Mailbox driver overview - Qualcomm Dragonwing Documentation

Mailbox provides a driver interface to exchange messages between mailbox clients and RTSS. Messages are stored in designated mailbox regions. The following figure shows exchange of messages between RTSS and APSS through mailbox:

The figure explains the mailbox driver features:

The following are the key components in the mailbox driver:

Linux HLOS mailbox server The Linux HLOS mailbox server (often called the HLOS mailbox resource manager or HLOS mailbox interface) is the user‑space or kernel‑space interface that lets Linux (HLOS) applications exchange messages with RTSS over the DDR region using the mailbox IPC mechanism. The Linux HLOS mailbox server acts as the HLOS mailbox resource manager and provides device nodes that:

Map the mailbox hardware (configured by the RTSS mailbox driver) into /dev/sail/* channels.
Offer notification APIs for HLOS applications.
Allow full duplex, structured IPC between Linux and RTSS/NHLOS over the mailbox IPC mechanism.

Typical usage flow (HLOS side) for a simple mailbox message read/write:

Open channel: Application opens Tx and Rx device nodes exposed by the mailbox resource manager.
Send: Application writes a structured message to the Tx device node.
Receive:
- Application blocks (or uses poll()/select()) on the Rx device node.
- Reads messages when notified (async or poll based).
Close: Application closes the mailbox handles; resource manager cleans up context.

Source code details:

sources/sail-mailbox/sail-mb-umd/include/sail_mailbox.h
sources/sail-mailbox/sail-mb-umd/src/sail_mbox_core.c, sail_mbox_lib.c, sail_mbox_lib.h, and sailmb_internal.c

RTSS mailbox external driver The mailbox external driver is the mechanism that creates and manages shared-memory channels between RTSS and APSS (HLOS).

Carves the shared DDR mailbox region into multiple regions such as region 0, region 1, and region 2.
Treats each region as a channel/subregion identified by a unique channel/subregion IDN.
Provides the buffering and notification infrastructure for message passing between RTSS and APSS.

This is the RTSS side counterpart to the HLOS mailbox server. The mailbox external driver manages the low-level shared-memory regions and IPC controller, while the HLOS communicates using /dev/sail/* nodes. Message flow and notifications:

RTSS and APSS share DDR mailbox regions managed by mailbox external driver (RTSS side) and the corresponding APSS/HLOS drivers.
When APSS or RTSS writes data into a region’s circular buffer, that data becomes available to the application.
To notify the application that new data is available or that a buffer has been consumed:
The mailbox external driver uses an IPCC to generate asynchronous events between APSS and RTSS.
On RTSS:

These IPC events trigger the mailbox external driver callbacks.
Typical RTOS IPC mechanisms are used to wake up or synchronize tasks that consume/produce messages in real time.

Source code details:

sail_proc/BSP/mailboxExt/public/mailboxExt_api.h
sail_proc/BSP/mailboxExt/src/mailboxExt_api.c, mailboxExt_config.c

Mailbox region - DDR DDR is an external memory used as shared memory for the mailbox driver to store the messages by Linux HLOS and RTSS.

Create a new mailbox channel

On RTSS side

Add client ID.
1. Add the new client ID to xMailboxExtclienttype in mailboxExt_api.h.
2. Place the entry between the QCMI start and end markers:

typedef enum
{
  MAILBOX_HLOSPMU = 0,
  MAILBOX_HLOSUSS,
  MAILBOX_HLOSTST,
  MAILBOX_CONSOLE,
  MAILBOX_OTA,
  MAILBOX_TMU,
  MAILBOX_PTP,
  MAILBOX_CAN,
  MAILBOX_POWER,
  MAILBOX_FUSE,
  MAILBOX_STZ,
  /* qcmi resource manager communication client entries goes here ..start*/
  /* qcmi resource manager communication client entries goes here ..end*/
  MAILBOX_SSM,
  MAILBOX_TSM,
  MAILBOX_EBMAX
}xMailboxExtclienttype;

