[PATCH v4 01/22] HID: THC: Add documentation

Even Xu
2025-01-06T02:32:09Z

Add Documentation/hid/intel-thc-hid.rst file to provide hardware
and software detail for intel THC drivers.

Co-developed-by: Sun Xinpeng <xinpeng.sun@intel.com>
Signed-off-by: Sun Xinpeng <xinpeng.sun@intel.com>
Signed-off-by: Even Xu <even.xu@intel.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
---
Documentation/hid/index.rst | 1 +
Documentation/hid/intel-thc-hid.rst | 568 ++++++++++++++++++++++++++++
2 files changed, 569 insertions(+)
create mode 100644 Documentation/hid/intel-thc-hid.rst

diff --git a/Documentation/hid/index.rst b/Documentation/hid/index.rst
index af02cf7cfa82..baf156b44b58 100644
--- a/Documentation/hid/index.rst
+++ b/Documentation/hid/index.rst
@@ -18,4 +18,5 @@ Human Interface Devices (HID)

hid-alps
intel-ish-hid
+ intel-thc-hid
amd-sfh-hid
diff --git a/Documentation/hid/intel-thc-hid.rst b/Documentation/hid/intel-thc-hid.rst
new file mode 100644
index 000000000000..6c417205ac6a
--- /dev/null
+++ b/Documentation/hid/intel-thc-hid.rst
@@ -0,0 +1,568 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Intel Touch Host Controller (THC)
+=================================
+
+Touch Host Controller is the name of the IP block in PCH that interface with Touch Devices (ex:
+touchscreen, touchpad etc.). It is comprised of 3 key functional blocks:
+
+- A natively half-duplex Quad I/O capable SPI master
+- Low latency I2C interface to support HIDI2C compliant devices
+- A HW sequencer with RW DMA capability to system memory
+
+It has a single root space IOSF Primary interface that supports transactions to/from touch devices.
+Host driver configures and controls the touch devices over THC interface. THC provides high
+bandwidth DMA services to the touch driver and transfers the HID report to host system main memory.
+
+Hardware sequencer within the THC is responsible for transferring (via DMA) data from touch devices
+into system memory. A ring buffer is used to avoid data loss due to asynchronous nature of data
+consumption (by host) in relation to data production (by touch device via DMA).
+
+Unlike other common SPI/I2C controllers, THC handles the HID device data interrupt and reset
+signals directly.
+
+1. Overview
+===========
+
+1.1 THC software/hardware stack
+-------------------------------
+
+Below diagram illustrates the high-level architecture of THC software/hardware stack, which is fully
+capable of supporting HIDSPI/HIDI2C protocol in Linux OS.
+
+::
+
+ ----------------------------------------------
+ | +-----------------------------------+ |
+ | | Input Device | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | HID Multi-touch Driver | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | HID Core | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | THC QuickSPI/QuickI2C Driver | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | THC Hardware Driver | |
+ | +-----------------------------------+ |
+ | +----------------+ +----------------+ |
+ | SW | PCI Bus Driver | | ACPI Resource | |
+ | +----------------+ +----------------+ |
+ ----------------------------------------------
+ ----------------------------------------------
+ | +-----------------------------------+ |
+ | HW | PCI Bus | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | THC Controller | |
+ | +-----------------------------------+ |
+ | +-----------------------------------+ |
+ | | Touch IC | |
+ | +-----------------------------------+ |
+ ----------------------------------------------
+
+Touch IC (TIC), also as known as the Touch devices (touchscreen or touchpad). The discrete analog
+components that sense and transfer either discrete touch data or heatmap data in the form of HID
+reports over the SPI/I2C bus to the THC Controller on the host.
+
+THC Host Controller, which is a PCI device HBA (host bus adapter), integrated into the PCH, that
+serves as a bridge between the Touch ICs and the host.
