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Intel Integrated Sensor Hub (ISH)
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===============================
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A sensor hub enables the ability to offload sensor polling and algorithm
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processing to a dedicated low power co-processor. This allows the core
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processor to go into low power modes more often, resulting in the increased
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battery life.
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There are many vendors providing external sensor hubs confirming to HID
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Sensor usage tables, and used in several tablets, 2 in 1 convertible laptops
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and embedded products. Linux had this support since Linux 3.9.
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Intel® introduced integrated sensor hubs as a part of the SoC starting from
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Cherry Trail and now supported on multiple generations of CPU packages. There
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are many commercial devices already shipped with Integrated Sensor Hubs (ISH).
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These ISH also comply to HID sensor specification, but the difference is the
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transport protocol used for communication. The current external sensor hubs
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mainly use HID over i2C or USB. But ISH doesn't use either i2c or USB.
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1. Overview
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Using a analogy with a usbhid implementation, the ISH follows a similar model
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for a very high speed communication:
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----------------- ----------------------
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| USB HID | --> | ISH HID |
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----------------- ----------------------
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----------------- ----------------------
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| USB protocol | --> | ISH Transport |
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----------------- ----------------------
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----------------- ----------------------
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| EHCI/XHCI | --> | ISH IPC |
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----------------- ----------------------
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PCI PCI
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----------------- ----------------------
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|Host controller| --> | ISH processor |
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----------------- ----------------------
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USB Link
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----------------- ----------------------
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| USB End points| --> | ISH Clients |
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----------------- ----------------------
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Like USB protocol provides a method for device enumeration, link management
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and user data encapsulation, the ISH also provides similar services. But it is
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very light weight tailored to manage and communicate with ISH client
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applications implemented in the firmware.
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The ISH allows multiple sensor management applications executing in the
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firmware. Like USB endpoints the messaging can be to/from a client. As part of
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enumeration process, these clients are identified. These clients can be simple
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HID sensor applications, sensor calibration application or senor firmware
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update application.
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The implementation model is similar, like USB bus, ISH transport is also
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implemented as a bus. Each client application executing in the ISH processor
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is registered as a device on this bus. The driver, which binds each device
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(ISH HID driver) identifies the device type and registers with the hid core.
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2. ISH Implementation: Block Diagram
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---------------------------
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| User Space Applications |
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---------------------------
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----------------IIO ABI----------------
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--------------------------
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| IIO Sensor Drivers |
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--------------------------
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--------------------------
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| IIO core |
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--------------------------
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--------------------------
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| HID Sensor Hub MFD |
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--------------------------
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--------------------------
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| HID Core |
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--------------------------
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--------------------------
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| HID over ISH Client |
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--------------------------
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--------------------------
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| ISH Transport (ISHTP) |
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--------------------------
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--------------------------
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| IPC Drivers |
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--------------------------
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OS
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---------------- PCI -----------------
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Hardware + Firmware
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----------------------------
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| ISH Hardware/Firmware(FW) |
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----------------------------
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3. High level processing in above blocks
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3.1 Hardware Interface
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The ISH is exposed as "Non-VGA unclassified PCI device" to the host. The PCI
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product and vendor IDs are changed from different generations of processors. So
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the source code which enumerate drivers needs to update from generation to
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generation.
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3.2 Inter Processor Communication (IPC) driver
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Location: drivers/hid/intel-ish-hid/ipc
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The IPC message used memory mapped I/O. The registers are defined in
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hw-ish-regs.h.
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3.2.1 IPC/FW message types
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There are two types of messages, one for management of link and other messages
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are to and from transport layers.
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TX and RX of Transport messages
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A set of memory mapped register offers support of multi byte messages TX and
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RX (E.g.IPC_REG_ISH2HOST_MSG, IPC_REG_HOST2ISH_MSG). The IPC layer maintains
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internal queues to sequence messages and send them in order to the FW.
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Optionally the caller can register handler to get notification of completion.
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A door bell mechanism is used in messaging to trigger processing in host and
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client firmware side. When ISH interrupt handler is called, the ISH2HOST
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doorbell register is used by host drivers to determine that the interrupt
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is for ISH.
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Each side has 32 32-bit message registers and a 32-bit doorbell. Doorbell
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register has the following format:
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Bits 0..6: fragment length (7 bits are used)
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Bits 10..13: encapsulated protocol
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Bits 16..19: management command (for IPC management protocol)
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Bit 31: doorbell trigger (signal H/W interrupt to the other side)
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Other bits are reserved, should be 0.
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3.2.2 Transport layer interface
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To abstract HW level IPC communication, a set of callbacks are registered.
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The transport layer uses them to send and receive messages.
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Refer to struct ishtp_hw_ops for callbacks.
