High Speed Synchronous Serial Interface (HSI)

Introduction

High Speed Syncronous Interface (HSI) is a fullduplex, low latency protocol, that is optimized for die-level interconnect between an Application Processor and a Baseband chipset. It has been specified by the MIPI alliance in 2003 and implemented by multiple vendors since then.

The HSI interface supports full duplex communication over multiple channels (typically 8) and is capable of reaching speeds up to 200 Mbit/s.

The serial protocol uses two signals, DATA and FLAG as combined data and clock signals and an additional READY signal for flow control. An additional WAKE signal can be used to wakeup the chips from standby modes. The signals are commonly prefixed by AC for signals going from the application die to the cellular die and CA for signals going the other way around.

+------------+                                 +---------------+
|  Cellular  |                                 |  Application  |
|    Die     |                                 |      Die      |
|            | - - - - - - CAWAKE - - - - - - >|               |
|           T|------------ CADATA ------------>|R              |
|           X|------------ CAFLAG ------------>|X              |
|            |<----------- ACREADY ------------|               |
|            |                                 |               |
|            |                                 |               |
|            |< - - - - -  ACWAKE - - - - - - -|               |
|           R|<----------- ACDATA -------------|T              |
|           X|<----------- ACFLAG -------------|X              |
|            |------------ CAREADY ----------->|               |
|            |                                 |               |
|            |                                 |               |
+------------+                                 +---------------+

HSI Subsystem in Linux

In the Linux kernel the hsi subsystem is supposed to be used for HSI devices. The hsi subsystem contains drivers for hsi controllers including support for multi-port controllers and provides a generic API for using the HSI ports.

It also contains HSI client drivers, which make use of the generic API to implement a protocol used on the HSI interface. These client drivers can use an arbitrary number of channels.

hsi-char Device

Each port automatically registers a generic client driver called hsi_char, which provides a charecter device for userspace representing the HSI port. It can be used to communicate via HSI from userspace. Userspace may configure the hsi_char device using the following ioctl commands:

HSC_RESET

flush the HSI port

HSC_SET_PM

enable or disable the client.

HSC_SEND_BREAK

send break

HSC_SET_RX

set RX configuration

HSC_GET_RX

get RX configuration

HSC_SET_TX

set TX configuration

HSC_GET_TX

get TX configuration

The kernel HSI API

struct hsi_channel

channel resource used by the hsi clients

Definition

struct hsi_channel {
  unsigned int    id;
  const char      *name;
};

Members

id

Channel number

name

Channel name

struct hsi_config

Configuration for RX/TX HSI modules

Definition

struct hsi_config {
  unsigned int            mode;
  struct hsi_channel      *channels;
  unsigned int            num_channels;
  unsigned int            num_hw_channels;
  unsigned int            speed;
  union {
    unsigned int    flow;
    unsigned int    arb_mode;
  };
};

Members

mode

Bit transmission mode (STREAM or FRAME)

channels

Channel resources used by the client

num_channels

Number of channel resources

num_hw_channels

Number of channels the transceiver is configured for [1..16]

speed

Max bit transmission speed (Kbit/s)

{unnamed_union}

anonymous

flow

RX flow type (SYNCHRONIZED or PIPELINE)

arb_mode

Arbitration mode for TX frame (Round robin, priority)

struct hsi_board_info

HSI client board info

Definition

struct hsi_board_info {
  const char              *name;
  unsigned int            hsi_id;
  unsigned int            port;
  struct hsi_config       tx_cfg;
  struct hsi_config       rx_cfg;
  void *platform_data;
  struct dev_archdata     *archdata;
};

Members

name

Name for the HSI device

hsi_id

HSI controller id where the client sits

port

Port number in the controller where the client sits

tx_cfg

HSI TX configuration

rx_cfg

HSI RX configuration

platform_data

Platform related data

archdata

Architecture-dependent device data

struct hsi_client

HSI client attached to an HSI port

Definition

struct hsi_client {
  struct device           device;
  struct hsi_config       tx_cfg;
  struct hsi_config       rx_cfg;
};

Members

device

Driver model representation of the device

tx_cfg

HSI TX configuration

rx_cfg

HSI RX configuration

struct hsi_client_driver

Driver associated to an HSI client

Definition

struct hsi_client_driver {
  struct device_driver    driver;
};

Members

driver

Driver model representation of the driver

struct hsi_msg

HSI message descriptor

Definition

struct hsi_msg {
  struct list_head        link;
  struct hsi_client       *cl;
  struct sg_table         sgt;
  void *context;
  void (*complete)(struct hsi_msg *msg);
  void (*destructor)(struct hsi_msg *msg);
  int status;
  unsigned int            actual_len;
  unsigned int            channel;
  unsigned int            ttype:1;
  unsigned int            break_frame:1;
};