+
+THC Hardware Driver, provides THC hardware operation APIs for above QuickSPI/QuickI2C driver, it
+accesses THC MMIO registers to configure and control THC hardware.
+
+THC QuickSPI/QuickI2C driver, also as known as HIDSPI/HIDI2C driver, is registered as a HID
+low-level driver that manages the THC Controller and implements HIDSPI/HIDI2C protocol.
+
+
+1.2 THC hardware diagram
+------------------------
+Below diagram shows THC hardware components::
+
+ ---------------------------------
+ | THC Controller |
+ | +---------------------------+ |
+ | | PCI Config Space | |
+ | +---------------------------+ |
+ | +---------------------------+ |
+ | + MMIO Registers | |
+ | +---------------------------+ |
+ +---------------+ | +------------+ +------------+ |
+ | System Memory +---+--+ DMA | | PIO | |
+ +---------------+ | +------------+ +------------+ |
+ | +---------------------------+ |
+ | | HW Sequencer | |
+ | +---------------------------+ |
+ | +------------+ +------------+ |
+ | | SPI/I2C | | GPIO | |
+ | | Controller | | Controller | |
+ | +------------+ +------------+ |
+ ---------------------------------
+
+As THC is exposed as a PCI devices, so it has standard PCI config space registers for PCI
+enumeration and configuration.
+
+MMIO Registers, which provide registers access for driver to configure and control THC hardware,
+the registers include several categories: Interrupt status and control, DMA configure,
+PIO (Programmed I/O, defined in section 3.2) status and control, SPI bus configure, I2C subIP
+status and control, reset status and control...
+
+THC provides two ways for driver to communicate with external Touch ICs: PIO and DMA.
+PIO can let driver manually write/read data to/from Touch ICs, instead, THC DMA can
+automatically write/read data without driver involved.
+
+HW Sequencer includes THC major logic, it gets instruction from MMIO registers to control
+SPI bus and I2C bus to finish a bus data transaction, it also can automatically handle
+Touch ICs interrupt and start DMA receive/send data from/to Touch ICs according to interrupt
+type. That means THC HW Sequencer understands HIDSPI/HIDI2C transfer protocol, and handle
+the communication without driver involved, what driver needs to do is just configure the THC
+properly, and prepare the formatted data packet or handle received data packet.
+
+As THC supports HIDSPI/HIDI2C protocols, it has SPI controller and I2C subIP in it to expose
+SPI bus and I2C bus. THC also integrates a GPIO controller to provide interrupt line support
+and reset line support.
+
+2. THC Hardware Interface
+=========================
+
+2.1 Host Interface
+------------------
+
+THC is exposed as "PCI Digitizer device" to the host. The PCI product and device IDs are
+changed from different generations of processors. So the source code which enumerates drivers
+needs to update from generation to generation.
+
+
+2.2 Device Interface
+--------------------
+
+THC supports two types of bus for Touch IC connection: Enhanced SPI bus and I2C bus.
+
+2.2.1 SPI Port
+~~~~~~~~~~~~~~
+
+When PORT_TYPE = 00b in MMIO registers, THC uses SPI interfaces to communicate with external
+Touch IC. THC enhanced SPI Bus supports different SPI modes: standard Single IO mode,
+Dual IO mode and Quad IO mode.
+
+In Single IO mode, THC drives MOSI line to send data to Touch ICs, and receives data from Touch
+ICs data from MISO line. In Dual IO mode, THC drivers MOSI and MISO both for data sending, and
+also receives the data on both line. In Quad IO mode, there are other two lines (IO2 and IO3)
+are added, THC drives MOSI (IO0), MISO (IO1), IO2 and IO3 at the same time for data sending, and
+also receives the data on those 4 lines. Driver needs to configure THC in different mode by
+setting different opcode.
+
+Beside IO mode, driver also needs to configure SPI bus speed. THC supports up to 42MHz SPI clock
+on Intel Lunar Lake platform.
