223 lines
8.5 KiB
Plaintext
223 lines
8.5 KiB
Plaintext
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Introduction
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============
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QUALCOMM MSM Interface (QMI) defines the interface between MSM and
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attached Terminal Equipment (TE). RmNet interface is a new logical
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device in QMI framework for data services. RmNet in accordance with
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QMI architecture defines channels for control and data transfers and
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for example it uses Data I/O channel for IP data transfer and control
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I/O channel for QMI messaging (functionality similar to AT commands).
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RmNet may be used in place of legacy USB modem interface.
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Tethered networking is one of the function from MSM which can also be
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supported using QMI protocol. There are other standard protocols exists
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such as CDC-ECM and Windows proprietary RNDIS. On the host-side system,
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the gadget rmnet device looks like a ethernet adapter.
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Hardware description
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====================
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QMI is a messaging protocol to expose various functionalities of MSM
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and one of the functionality to be tethered networking which is being
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exposed over QMI using RmNet protocol. This usb gadget has one bulk-in,
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one bulk-out and one interrupt-in endpoint.
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Design:
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=======
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RmNet function driver design follows two approaches:
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Approach 1:
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-----------
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Single function driver is used to communicate with
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Modem(both for data and control). Most of the initial
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MSM targets are following this approach.
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The main disadvantage with this approach is there are
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multiple RmNet drivers for any change in DATA and Control
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Layer. There is no re-use in the code.
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Approach 2:
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-----------
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RmNet driver is divided into 3 components
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1. USB:
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This component has the functionality to deal with composite layer.
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Allocates Interfaces, Endpoints, listens to connect/disconnect
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interrupts and gives connect/disconnect notifications to DATA and
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CONTROL modules.
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2. Data:
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This component talks to modem to transfer IP data. Usually DATA
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and CONTROL go over same channel. However, to achieve higher
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data rates new transport channel for DATA may be used.
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3. Control:
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This component talks to modem to transfer rmnet control data.
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Software description
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====================
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The RmNet suports following data and control transports:
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as follows:
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1. SMD Interface
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2. SDIO Interface
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3. BAM Interface
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4. SMD Control Interface
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SMD interface uses the Shared memory for the RmNet driver to communicate
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with the MSM modem processor.
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SDIO interface acts as a link for communication of RmNet driver with the
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MDM modem processor.
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USB INTERACTION:
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----------------
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The RmNet function driver binds with the USB using the struct usb_function.
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The function is added using the usb_function_add().
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Control Transfers: The RmNet handles two Class-specific control
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transfers: SEND_ENCAPSULATED_COMMAND and GET_ENCAPSULATED_RESPONSE.
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The asynchronous protocol QMI which consists of the QMI requests/responses
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is used for handling the transfers between the RmNet and the Host where the
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host sends a new QMI request before receiving the response for the current
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QMI request.
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Control & Data flow:
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1. Host issues a SEND_ENCAPSULATED command to send a QMI request.
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SMD: If the SMD control channel has enough room to accomodate a QMI request,
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it is written into the SMD buffer. Otherwise, append/add that request to
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qmi_request queue. A tasklet is scheduled to drain all QMI requests in
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qmi_request queue.
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SDIO: Add each request in the qmi_request queue and is processed until
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the queue is empty.
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2. Append/Add QMI response from modem to qmi_response queue.
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A notification on an interrupt end point is used to communicate the QMI
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response to host.
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3. Host issues a GET_ENCAPSULATED command to retrieve the QMI response.
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The response from qmi_response queue will be sent to the host.
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4. After the connection is fully established data can be sent to
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bulk-out endpoint and data can be received from bulk-in endpoint.
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5. Host can send QMI requests even after the connection is established.
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RmNet gadget driver is completely unaware of QMI/IP protocol. It just
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acts as a bridge between the modem and the PC.
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All the request/response queues in the driver can be accessed either
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from tasklet/workqueue or from interrupt context (either usb or smd/sdio
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interrupt handler). Hence a spinlock is used to protect all data/control req
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lists.