Add channel IDs.

a. Add RTSS Receiver (Rx), Sender (Tx), or both Rx-Tx ID pairs to mailboxEBChanType_e in mailboxExt_config.h. b. Insert the IDs within the QCMI start and end markers:

typedef enum
{
        EB_CHAN_PMU_RX_IDN,
        EB_CHAN_PMU_TX_IDN,
        EB_CHAN_ULS_RX_IDN,
        EB_CHAN_ULS_TX_IDN,
        EB_CHAN_TST_RX_IDN,
        EB_CHAN_TST_TX_IDN,
        EB_CHAN_CONSOLE_TX_IDN,
        EB_CHAN_OTA_RX_IDN,
        EB_CHAN_OTA_TX_IDN,
        EB_CHAN_TMU_RX_IDN,
        EB_CHAN_TMU_TX_IDN,
        EB_CHAN_PTP_RX_IDN,
        EB_CHAN_PTP_TX_IDN,
        EB_CHAN_CAN_RX_IDN,
        EB_CHAN_CAN_TX_IDN,
        EB_CHAN_POWER_TX_IDN,
        EB_CHAN_FUSE_RX_IDN,
        EB_CHAN_FUSE_TX_IDN,
        EB_CHAN_STZ_RX_IDN,
        EB_CHAN_STZ_TX_IDN,
        /* qcmi resource manager communication client chan entries goes here following placeholder entries always be at the end */
        EB_CHAN_SSM_RX_IDN,     /* SSM RX */
        EB_CHAN_SSM_TX_IDN,     /* SSM TX */
        EB_CHAN_TSM_RX_IDN,     /* TSM RX */
        EB_CHAN_TSM_TX_IDN,     /* TSM TX */
        EB_CHAN_MAX,
}mailboxEBChanType_e;

Update subregion count.

a. Update mailboxEB_QCMI_NUMSUBREGION in mailboxExt_config.h to reflect the new channel:

/* Number of SubRegions in the HLOS general mailbox */
#define mailboxEB_QCMI_NUMSUBREGION 20U

For an Rx-Tx pair, the number of subregions is 2, so set mailboxEB_QCMI_NUMSUBREGION to 22.
For Rx or Tx only, the number of subregions is 1, so set mailboxEB_QCMI_NUMSUBREGION to 21.

Define a new channel configuration entry.

a. Add a new channel configuration entry in the mailboxEB_DATA static resource table:

static mbsub_rgn_config_t Mailbox_QCMISubRegionCfg[EB_CHAN_MAX]

This entry must follow the updated channel index numbering defined in the xMailboxExtClientType enum. Structure definition of mbsub_rgn_config_t:

typedef struct __attribute__((packed))
{
int8    name[MB_NAME_SZ];    // mailbox sub-region name
uint32  sender;                            // mailbox sender
uint32  receiver;                         //  mailbox receiver
uint32  prot;                                 // mailbox protocol
uint32  sig;                                   // Mailbox signal Tx-Rx (Unique)
uint32  mode;                             // Mailbox Read Mode (RD) or Write Mode (WR)                           RTSS consider RD =1; WR = 0;
uint32  prio;                             // channel priority 0: means default Max , 1 : Max -1 , 2: Max-2 so on upto max 65
uint32  start_addr_offset;         // offset of sub-region startaddress.
uint32  end_addr_offset;           // offset of sub-region end address.
uint32  item_max_num;              // Sub-region maximum item number
uint32  item_max_size;             // Sub-region maximum item size
} mbsub_rgn_config_t;

See the mailboxExt_config.c file for examples of existing channel configurations. The MAILBOX_CONSOLE entry with index value 3 in the xMailboxExtClientType enum is defined as follows:

mailboxEB_DATA static mbsub_rgn_config_t Mailbox_QCMISubRegionCfg[EB_CHAN_MAX] =
{