+
+For THC sending data to Touch IC, the data flow on SPI bus::
+
+ | --------------------THC sends---------------------------------|
+ <8Bits OPCode><24Bits Slave Address><Data><Data><Data>...........
+
+For THC receiving data from Touch IC, the data flow on SPI bus::
+
+ | ---------THC Sends---------------||-----Touch IC sends--------|
+ <8Bits OPCode><24Bits Slave Address><Data><Data><Data>...........
+
+2.2.2 I2C Port
+~~~~~~~~~~~~~~
+
+THC also integrates I2C controller in it, it's called I2C SubSystem. When PORT_TYPE = 01, THC
+is configured to I2C mode. Comparing to SPI mode which can be configured through MMIO registers
+directly, THC needs to use PIO read (by setting SubIP read opcode) to I2C subIP APB registers'
+value and use PIO write (by setting SubIP write opcode) to do a write operation.
+
+2.2.3 GPIO interface
+~~~~~~~~~~~~~~~~~~~~
+
+THC also includes two GPIO pins, one for interrupt and the other for device reset control.
+
+Interrupt line can be configured to either level triggerred or edge triggerred by setting MMIO
+Control register.
+
+Reset line is controlled by BIOS (or EFI) through ACPI _RST method, driver needs to call this
+device ACPI _RST method to reset touch IC during initialization.
+
+3. High level concept
+=====================
+
+3.1 Opcode
+----------
+
+Opcode (operation code) is used to tell THC or Touch IC what the operation will be, such as PIO
+read or PIO write.
+
+When THC is configured to SPI mode, opcodes are used for determining the read/write IO mode.
+There are some OPCode examples for SPI IO mode:
+
+======= ==============================
+opcode Corresponding SPI command
+======= ==============================
+0x0B Read Single I/O
+0x02 Write Single I/O
+0xBB Read Dual I/O
+0xB2 Write Dual I/O
+0xEB Read Quad I/O
+0xE2 Write Quad I/O
+======= ==============================
+
+In general, different touch IC has different OPCode definition. According to HIDSPI
+protocol whitepaper, those OPCodes are defined in device ACPI table, and driver needs to
+query those information through OS ACPI APIs during driver initialization, then configures
+THC MMIO OPCode registers with correct setting.
+
+When THC is working in I2C mode, opcodes are used to tell THC what's the next PIO type:
+I2C SubIP APB register read, I2C SubIP APB register write, I2C touch IC device read,
+I2C touch IC device write, I2C touch IC device write followed by read.
+
+Here are the THC pre-defined opcodes for I2C mode:
+
+======= =================================================== ===========
+opcode Corresponding I2C command Address
+======= =================================================== ===========
+0x12 Read I2C SubIP APB internal registers 0h - FFh
+0x13 Write I2C SubIP APB internal registers 0h - FFh
+0x14 Read external Touch IC through I2C bus N/A
+0x18 Write external Touch IC through I2C bus N/A
+0x1C Write then read external Touch IC through I2C bus N/A
+======= =================================================== ===========
+
+3.2 PIO
+-------
+
+THC provides a programmed I/O (PIO) access interface for the driver to access the touch IC's
+configuration registers, or access I2C subIP's configuration registers. To use PIO to perform
+I/O operations, driver should pre-program PIO control registers and PIO data registers and kick
+off the sequencing cycle. THC uses different PIO opcodes to distinguish different PIO
+operations (PIO read/write/write followed by read).
+
+If there is a Sequencing Cycle In Progress and an attempt is made to program any of the control,
+address, or data register the cycle is blocked and a sequence error will be encountered.
+
+A status bit indicates when the cycle has completed allowing the driver to know when read results
+can be checked and/or when to initiate a new command. If enabled, the cycle done assertion can
+interrupt driver with an interrupt.
+
+Because THC only has 16 FIFO registers for PIO, so all the data transfer through PIO shouldn't
+exceed 64 bytes.