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3.3 ISH Transport layer
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Location: drivers/hid/intel-ish-hid/ishtp/
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3.3.1 A Generic Transport Layer
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The transport layer is a bi-directional protocol, which defines:
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- Set of commands to start, stop, connect, disconnect and flow control
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(ishtp/hbm.h) for details
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- A flow control mechanism to avoid buffer overflows
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This protocol resembles bus messages described in the following document:
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http://www.intel.com/content/dam/www/public/us/en/documents/technical-\
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specifications/dcmi-hi-1-0-spec.pdf "Chapter 7: Bus Message Layer"
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3.3.2 Connection and Flow Control Mechanism
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Each FW client and a protocol is identified by an UUID. In order to communicate
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to a FW client, a connection must be established using connect request and
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response bus messages. If successful, a pair (host_client_id and fw_client_id)
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will identify the connection.
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Once connection is established, peers send each other flow control bus messages
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independently. Every peer may send a message only if it has received a
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flow-control credit before. Once it sent a message, it may not send another one
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before receiving the next flow control credit.
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Either side can send disconnect request bus message to end communication. Also
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the link will be dropped if major FW reset occurs.
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3.3.3 Peer to Peer data transfer
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Peer to Peer data transfer can happen with or without using DMA. Depending on
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the sensor bandwidth requirement DMA can be enabled by using module parameter
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ishtp_use_dma under intel_ishtp.
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Each side (host and FW) manages its DMA transfer memory independently. When an
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ISHTP client from either host or FW side wants to send something, it decides
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whether to send over IPC or over DMA; for each transfer the decision is
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independent. The sending side sends DMA_XFER message when the message is in
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the respective host buffer (TX when host client sends, RX when FW client
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sends). The recipient of DMA message responds with DMA_XFER_ACK, indicating
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the sender that the memory region for that message may be reused.
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DMA initialization is started with host sending DMA_ALLOC_NOTIFY bus message
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(that includes RX buffer) and FW responds with DMA_ALLOC_NOTIFY_ACK.
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Additionally to DMA address communication, this sequence checks capabilities:
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if thw host doesn't support DMA, then it won't send DMA allocation, so FW can't
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send DMA; if FW doesn't support DMA then it won't respond with
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DMA_ALLOC_NOTIFY_ACK, in which case host will not use DMA transfers.
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Here ISH acts as busmaster DMA controller. Hence when host sends DMA_XFER,
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it's request to do host->ISH DMA transfer; when FW sends DMA_XFER, it means
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that it already did DMA and the message resides at host. Thus, DMA_XFER
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and DMA_XFER_ACK act as ownership indicators.
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At initial state all outgoing memory belongs to the sender (TX to host, RX to
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FW), DMA_XFER transfers ownership on the region that contains ISHTP message to
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the receiving side, DMA_XFER_ACK returns ownership to the sender. A sender
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needs not wait for previous DMA_XFER to be ack'ed, and may send another message
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as long as remaining continuous memory in its ownership is enough.
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In principle, multiple DMA_XFER and DMA_XFER_ACK messages may be sent at once
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(up to IPC MTU), thus allowing for interrupt throttling.
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Currently, ISH FW decides to send over DMA if ISHTP message is more than 3 IPC
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fragments and via IPC otherwise.
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3.3.4 Ring Buffers
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When a client initiate a connection, a ring or RX and TX buffers are allocated.
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The size of ring can be specified by the client. HID client set 16 and 32 for
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TX and RX buffers respectively. On send request from client, the data to be
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sent is copied to one of the send ring buffer and scheduled to be sent using
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bus message protocol. These buffers are required because the FW may have not
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have processed the last message and may not have enough flow control credits
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to send. Same thing holds true on receive side and flow control is required.
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3.3.5 Host Enumeration
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The host enumeration bus command allow discovery of clients present in the FW.
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There can be multiple sensor clients and clients for calibration function.
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To ease in implantation and allow independent driver handle each client
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this transport layer takes advantage of Linux Bus driver model. Each
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client is registered as device on the the transport bus (ishtp bus).
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Enumeration sequence of messages:
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- Host sends HOST_START_REQ_CMD, indicating that host ISHTP layer is up.
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- FW responds with HOST_START_RES_CMD
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- Host sends HOST_ENUM_REQ_CMD (enumerate FW clients)
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- FW responds with HOST_ENUM_RES_CMD that includes bitmap of available FW
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client IDs
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- For each FW ID found in that bitmap host sends
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HOST_CLIENT_PROPERTIES_REQ_CMD
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- FW responds with HOST_CLIENT_PROPERTIES_RES_CMD. Properties include UUID,
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max ISHTP message size, etc.