Members

link

Free to use by the current descriptor owner

cl

HSI device client that issues the transfer

sgt

Head of the scatterlist array

context

Client context data associated to the transfer

complete

Transfer completion callback

destructor

Destructor to free resources when flushing

status

Status of the transfer when completed

actual_len

Actual length of data transferred on completion

channel

Channel were to TX/RX the message

ttype

Transfer type (TX if set, RX otherwise)

break_frame

if true HSI will send/receive a break frame. Data buffers are ignored in the request.

struct hsi_port

HSI port device

Definition

struct hsi_port {
  struct device                   device;
  struct hsi_config               tx_cfg;
  struct hsi_config               rx_cfg;
  unsigned int                    num;
  unsigned int                    shared:1;
  int claimed;
  struct mutex                    lock;
  int (*async)(struct hsi_msg *msg);
  int (*setup)(struct hsi_client *cl);
  int (*flush)(struct hsi_client *cl);
  int (*start_tx)(struct hsi_client *cl);
  int (*stop_tx)(struct hsi_client *cl);
  int (*release)(struct hsi_client *cl);
  struct blocking_notifier_head   n_head;
};

Members

device

Driver model representation of the device

tx_cfg

Current TX path configuration

rx_cfg

Current RX path configuration

num

Port number

shared

Set when port can be shared by different clients

claimed

Reference count of clients which claimed the port

lock

Serialize port claim

async

Asynchronous transfer callback

setup

Callback to set the HSI client configuration

flush

Callback to clean the HW state and destroy all pending transfers

start_tx

Callback to inform that a client wants to TX data

stop_tx

Callback to inform that a client no longer wishes to TX data

release

Callback to inform that a client no longer uses the port

n_head

Notifier chain for signaling port events to the clients.

struct hsi_controller

HSI controller device

Definition

struct hsi_controller {
  struct device           device;
  struct module           *owner;
  unsigned int            id;
  unsigned int            num_ports;
  struct hsi_port         **port;
};

Members

device

Driver model representation of the device

owner

Pointer to the module owning the controller

id

HSI controller ID

num_ports

Number of ports in the HSI controller

port

Array of HSI ports

unsigned int hsi_id(struct hsi_client *cl)

Get HSI controller ID associated to a client

Parameters

struct hsi_client *cl

Pointer to a HSI client

Description

Return the controller id where the client is attached to

unsigned int hsi_port_id(struct hsi_client *cl)

Gets the port number a client is attached to

Parameters

struct hsi_client *cl

Pointer to HSI client

Description

Return the port number associated to the client

int hsi_setup(struct hsi_client *cl)

Configure the client’s port

Parameters

struct hsi_client *cl

Pointer to the HSI client

Description

When sharing ports, clients should either relay on a single client setup or have the same setup for all of them.

Return -errno on failure, 0 on success

int hsi_flush(struct hsi_client *cl)

Flush all pending transactions on the client’s port

Parameters

struct hsi_client *cl

Pointer to the HSI client

Description

This function will destroy all pending hsi_msg in the port and reset the HW port so it is ready to receive and transmit from a clean state.

Return -errno on failure, 0 on success

int hsi_async_read(struct hsi_client *cl, struct hsi_msg *msg)

Submit a read transfer

Parameters

struct hsi_client *cl

Pointer to the HSI client

struct hsi_msg *msg

HSI message descriptor of the transfer

Description

Return -errno on failure, 0 on success

int hsi_async_write(struct hsi_client *cl, struct hsi_msg *msg)

Submit a write transfer

Parameters

struct hsi_client *cl

Pointer to the HSI client

struct hsi_msg *msg

HSI message descriptor of the transfer

Description

Return -errno on failure, 0 on success

int hsi_start_tx(struct hsi_client *cl)

Signal the port that the client wants to start a TX

Parameters

struct hsi_client *cl

Pointer to the HSI client

Description

Return -errno on failure, 0 on success

int hsi_stop_tx(struct hsi_client *cl)

Signal the port that the client no longer wants to transmit

Parameters

struct hsi_client *cl

Pointer to the HSI client

Description

Return -errno on failure, 0 on success

void hsi_port_unregister_clients(struct hsi_port *port)

Unregister an HSI port

Parameters

struct hsi_port *port

The HSI port to unregister

void hsi_unregister_controller(struct hsi_controller *hsi)