+
+As DMA needs max packet size for transferring configuration, and the max packet size information
+always in HID device descriptor which needs THC driver to read it out from HID Device (Touch IC).
+So PIO typical use case is, before DMA initialization, write RESET command (PIO write), read
+RESET response (PIO read or PIO write followed by read), write Power ON command (PIO write), read
+device descriptor (PIO read).
+
+For how to issue a PIO operation, here is the steps which driver needs follow:
+
+- Program read/write data size in THC_SS_BC.
+- Program I/O target address in THC_SW_SEQ_DATA0_ADDR.
+- If write, program the write data in THC_SW_SEQ_DATA0..THC_SW_SEQ_DATAn.
+- Program the PIO opcode in THC_SS_CMD.
+- Set TSSGO = 1 to start the PIO write sequence.
+- If THC_SS_CD_IE = 1, SW will receives a MSI when the PIO is completed.
+- If read, read out the data in THC_SW_SEQ_DATA0..THC_SW_SEQ_DATAn.
+
+3.3 DMA
+-------
+
+THC has 4 DMA channels: Read DMA1, Read DMA2, Write DMA and Software DMA.
+
+3.3.1 Read DMA Channel
+~~~~~~~~~~~~~~~~~~~~~~
+
+THC has two Read DMA engines: 1st RxDMA (RxDMA1) and 2nd RxDMA (RxDMA2). RxDMA1 is reserved for
+raw data mode. RxDMA2 is used for HID data mode and it is the RxDMA engine currently driver uses
+for HID input report data retrieval.
+
+RxDMA's typical use case is auto receiving the data from Touch IC. Once RxDMA is enabled by
+software, THC will start auto-handling receiving logic.
+
+For SPI mode, THC RxDMA sequence is: when Touch IC triggers a interrupt to THC, THC reads out
+report header to identify what's the report type, and what's the report length, according to
+above information, THC reads out report body to internal FIFO and start RxDMA coping the data
+to system memory. After that, THC update interrupt cause register with report type, and update
+RxDMA PRD table read pointer, then trigger a MSI interrupt to notify driver RxDMA finishing
+data receiving.
+
+For I2C mode, THC RxDMA's behavior is a little bit different, because of HIDI2C protocol difference
+with HIDSPI protocol, RxDMA only be used to receive input report. The sequence is, when Touch IC
+triggers a interrupt to THC, THC first reads out 2 bytes from input report address to determine the
+packet length, then use this packet length to start a DMA reading from input report address for
+input report data. After that, THC update RxDMA PRD table read pointer, then trigger a MSI interrupt
+to notify driver input report data is ready in system memory.
+
+All above sequence is hardware automatically handled, all driver needs to do is configure RxDMA and
+waiting for interrupt ready then read out the data from system memory.
+
+3.3.2 Software DMA channel
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+THC supports a software triggerred RxDMA mode to read the touch data from touch IC. This SW RxDMA
+is the 3rd THC RxDMA engine with the similar functionalities as the existing two RxDMAs, the only
+difference is this SW RxDMA is triggerred by software, and RxDMA2 is triggerred by external Touch IC
+interrupt. It gives a flexiblity to software driver to use RxDMA read Touch IC data in any time.
+
+Before software starts a SW RxDMA, it shall stop the 1st and 2nd RxDMA, clear PRD read/write pointer
+and quiesce the device interrupt (THC_DEVINT_QUIESCE_HW_STS = 1), other operations are the same with
+RxDMA.
+
+3.3.3 Write DMA Channel
+~~~~~~~~~~~~~~~~~~~~~~~
+
+THC has one write DMA engine, which can be used for sending data to Touch IC automatically.
+According to HIDSPI and HIDI2C protocol, every time only one command can be sent to touch IC, and
+before last command is completely handled, next command cannot be sent, THC write DMA engine only
+supports single PRD table.
