
notify the device when there is new work to do.

Event Doorbell array: Associated with event context array, the Event Doorbell
(DB) registers are used by the host to notify the device when new events are
available.

Debug registers: A set of registers and counters used by the device to expose
debugging information like performance, functional, and stability to the host.

Data structures
---------------

All data structures used by MHI are in the host system memory. Using the
physical interface, the device accesses those data structures. MHI data
structures and data buffers in the host system memory regions are mapped for
the device.

Channel context array: All channel configurations are organized in channel
context data array.

Transfer rings: Used by the host to schedule work items for a channel. The
transfer rings are organized as a circular queue of Transfer Descriptors (TD).

Event context array: All event configurations are organized in the event context
data array.

Event rings: Used by the device to send completion and state transition messages
to the host

Command context array: All command configurations are organized in command
context data array.

Command rings: Used by the host to send MHI commands to the device. The command

Annotation

Implementation Notes