Unregister an HSI controller

Parameters

struct hsi_controller *hsi

The HSI controller to register

int hsi_register_controller(struct hsi_controller *hsi)

Register an HSI controller and its ports

Parameters

struct hsi_controller *hsi

The HSI controller to register

Description

Returns -errno on failure, 0 on success.

int hsi_register_client_driver(struct hsi_client_driver *drv)

Register an HSI client to the HSI bus

Parameters

struct hsi_client_driver *drv

HSI client driver to register

Description

Returns -errno on failure, 0 on success.

void hsi_put_controller(struct hsi_controller *hsi)

Free an HSI controller

Parameters

struct hsi_controller *hsi

Pointer to the HSI controller to freed

Description

HSI controller drivers should only use this function if they need to free their allocated hsi_controller structures before a successful call to hsi_register_controller. Other use is not allowed.

struct hsi_controller * hsi_alloc_controller(unsigned int n_ports, gfp_t flags)

Allocate an HSI controller and its ports

Parameters

unsigned int n_ports

Number of ports on the HSI controller

gfp_t flags

Kernel allocation flags

Description

Return NULL on failure or a pointer to an hsi_controller on success.

void hsi_free_msg(struct hsi_msg *msg)

Free an HSI message

Parameters

struct hsi_msg *msg

Pointer to the HSI message

Description

Client is responsible to free the buffers pointed by the scatterlists.

struct hsi_msg * hsi_alloc_msg(unsigned int nents, gfp_t flags)

Allocate an HSI message

Parameters

unsigned int nents

Number of memory entries

gfp_t flags

Kernel allocation flags

Description

nents can be 0. This mainly makes sense for read transfer. In that case, HSI drivers will call the complete callback when there is data to be read without consuming it.

Return NULL on failure or a pointer to an hsi_msg on success.

int hsi_async(struct hsi_client *cl, struct hsi_msg *msg)

Submit an HSI transfer to the controller

Parameters

struct hsi_client *cl

HSI client sending the transfer

struct hsi_msg *msg

The HSI transfer passed to controller

Description

The HSI message must have the channel, ttype, complete and destructor fields set beforehand. If nents > 0 then the client has to initialize also the scatterlists to point to the buffers to write to or read from.

HSI controllers relay on pre-allocated buffers from their clients and they do not allocate buffers on their own.

Once the HSI message transfer finishes, the HSI controller calls the complete callback with the status and actual_len fields of the HSI message updated. The complete callback can be called before returning from hsi_async.

Returns -errno on failure or 0 on success

int hsi_claim_port(struct hsi_client *cl, unsigned int share)

Claim the HSI client’s port

Parameters

struct hsi_client *cl

HSI client that wants to claim its port

unsigned int share

Flag to indicate if the client wants to share the port or not.

Description

Returns -errno on failure, 0 on success.

void hsi_release_port(struct hsi_client *cl)

Release the HSI client’s port

Parameters

struct hsi_client *cl

HSI client which previously claimed its port

int hsi_register_port_event(struct hsi_client *cl, void (*handler)(struct hsi_client *, unsigned long))

Register a client to receive port events

Parameters

struct hsi_client *cl

HSI client that wants to receive port events

void (*handler)(struct hsi_client *, unsigned long)

Event handler callback

Description

Clients should register a callback to be able to receive events from the ports. Registration should happen after claiming the port. The handler can be called in interrupt context.

Returns -errno on error, or 0 on success.

int hsi_unregister_port_event(struct hsi_client *cl)

Stop receiving port events for a client

Parameters

struct hsi_client *cl

HSI client that wants to stop receiving port events

Description

Clients should call this function before releasing their associated port.

Returns -errno on error, or 0 on success.

int hsi_event(struct hsi_port *port, unsigned long event)

Notifies clients about port events

Parameters

struct hsi_port *port

Port where the event occurred

unsigned long event

The event type

Description

Clients should not be concerned about wake line behavior. However, due to a race condition in HSI HW protocol, clients need to be notified about wake line changes, so they can implement a workaround for it.

Events: HSI_EVENT_START_RX - Incoming wake line high HSI_EVENT_STOP_RX - Incoming wake line down

Returns -errno on error, or 0 on success.

int hsi_get_channel_id_by_name(struct hsi_client *cl, char *name)

acquire channel id by channel name

Parameters

struct hsi_client *cl

HSI client, which uses the channel

char *name

name the channel is known under

Description

Clients can call this function to get the hsi channel ids similar to requesting IRQs or GPIOs by name. This function assumes the same channel configuration is used for RX and TX.

Returns -errno on error or channel id on success.