+
+What driver needs to do is, preparing PRD table and DMA buffer, then copy data to DMA buffer and
+update PRD table with buffer address and buffer length, then start write DMA. THC will
+automatically send the data to touch IC, and trigger a DMA completion interrupt once transferring
+is done.
+
+3.4 PRD
+-------
+
+Physical Region Descriptor (PRD) provides the memory mapping description for THC DMAs.
+
+3.4.1 PRD table and entry
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to improve physical DMA memory usage, modern drivers trend to allocate a virtually
+contiguous, but physically fragmented buffer of memory for each data buffer. Linux OS also
+provide SGL (scatter gather list) APIs to support this usage.
+
+THC uses PRD table (physical region descriptor) to support the corresponding OS kernel
+SGL that describes the virtual to physical buffer mapping.
+
+::
+
+ ------------------------ -------------- --------------
+ | PRD table base address +----+ PRD table #1 +-----+ PRD Entry #1 |
+ ------------------------ -------------- --------------
+ --------------
+ | PRD Entry #2 |
+ --------------
+ --------------
+ | PRD Entry #n |
+ --------------
+
+The read DMA engine supports multiple PRD tables held within a circular buffer that allow the THC
+to support multiple data buffers from the Touch IC. This allows host SW to arm the Read DMA engine
+with multiple buffers, allowing the Touch IC to send multiple data frames to the THC without SW
+interaction. This capability is required when the CPU processes touch frames slower than the
+Touch IC can send them.
+
+To simplify the design, SW assumes worst-case memory fragmentation. Therefore,each PRD table shall
+contain the same number of PRD entries, allowing for a global register (per Touch IC) to hold the
+number of PRD-entries per PRD table.
+
+SW allocates up to 128 PRD tables per Read DMA engine as specified in the THC_M_PRT_RPRD_CNTRL.PCD
+register field. The number of PRD tables should equal the number of data buffers.
+
+Max OS memory fragmentation will be at a 4KB boundary, thus to address 1MB of virtually contiguous
+memory 256 PRD entries are required for a single PRD Table. SW writes the number of PRD entries
+for each PRD table in the THC_M_PRT_RPRD_CNTRL.PTEC register field. The PRD entry's length must be
+multiple of 4KB except for the last entry in a PRD table.
+
+SW allocates all the data buffers and PRD tables only once at host initialization.
+
+3.4.2 PRD Write pointer and read pointer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As PRD tables are organized as a Circular Buffer (CB), a read pointer and a write pointer for a CB
+are needed.
+
+DMA HW consumes the PRD tables in the CB, one PRD entry at a time until the EOP bit is found set
+in a PRD entry. At this point HW increments the PRD read pointer. Thus, the read pointer points
+to the PRD which the DMA engine is currently processing. This pointer rolls over once the circular
+buffer's depth has been traversed with bit[7] the Rollover bit. E.g. if the DMA CB depth is equal
+to 4 entries (0011b), then the read pointers will follow this pattern (HW is required to honor
+this behavior): 00h 01h 02h 03h 80h 81h 82h 83h 00h 01h ...
+
+The write pointer is updated by SW. The write pointer points to location in the DMA CB, where the
+next PRD table is going to be stored. SW needs to ensure that this pointer rolls over once the
+circular buffer's depth has been traversed with Bit[7] as the rollover bit. E.g. if the DMA CB
+depth is equal to 5 entries (0100b), then the write pointers will follow this pattern (SW is
+required to honor this behavior): 00h 01h 02h 03h 04h 80h 81h 82h 83h 84h 00h 01h ..
+
+3.4.3 PRD descriptor structure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Intel THC uses PRD entry descriptor for every PRD entry. Every PRD entry descriptor occupies
+128 bits memories:
+
+=================== ======== ===============================================
+struct field bit(s) description
+=================== ======== ===============================================
+dest_addr 53..0 destination memory address, as every entry
+ is 4KB, ignore lowest 10 bits of address.
