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diff --git a/Documentation/mhi/mhi.rst b/Documentation/mhi/mhi.rst new file mode 100644 index 0000000000..803ff84f7d --- /dev/null +++ b/Documentation/mhi/mhi.rst @@ -0,0 +1,218 @@ +.. 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 +rings are organized as a circular queue of Command Descriptors (CD). + +Channels +-------- + +MHI channels are logical, unidirectional data pipes between a host and a device. +The concept of channels in MHI is similar to endpoints in USB. MHI supports up +to 256 channels. However, specific device implementations may support less than +the maximum number of channels allowed. + +Two unidirectional channels with their associated transfer rings form a +bidirectional data pipe, which can be used by the upper-layer protocols to +transport application data packets (such as IP packets, modem control messages, +diagnostics messages, and so on). Each channel is associated with a single +transfer ring. + +Transfer rings +-------------- + +Transfers between the host and device are organized by channels and defined by +Transfer Descriptors (TD). TDs are managed through transfer rings, which are +defined for each channel between the device and host and reside in the host +memory. TDs consist of one or more ring elements (or transfer blocks):: + + [Read Pointer (RP)] ----------->[Ring Element] } TD + [Write Pointer (WP)]- [Ring Element] + - [Ring Element] + --------->[Ring Element] + [Ring Element] + +Below is the basic usage of transfer rings: + +* Host allocates memory for transfer ring. +* Host sets the base pointer, read pointer, and write pointer in corresponding + channel context. +* Ring is considered empty when RP == WP. +* Ring is considered full when WP + 1 == RP. +* RP indicates the next element to be serviced by the device. +* When the host has a new buffer to send, it updates the ring element with + buffer information, increments the WP to the next element and rings the + associated channel DB. + +Event rings +----------- + +Events from the device to host are organized in event rings and defined by Event +Descriptors (ED). Event rings are used by the device to report events such as +data transfer completion status, command completion status, and state changes +to the host. Event rings are the array of EDs that resides in the host +memory. EDs consist of one or more ring elements (or transfer blocks):: + + [Read Pointer (RP)] ----------->[Ring Element] } ED + [Write Pointer (WP)]- [Ring Element] + - [Ring Element] + --------->[Ring Element] + [Ring Element] + +Below is the basic usage of event rings: + +* Host allocates memory for event ring. +* Host sets the base pointer, read pointer, and write pointer in corresponding + channel context. +* Both host and device has a local copy of RP, WP. +* Ring is considered empty (no events to service) when WP + 1 == RP. +* Ring is considered full of events when RP == WP. +* When there is a new event the device needs to send, the device updates ED + pointed by RP, increments the RP to the next element and triggers the + interrupt. + +Ring Element +------------ + +A Ring Element is a data structure used to transfer a single block +of data between the host and the device. Transfer ring element types contain a +single buffer pointer, the size of the buffer, and additional control +information. Other ring element types may only contain control and status +information. For single buffer operations, a ring descriptor is composed of a +single element. For large multi-buffer operations (such as scatter and gather), +elements can be chained to form a longer descriptor. + +MHI Operations +============== + +MHI States +---------- + +MHI_STATE_RESET +~~~~~~~~~~~~~~~ +MHI is in reset state after power-up or hardware reset. The host is not allowed +to access device MMIO register space. + +MHI_STATE_READY +~~~~~~~~~~~~~~~ +MHI is ready for initialization. The host can start MHI initialization by +programming MMIO registers. + +MHI_STATE_M0 +~~~~~~~~~~~~ +MHI is running and operational in the device. The host can start channels by +issuing channel start command. + +MHI_STATE_M1 +~~~~~~~~~~~~ +MHI operation is suspended by the device. This state is entered when the +device detects inactivity at the physical interface within a preset time. + +MHI_STATE_M2 +~~~~~~~~~~~~ +MHI is in low power state. MHI operation is suspended and the device may +enter lower power mode. + +MHI_STATE_M3 +~~~~~~~~~~~~ +MHI operation stopped by the host. This state is entered when the host suspends +MHI operation. + +MHI Initialization +------------------ + +After system boots, the device is enumerated over the physical interface. +In the case of PCIe, the device is enumerated and assigned BAR-0 for +the device's MMIO register space. To initialize the MHI in a device, +the host performs the following operations: + +* Allocates the MHI context for event, channel and command arrays. +* Initializes the context array, and prepares interrupts. +* Waits until the device enters READY state. +* Programs MHI MMIO registers and sets device into MHI_M0 state. +* Waits for the device to enter M0 state. + +MHI Data Transfer +----------------- + +MHI data transfer is initiated by the host to transfer data to the device. +Following are the sequence of operations performed by the host to transfer +data to device: + +* Host prepares TD with buffer information. +* Host increments the WP of the corresponding channel transfer ring. +* Host rings the channel DB register. +* Device wakes up to process the TD. +* Device generates a completion event for the processed TD by updating ED. +* Device increments the RP of the corresponding event ring. +* Device triggers IRQ to wake up the host. +* Host wakes up and checks the event ring for completion event. +* Host updates the WP of the corresponding event ring to indicate that the + data transfer has been completed successfully. + |