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- Once host received properties for that last discovered client, it considers
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ISHTP device fully functional (and allocates DMA buffers)
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3.4 HID over ISH Client
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Location: drivers/hid/intel-ish-hid
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The ISHTP client driver is responsible for:
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- enumerate HID devices under FW ISH client
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- Get Report descriptor
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- Register with HID core as a LL driver
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- Process Get/Set feature request
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- Get input reports
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3.5 HID Sensor Hub MFD and IIO sensor drivers
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The functionality in these drivers is the same as an external sensor hub.
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Refer to
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Documentation/hid/hid-sensor.txt for HID sensor
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Documentation/ABI/testing/sysfs-bus-iio for IIO ABIs to user space
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3.6 End to End HID transport Sequence Diagram
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HID-ISH-CLN ISHTP IPC HW
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| | |-----WAKE UP------------------>|
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| | |-----HOST READY--------------->|
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| | |<----MNG_RESET_NOTIFY_ACK----- |
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| |<----ISHTP_START------ | |
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| |<-----------------HOST_START_RES_CMD-------------------|
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| |------------------QUERY_SUBSCRIBER-------------------->|
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| |------------------HOST_ENUM_REQ_CMD------------------->|
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| |<-----------------HOST_ENUM_RES_CMD--------------------|
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| |------------------HOST_CLIENT_PROPERTIES_REQ_CMD------>|
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| |<-----------------HOST_CLIENT_PROPERTIES_RES_CMD-------|
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| Create new device on in ishtp bus | |
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| |------------------HOST_CLIENT_PROPERTIES_REQ_CMD------>|
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| |<-----------------HOST_CLIENT_PROPERTIES_RES_CMD-------|
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| Create new device on in ishtp bus | |
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| |--Repeat HOST_CLIENT_PROPERTIES_REQ_CMD-till last one--|
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probed()
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|----ishtp_cl_connect-->|----------------- CLIENT_CONNECT_REQ_CMD-------------->|
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| |<----------------CLIENT_CONNECT_RES_CMD----------------|
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|register event callback| | |
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|ishtp_cl_send(
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HOSTIF_DM_ENUM_DEVICES) |----------fill ishtp_msg_hdr struct write to HW----- >|
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| | |<-----IRQ(IPC_PROTOCOL_ISHTP---|
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|<--ENUM_DEVICE RSP-----| | |
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for each enumerated device
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|ishtp_cl_send(
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HOSTIF_GET_HID_DESCRIPTOR |----------fill ishtp_msg_hdr struct write to HW--- >|
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...Response
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for each enumerated device
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|ishtp_cl_send(
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HOSTIF_GET_REPORT_DESCRIPTOR |----------fill ishtp_msg_hdr struct write to HW- >|
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hid_allocate_device
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hid_add_device | | |
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3.7 ISH Debugging
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To debug ISH, event tracing mechanism is used. To enable debug logs
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echo 1 > /sys/kernel/debug/tracing/events/intel_ish/enable
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cat sys/kernel/debug/tracing/trace
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3.8 ISH IIO sysfs Example on Lenovo thinkpad Yoga 260
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root@otcpl-ThinkPad-Yoga-260:~# tree -l /sys/bus/iio/devices/
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/sys/bus/iio/devices/
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├── iio:device0 -> ../../../devices/0044:8086:22D8.0001/HID-SENSOR-200073.9.auto/iio:device0
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│ ├── buffer
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│ │ ├── enable
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│ │ ├── length
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│ │ └── watermark
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...
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│ ├── in_accel_hysteresis
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│ ├── in_accel_offset
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│ ├── in_accel_sampling_frequency
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│ ├── in_accel_scale
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│ ├── in_accel_x_raw
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│ ├── in_accel_y_raw
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│ ├── in_accel_z_raw
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│ ├── name
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│ ├── scan_elements
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│ │ ├── in_accel_x_en
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│ │ ├── in_accel_x_index
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│ │ ├── in_accel_x_type
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│ │ ├── in_accel_y_en
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│ │ ├── in_accel_y_index
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│ │ ├── in_accel_y_type
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│ │ ├── in_accel_z_en
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│ │ ├── in_accel_z_index
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│ │ └── in_accel_z_type
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...
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│ │ ├── devices
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│ │ │ │ ├── buffer
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│ │ │ │ │ ├── enable
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│ │ │ │ │ ├── length
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│ │ │ │ │ └── watermark
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│ │ │ │ ├── dev
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│ │ │ │ ├── in_intensity_both_raw
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│ │ │ │ ├── in_intensity_hysteresis
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│ │ │ │ ├── in_intensity_offset
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│ │ │ │ ├── in_intensity_sampling_frequency
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│ │ │ │ ├── in_intensity_scale
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│ │ │ │ ├── name
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│ │ │ │ ├── scan_elements
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│ │ │ │ │ ├── in_intensity_both_en
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│ │ │ │ │ ├── in_intensity_both_index
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│ │ │ │ │ └── in_intensity_both_type
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│ │ │ │ ├── trigger
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│ │ │ │ │ └── current_trigger
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...