+reserved1 54..62 reserved
+int_on_completion 63 completion interrupt enable bit, if this bit
+ set it means THC will trigger a completion
+ interrupt. This bit is set by SW driver.
+len 87..64 how many bytes of data in this entry.
+end_of_prd 88 end of PRD table bit, if this bit is set,
+ it means this entry is last entry in this PRD
+ table. This bit is set by SW driver.
+hw_status 90..89 HW status bits
+reserved2 127..91 reserved
+=================== ======== ===============================================
+
+And one PRD table can include up to 256 PRD entries, as every entries is 4K bytes, so every
+PRD table can describe 1M bytes memory.
+
+.. code-block:: c
+
+ struct thc_prd_table {
+ struct thc_prd_entry entries[PRD_ENTRIES_NUM];
+ };
+
+In general, every PRD table means one HID touch data packet. Every DMA engine can support
+up to 128 PRD tables (except write DMA, write DMA only has one PRD table). SW driver is responsible
+to get max packet length from touch IC, and use this max packet length to create PRD entries for
+each PRD table.
+
+4. HIDSPI support (QuickSPI)
+============================
+
+Intel THC is total compatible with HIDSPI protocol, THC HW sequenser can accelerate HIDSPI
+protocol transferring.
+
+4.1 Reset Flow
+--------------
+
+- Call ACPI _RST method to reset Touch IC device.
+- Read the reset response from TIC through PIO read.
+- Issue a command to retrieve device descriptor from Touch IC through PIO write.
+- Read the device descriptor from Touch IC through PIO read.
+- If the device descriptor is valid, allocate DMA buffers and configure all DMA channels.
+- Issue a command to retrieve report descriptor from Touch IC through DMA.
+
+4.2 Input Report Data Flow
+--------------------------
+
+Basic Flow:
+
+- Touch IC interrupts the THC Controller using an in-band THC interrupt.
+- THC Sequencer reads the input report header by transmitting read approval as a signal
+ to the Touch IC to prepare for host to read from the device.
+- THC Sequencer executes a Input Report Body Read operation corresponding to the value
+ reflected in “Input Report Length” field of the Input Report Header.
+- THC DMA engine begins fetching data from the THC Sequencer and writes to host memory
+ at PRD entry 0 for the current CB PRD table entry. This process continues until the
+ THC Sequencer signals all data has been read or the THC DMA Read Engine reaches the
+ end of it's last PRD entry (or both).
+- The THC Sequencer checks for the “Last Fragment Flag” bit in the Input Report Header.
+ If it is clear, the THC Sequencer enters an idle state.
+- If the “Last Fragment Flag” bit is enabled the THC Sequencer enters End-of-Frame Processing.
+
+THC Sequencer End of Frame Processing:
+
+- THC DMA engine increments the read pointer of the Read PRD CB, sets EOF interrupt status
+ in RxDMA2 register (THC_M_PRT_READ_DMA_INT_STS_2).
+- If THC EOF interrupt is enabled by the driver in the control register (THC_M_PRT_READ_DMA_CNTRL_2),
+ generates interrupt to software.
+
+Sequence of steps to read data from RX DMA buffer:
+
+- THC QuickSPI driver checks CB write Ptr and CB read Ptr to identify if any data frame in DMA
+ circular buffers.
+- THC QuickSPI driver gets first unprocessed PRD table.
+- THC QuickSPI driver scans all PRD entries in this PRD table to calculate the total frame size.
+- THC QuickSPI driver copies all frame data out.
+- THC QuickSPI driver checks the data type according to input report body, and calls related
+ callbacks to process the data.
+- THC QuickSPI driver updates write Ptr.
+
+4.3 Output Report Data Flow
+---------------------------
+
+Generic Output Report Flow:
+
+- HID core calls raw_request callback with a request to THC QuickSPI driver.
+- THC QuickSPI Driver converts request provided data into the output report packet and copies it
+ to THC's write DMA buffer.