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SMD Interface:
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--------------
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1. Each QMI request/response can at most be 2048 bytes. Eight 2KB buffers
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are allocated using kmalloc for storing maximum of 8 requests/responses.
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2. Four 2KB buffers are allocated using kmalloc for data transfers on
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each bulk endpoint.
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Data structures:
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struct qmi_buf - Buffer to handle QMI requests & responses
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struct rmnet_smd_info - Control & Data SMD channel private data
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struct rmnet_dev - Endpoint and driver specific data
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Workqueues:
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rmnet_connect_work - Called on device connection.
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Opens SMD channels; enables endpoints
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rmnet_disconnect_work - Called on device disconnection.
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Closes SMD channels.
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Tasklets:
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rmnet_control_rx_tlet
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rmnet_control_tx_tlet - Control transfer data reception and transmission
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handler
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rmnet_data_rx_tlet
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rmnet_data_tx_tlet - Data transfer data reception and transmission handler
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SMD control interface
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----------------------
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This function driver implements exchnage of control informtion with
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modem over SMD. Uses smd_read/write commands to read or write rmnet
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ctrl packets. Exposes a call back function to usb component to write
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control packet and at the same time call a call back usb component
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callback to send data to usb host.
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Data structures and Interfaces are very similar to control interfaces
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explained in "SMD Interface"
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BAM MUX interface
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------------------
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BAM Mux interface is very similar to SDIO MUX interface. However there
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are differences in the way BAM and SDIO operate but all of the details
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are masked by MUX Driver.
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Refer to the SDIO interfaces for more information on data structures
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SDIO Interface:
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---------------
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1. Each QMI request/response buffer is allocated depending on the size
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of data to be transmitted for the request/response.
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2. A 2KB network buffer is allocated for data transfer on bulk-out
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endpoint. The SDIO allocates the required buffer for data transfers
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on an bulk-in endpoint.
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Data structures:
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struct rmnet_sdio_qmi_buf - Buffer to handle QMI requests/responses.
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struct rmnet_dev - Endpoint and driver specific data
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Workqueues:
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rmnet_connect_work - Device connection handler. Opens SDIO
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channels; enables and allocate buffer for
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endpoints
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rmnet_disconnect_work - Device disconnection handler. Closes
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SDIO channels; Frees allocated buffers.
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rmnet_control_rx_work - Control data reception handler.
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rmnet_data_rx_work - Network data reception handler.
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Two SMD/SDIO channels (control and data) are used as communication channels
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between Modem and Apps processor. The driver opens the SMD/SDIO channels
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on USB device connection. Data is either read from/written to the channels
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as one complete packet.
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SMD/SDIO provides a notification whenever the Modem processor completes
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read/write of a packet. Based on these SMD/SDIO notifications all the
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pending read/write requests will be handled. Tasklets(SMD)/Workqueues(SDIO)
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are used to get the requests done.
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There is another variant of rmnet driver called rmnet_smd_sdio which is used
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on some boards. This driver allows the transport (SMD/SDIO) to be chosen
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at runtime. This is required because either MDM processor or MODEM processor
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is only active at a time for data transfers. As SMD and SDIO interfaces
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are different, different endpoint completion handlers are used. This driver
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leverage the existing rmnet over smd and rmnet over sdio drivers. The control
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messages (QMI) always routed over SDIO. After the control messages exchange,
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user space will come to know about the available data transport (SMD/SDIO).
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User space notify the same to driver and the corresponding transport is
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activated. It is assumed that transport will not change while a USB cable
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is connected.
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Rmnet over SMD and rmnet over SDIO doesn't expose any of its interfaces to
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either kernelspace or userspace. But rmnet over smd/sdio expose a sysfs
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interface for userspace to notify the available transport to driver.
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The sysfs file can be found at
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/sys/class/usb_composite/rmnet_smd_sdio/transport
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The below command activates the SMD transport
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echo 0 > /sys/class/usb_composite/rmnet_smd_sdio/transport
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The below command activates the SDIO transport
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echo 1 > /sys/class/usb_composite/rmnet_smd_sdio/transport
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-EINVAL is returned if a write is attempted to transport when a USB cable
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is not connected.
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