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

srinivas pandruvada
2025-01-06T13:05:36Z

On Mon, 2025-01-06 at 10:31 +0800, Even Xu wrote:
> 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>
> ---

Change logs are missing?
Also didn't receive any cover letter with any changes?

Thanks,
Srinivas

>  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