+- Start TxDMA to complete the write operation.
+
+5. HIDI2C support (QuickI2C)
+============================
+
+5.1 Reset Flow
+--------------
+
+- Read device descriptor from Touch IC device through PIO write followed by read.
+- If the device descriptor is valid, allocate DMA buffers and configure all DMA channels.
+- Use PIO or TxDMA to write a SET_POWER request to TIC's command register, and check if the
+ write operation is successfully completed.
+- Use PIO or TxDMA to write a RESET request to TIC's command register. If the write operation
+ is successfully completed, wait for reset response from TIC.
+- Use SWDMA to read report descriptor through TIC's report descriptor register.
+
+5.2 Input Report Data Flow
+--------------------------
+
+Basic Flow:
+
+- Touch IC asserts the interrupt indicating that it has an interrupt to send to HOST.
+ THC Sequencer issues a READ request over the I2C bus. The HIDI2C device returns the
+ first 2 bytes from the HIDI2C device which contains the length of the received data.
+- THC Sequencer continues the Read operation as per the size of data indicated in the
+ length field.
+- THC DMA engine begins fetching data from the THC Sequencer and writes to host memory
+ at PRD entry 0 for the current CB PRD table entry. THC writes 2Bytes for length field
+ plus the remaining data to RxDMA buffer. This process continues until the THC Sequencer
+ signals all data has been read or the THC DMA Read Engine reaches the end of it's last
+ PRD entry (or both).
+- THC Sequencer enters End-of-Input Report Processing.
+- If the device has no more input reports to send to the host, it de-asserts the interrupt
+ line. For any additional input reports, device keeps the interrupt line asserted and
+ steps 1 through 4 in the flow are repeated.
+
+THC Sequencer End of Input Report Processing:
+
+- THC DMA engine increments the read pointer of the Read PRD CB, sets EOF interrupt status
+ in RxDMA 2 register (THC_M_PRT_READ_DMA_INT_STS_2).
+- If THC EOF interrupt is enabled by the driver in the control register
+ (THC_M_PRT_READ_DMA_CNTRL_2), generates interrupt to software.
+
+Sequence of steps to read data from RX DMA buffer:
+
+- THC QuickI2C driver checks CB write Ptr and CB read Ptr to identify if any data frame in DMA
+ circular buffers.
+- THC QuickI2C driver gets first unprocessed PRD table.
+- THC QuickI2C driver scans all PRD entries in this PRD table to calculate the total frame size.
+- THC QuickI2C driver copies all frame data out.
+- THC QuickI2C driver call hid_input_report to send the input report content to HID core, which
+ includes Report ID + Report Data Content (remove the length field from the original report
+ data).
+- THC QuickI2C driver updates write Ptr.
+
+5.3 Output Report Data Flow
+---------------------------
+
+Generic Output Report Flow:
+
+- HID core call THC QuickI2C raw_request callback.
+- THC QuickI2C uses PIO or TXDMA to write a SET_REPORT request to TIC's command register. Report
+ type in SET_REPORT should be set to Output.
+- THC QuickI2C programs TxDMA buffer with TX Data to be written to TIC's data register. The first
+ 2 bytes should indicate the length of the report followed by the report contents including
+ Report ID.
+
+6. THC Debugging
+================
+
+To debug THC, event tracing mechanism is used. To enable debug logs::
+
+ echo 1 > /sys/kernel/debug/tracing/events/intel_thc/enable
+ cat /sys/kernel/debug/tracing/trace
+
+7. Reference
+============
+- HIDSPI: https://download.microsoft.com/download/c/a/0/ca07aef3-3e10-4022-b1e9-c98cea99465d/HidSpiProtocolSpec.pdf
+- HIDI2C: https://download.microsoft.com/download/7/d/d/7dd44bb7-2a7a-4505-ac1c-7227d3d96d5b/hid-over-i2c-protocol-spec-v1-0.docx
--
2.40.1