   * Description: sail debug log logger write channel used to send
     data to MD(HLOS\UEFI)
     */
    {
        "/dev/sail/log",
          (uint32)IPCC_C_APPS,
      (uint32)IPCC_C_SAIL0,
      (uint32)IPCC_P_COMPUTEL1,
      mailboxEB_CHAN_CONSOLE_TX_IDN_SIGID,
          MB_CHAN_WR_MODE,
      mailboxEB_QCMI_PRIORITY,
      MB_DEFAULT_CHAN_OFFSET,
      MB_DEFAULT_CHAN_OFFSET,
      mailboxEB_CHAN_CONSOLE_TX_IDN_NUMITEM,
      mailboxEB_CHAN_CONSOLE_TX_IDN_ITEMSIZE
    },

Note

The name field should have three characters, with suffix 0 for read and 1 for write channels.
If a single Rx or Tx ID is mapped to the xMailboxExtClientType client entry, initialize other channel entries with default values, see MAILBOX_CONSOLE example

Add entry in xMailboxClientRec table. a. Update the mailboxEB_CONST table:

const xMailboxEBClientRec_t xMailboxClientRec[MAILBOX_EBMAX]

Structure definition of xMailboxEBClientRec_t:

typedef struct
{
xMailboxEBChanRec_t ChanRec[MB_CHAN_MAX_TYP;
}xMailboxEBClientRec_t;

Structure definition of xMailboxEBChanRec_t:

/* chan descr type */
typedef struct
{
mailboxEBIDNType_e MbId;
mailboxEBChanType_e ChanId;
uint32 MbAddress;
uint32 *pDbgCnt;
xMailboxExtclienttypeClient;
const mbsub_rgn_config_t *pRgnCfg;
}xMailboxEBChanRec_t;
/* client chan descrtype  */

For the /dev/sail/log channel, the client with MAILBOX_CONSOLE client ID contains the following configuration entries, where only the Tx (write) channel is defined.

{
  /* rd chan : NA default init */
  (mailboxEBIDNType_e)0xFF,
  (mailboxEBChanType_e)0xFF,
  0,
  NULL,
  (xMailboxExtclienttype)0xFF,
  NULL,
  /* wr chan */
  MAILBOX_QCMI_IDN,
  EB_CHAN_CONSOLE_TX_IDN,
  mailboxEB_QCMI_ADDR,
  &ulMailbox_DbgCnt[EB_CHAN_CONSOLE_TX_IDN],
  MAILBOX_CONSOLE,
  &Mailbox_QCMISubRegionCfg[EB_CHAN_CONSOLE_TX_IDN]
},

Protocol and channel mapping: The following table lists the protocol signals, sender, receiver, and channel support for integrator reference when adding a new channel in the mailboxExt driver.

Supported protocol	Signals	Sender	Receiver	Channel supported
`IPCC_P_COMPUTEL 1`	32	`IPCC_C_APPS`	One of the RTSS cores, for example, `IPCC_C_SAIL0` or `IPCC_C_SAIL1`	32

Select the protocol, sender, and receiver according to the table.
RTSS cores use logical number suffixes appended to macros, for example, IPCC_C_SAIL0 for core 0.
The sender column indicates the sender client line used by APSS to link with the RTSS domain receiver client. Each sender client provides the number of signals specified in the table based on protocol selection.

Reference: The mailbox_test application is location at sail_proc\BSP\tests\sailsw1\mailbox_test\src\MBtests_sailsw1.c. On APPS side

Add a new channel entry in the devicetree.

