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+What: /dev/jtag[0-9]+
+Date: July 2018
+KernelVersion: 4.20
+Contact: oleksandrs@mellanox.com
+Description:
+ The misc device files /dev/jtag* are the interface
+ between JTAG controller interface and userspace.
+
+ The ioctl(2)-based ABI is defined and documented in
+ [include/uapi]<linux/jtag.h>.
+
+ The following file operations are supported:
+
+ open(2)
+ Opens and allocates file descriptor.
+
+ ioctl(2)
+ Initiate various actions.
+ See the inline documentation in [include/uapi]<linux/jtag.h>
+ for descriptions of all ioctls.
+
+Users:
+ userspace tools which wants to access to JTAG bus
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+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Joint Test Action Group (JTAG)
+==============================
+
+.. toctree::
+ :maxdepth: 1
+
+ jtag-summary
+ jtagdev
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
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+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
+Linux kernel JTAG support
+====================================
+
+Introduction to JTAG
+====================
+
+JTAG is an industry standard for verifying hardware. JTAG provides access to
+many logic signals of a complex integrated circuit, including the device pins.
+
+A JTAG interface is a special interface added to a chip.
+Depending on the version of JTAG, two, four, or five pins are added.
+
+The connector pins are:
+ * TDI (Test Data In)
+ * TDO (Test Data Out)
+ * TCK (Test Clock)
+ * TMS (Test Mode Select)
+ * TRST (Test Reset) optional
+
+JTAG interface is designed to have two parts - basic core driver and
+hardware specific driver. The basic driver introduces a general interface
+which is not dependent of specific hardware. It provides communication
+between user space and hardware specific driver.
+Each JTAG device is represented as a char device from (jtag0, jtag1, ...).
+Access to a JTAG device is performed through IOCTL calls.
+
+Call flow example:
+::
+
+ User: open -> /dev/jatgX -> JTAG core driver -> JTAG hardware specific driver
+ User: ioctl -> /dev/jtagX -> JTAG core driver -> JTAG hardware specific driver
+ User: close -> /dev/jatgX -> JTAG core driver -> JTAG hardware specific driver
+
+
+THANKS TO
+---------
+Contributors to Linux-JTAG discussions include (in alphabetical order,
+by last name):
+
+- Omar Castro
+- Ernesto Corona
+- Steven Filary
+- Vadim Pasternak
+- Jiri Pirko
+- Oleksandr Shamray
+- Billy Tsai
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+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+JTAG userspace API
+==================
+JTAG controller devices can be accessed through a character misc-device.
+
+Each JTAG controller interface can be accessed by using /dev/jtagN.
+
+JTAG system calls set:
+ * SIR (Scan Instruction Register, IEEE 1149.1 Instruction Register scan);
+ * SDR (Scan Data Register, IEEE 1149.1 Data Register scan);
+ * RUNTEST (Forces the IEEE 1149.1 bus to a run state for a specified number of clocks.
+
+open(), close()
+---------------
+Open/Close device:
+::
+
+ jtag_fd = open("/dev/jtag0", O_RDWR);
+ close(jtag_fd);
+
+ioctl()
+-------
+All access operations to JTAG devices are performed through ioctl interface.
+The IOCTL interface supports these requests:
+::
+
+ JTAG_SIOCSTATE - Force JTAG state machine to go into a TAPC state
+ JTAG_SIOCFREQ - Set JTAG TCK frequency
+ JTAG_GIOCFREQ - Get JTAG TCK frequency
+ JTAG_IOCXFER - send/receive JTAG data Xfer
+ JTAG_GIOCSTATUS - get current JTAG TAP state
+ JTAG_SIOCMODE - set JTAG mode flags.
+ JTAG_IOCBITBANG - JTAG bitbang low level control.
+ JTAG_SIOCTRST - Set TRST pin for JTAG.