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│ │ │ │ ├── buffer
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│ │ │ │ │ ├── enable
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│ │ │ │ │ ├── length
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│ │ │ │ │ └── watermark
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│ │ │ │ ├── dev
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│ │ │ │ ├── in_magn_hysteresis
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│ │ │ │ ├── in_magn_offset
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│ │ │ │ ├── in_magn_sampling_frequency
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│ │ │ │ ├── in_magn_scale
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│ │ │ │ ├── in_magn_x_raw
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│ │ │ │ ├── in_magn_y_raw
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│ │ │ │ ├── in_magn_z_raw
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│ │ │ │ ├── in_rot_from_north_magnetic_tilt_comp_raw
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│ │ │ │ ├── in_rot_hysteresis
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│ │ │ │ ├── in_rot_offset
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│ │ │ │ ├── in_rot_sampling_frequency
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│ │ │ │ ├── in_rot_scale
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│ │ │ │ ├── name
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...
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│ │ │ │ ├── scan_elements
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│ │ │ │ │ ├── in_magn_x_en
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│ │ │ │ │ ├── in_magn_x_index
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│ │ │ │ │ ├── in_magn_x_type
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│ │ │ │ │ ├── in_magn_y_en
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│ │ │ │ │ ├── in_magn_y_index
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│ │ │ │ │ ├── in_magn_y_type
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│ │ │ │ │ ├── in_magn_z_en
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│ │ │ │ │ ├── in_magn_z_index
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│ │ │ │ │ ├── in_magn_z_type
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│ │ │ │ │ ├── in_rot_from_north_magnetic_tilt_comp_en
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│ │ │ │ │ ├── in_rot_from_north_magnetic_tilt_comp_index
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│ │ │ │ │ └── in_rot_from_north_magnetic_tilt_comp_type
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│ │ │ │ ├── trigger
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│ │ │ │ │ └── current_trigger
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...
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│ │ │ │ ├── buffer
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│ │ │ │ │ ├── enable
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│ │ │ │ │ ├── length
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│ │ │ │ │ └── watermark
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│ │ │ │ ├── dev
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│ │ │ │ ├── in_anglvel_hysteresis
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│ │ │ │ ├── in_anglvel_offset
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│ │ │ │ ├── in_anglvel_sampling_frequency
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│ │ │ │ ├── in_anglvel_scale
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│ │ │ │ ├── in_anglvel_x_raw
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│ │ │ │ ├── in_anglvel_y_raw
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│ │ │ │ ├── in_anglvel_z_raw
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│ │ │ │ ├── name
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│ │ │ │ ├── scan_elements
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│ │ │ │ │ ├── in_anglvel_x_en
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│ │ │ │ │ ├── in_anglvel_x_index
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│ │ │ │ │ ├── in_anglvel_x_type
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│ │ │ │ │ ├── in_anglvel_y_en
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│ │ │ │ │ ├── in_anglvel_y_index
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│ │ │ │ │ ├── in_anglvel_y_type
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│ │ │ │ │ ├── in_anglvel_z_en
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│ │ │ │ │ ├── in_anglvel_z_index
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│ │ │ │ │ └── in_anglvel_z_type
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│ │ │ │ ├── trigger
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│ │ │ │ │ └── current_trigger
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...
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│ │ │ │ ├── buffer
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│ │ │ │ │ ├── enable
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│ │ │ │ │ ├── length
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│ │ │ │ │ └── watermark
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│ │ │ │ ├── dev
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│ │ │ │ ├── in_anglvel_hysteresis
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│ │ │ │ ├── in_anglvel_offset
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│ │ │ │ ├── in_anglvel_sampling_frequency
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│ │ │ │ ├── in_anglvel_scale
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│ │ │ │ ├── in_anglvel_x_raw
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│ │ │ │ ├── in_anglvel_y_raw
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│ │ │ │ ├── in_anglvel_z_raw
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│ │ │ │ ├── name
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│ │ │ │ ├── scan_elements
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│ │ │ │ │ ├── in_anglvel_x_en
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│ │ │ │ │ ├── in_anglvel_x_index
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│ │ │ │ │ ├── in_anglvel_x_type
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│ │ │ │ │ ├── in_anglvel_y_en
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│ │ │ │ │ ├── in_anglvel_y_index
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│ │ │ │ │ ├── in_anglvel_y_type
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│ │ │ │ │ ├── in_anglvel_z_en
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│ │ │ │ │ ├── in_anglvel_z_index
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│ │ │ │ │ └── in_anglvel_z_type
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│ │ │ │ ├── trigger
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│ │ │ │ │ └── current_trigger
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...
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