Update the devicetree with the new channel configuration:

sail_mailbox: sail-mailbox@90d80000 {
  compatible = "qcom,sail-mailbox";
  reg = <0x0 0x01FFE02C 0x0 0x10>,
  <0x0 0x01FFD018 0x0 0x10>,
  <0x0 0x17C0000C 0x0 0x04>;
  mboxes =  <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x2>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x3>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x4>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x5>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x6>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0X7>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL2 0X8>,
            <&ipcc_computeL1 IPCC_CLIENT_SAIL0 0X9>;
            +<&ipcc_computeL1 IPCC_CLIENT_SAIL0 0XA>  //Example new entry for new  channel created at RTSS side
  memory-region = <&sail_mailbox_mem>,
                  <&sail_ota_mem>;
                interrupt-parent=<&ipcc_computeL1>;
  interrupts =  <IPCC_CLIENT_SAIL0 0x2 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0x3 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0x4 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0x5 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0x6 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0X7 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL2 0X8 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL1 0X9 IRQ_TYPE_EDGE_RISING>,
                <IPCC_CLIENT_SAIL0 0XA IRQ_TYPE_EDGE_RISING>; //example entry for newly created channel at sail side
  sail-handshake-delay = <50000>;
  status = "okay";
         };

Note
&ipcc_computeL1 IPCC_CLIENT_SAIL0 0xA indicates:

ipcc_computeL1: Protocol name (currently only this is supported).

IPCC_CLIENT_SAIL1: RTSS core number.

0XA: Channel number.

Example mapping:
<&ipcc_computeL1 IPCC_CLIENT_SAIL0 0x3>, <IPCC_CLIENT_SAIL0 0x3 IRQ_TYPE_EDGE_RISING>
This corresponds to the /dev/sail/log, mapped to MAILBOX_CONSOLE with index value 3 in the xMailboxExtClientType enum.

References:

saildbg
- Location sail-mailbox/reference_apps/sail_dbg/src/saildbg.c
- Key APIs:
  - open_sail_mailbox(): Opens the mailbox channel for TX and RX.
  - write_sail_mailbox(): Writes a message to the TX channel.
  - read_sail_mailbox(): Reads a message from the RX channel.
  - close_sail_mailbox(): Closes the mailbox channel for TX and RX.
- Structures:
  - struct SailClientDataType *pTxClientData: TX structure for transmitting messages.
  - struct SailClientDataType *pRxClientData: RX structure for receiving messages.
- Mailbox test application: Mailbox test application: saildbg
sail_console_chan`:
- Location: sail-mailbox/unit_test_apps/sail_console_chan_app.c
- Mailbox test application: Mailbox test application: sail_console_chan_app

RTSS mailbox demo application

The sailmb_demo reference application demonstrates bidirectional mailbox communication between the APSS, which runs Linux, and RTSS, which runs Free RTOS. The application sends a message from APSS to an RTSS CPU core and receives a response from that RTSS CPU core. A successful message exchange verifies the following:

The RTSS firmware has initialized the shared mailbox structure in DDR.
The Linux mailbox driver has loaded, mapped the shared memory, and completed the hardware handshake with RTSS.
The APSS application can write a message to shared memory and trigger an interrupt to RTSS.
RTSS can receive the interrupt, read the message, write a response, and trigger an interrupt to APSS.
APSS can receive the interrupt, read the response, and display it on the console.

The application communicates with four RTSS CPU cores. Each core uses a dedicated transmit and receive channel pair. The mailbox channel mappings in the following table are from the APSS perspective:

RTSS core	APSS transmit channel	APSS receive channel	RTSS task	IPCC signal	Buffer capacity	Message size
Core 0	`/dev/sail/cz0`	`/dev/sail/cz1`	xCore0Task	Signal 13	10 messages	64 bytes per message
Core 1	`/dev/sail/co0`	`/dev/sail/co1`	xCore1Task	Signal 14	10 messages	64 bytes per message
Core 2	`/dev/sail/ct0`	`/dev/sail/ct1`	xCore2Task	Signal 15	10 messages	64 bytes per message
Core 3	`/dev/sail/cth0`	`/dev/sail/cth1`	xCore3Task	Signal 16	10 messages	64 bytes per message

To run the RTSS mailbox demo application, see Mailbox test application: sailmb_demo.

​Create a new mailbox channel

​RTSS mailbox demo application

Create a new mailbox channel

RTSS mailbox demo application