+
+JTAG_SIOCFREQ
+~~~~~~~~~~~~~
+Set JTAG clock speed:
+::
+
+ unsigned int jtag_fd;
+ ioctl(jtag_fd, JTAG_SIOCFREQ, &frq);
+
+JTAG_GIOCFREQ
+~~~~~~~~~~~~~
+Get JTAG clock speed:
+::
+
+ unsigned int jtag_fd;
+ ioctl(jtag_fd, JTAG_GIOCFREQ, &frq);
+
+JTAG_SIOCSTATE
+~~~~~~~~~~~~~~
+Force JTAG state machine to go into a TAPC state
+::
+
+ struct jtag_tap_state {
+ __u8 reset;
+ __u8 from;
+ __u8 endstate;
+ __u32 tck;
+ };
+
+reset: one of below options
+::
+
+ JTAG_NO_RESET - go through selected endstate from current state
+ JTAG_FORCE_RESET - go through TEST_LOGIC/RESET state before selected endstate
+
+endstate: any state listed in jtag_tapstate enum
+::
+
+ enum jtag_tapstate {
+ JTAG_STATE_TLRESET,
+ JTAG_STATE_IDLE,
+ JTAG_STATE_SELECTDR,
+ JTAG_STATE_CAPTUREDR,
+ JTAG_STATE_SHIFTDR,
+ JTAG_STATE_EXIT1DR,
+ JTAG_STATE_PAUSEDR,
+ JTAG_STATE_EXIT2DR,
+ JTAG_STATE_UPDATEDR,
+ JTAG_STATE_SELECTIR,
+ JTAG_STATE_CAPTUREIR,
+ JTAG_STATE_SHIFTIR,
+ JTAG_STATE_EXIT1IR,
+ JTAG_STATE_PAUSEIR,
+ JTAG_STATE_EXIT2IR,
+ JTAG_STATE_UPDATEIR
+ };
+
+tck: clock counter
+
+Example:
+::
+
+ struct jtag_tap_state tap_state;
+
+ tap_state.endstate = JTAG_STATE_IDLE;
+ tap_state.reset = 0;
+ tap_state.tck = data_p->tck;
+ usleep(25 * 1000);
+ ioctl(jtag_fd, JTAG_SIOCSTATE, &tap_state);
+
+JTAG_GIOCSTATUS
+~~~~~~~~~~~~~~~
+Get JTAG TAPC current machine state
+::
+
+ unsigned int jtag_fd;
+ jtag_tapstate tapstate;
+ ioctl(jtag_fd, JTAG_GIOCSTATUS, &tapstate);
+
+JTAG_IOCXFER
+~~~~~~~~~~~~
+Send SDR/SIR transaction
+::
+
+ struct jtag_xfer {
+ __u8 type;
+ __u8 direction;
+ __u8 from;
+ __u8 endstate;
+ __u32 padding;
+ __u32 length;
+ __u64 tdio;
+ };
+
+type: transfer type - JTAG_SIR_XFER/JTAG_SDR_XFER
+
+direction: xfer direction - JTAG_READ_XFER/JTAG_WRITE_XFER/JTAG_READ_WRITE_XFER
+
+from: jtag_tapstate enum representing the initial tap state of the chain before xfer.
+
+endstate: end state after transaction finish any of jtag_tapstate enum
+
+padding: padding configuration. See the following table with bitfield descriptions.
+
+=============== ========= ======= =====================================================
+Bit Field Bit begin Bit end Description
+=============== ========= ======= =====================================================
+rsvd 25 31 Reserved, not used
+pad data 24 24 Value used for pre and post padding. Either 1 or 0.
+post pad count 12 23 Number of padding bits to be executed after transfer.
+pre pad count 0 11 Number of padding bit to be executed before transfer.
+=============== ========= ======= =====================================================
+
+length: xfer data length in bits
+
+tdio : xfer data array
+
+Example:
+::
+
+ struct jtag_xfer xfer;
+ static char buf[64];
+ static unsigned int buf_len = 0;
+ [...]
+ xfer.type = JTAG_SDR_XFER;
+ xfer.tdio = (__u64)buf;
+ xfer.length = buf_len;
+ xfer.from = JTAG_STATE_TLRESET;
+ xfer.endstate = JTAG_STATE_IDLE;
+
+ if (is_read)
+ xfer.direction = JTAG_READ_XFER;
+ else if (is_write)
+ xfer.direction = JTAG_WRITE_XFER;
+ else
+ xfer.direction = JTAG_READ_WRITE_XFER;
+
+ ioctl(jtag_fd, JTAG_IOCXFER, &xfer);
+
+JTAG_SIOCMODE
+~~~~~~~~~~~~~
+If hardware driver can support different running modes you can change it.
+
+Example:
+::
+
+ struct jtag_mode mode;
+ mode.feature = JTAG_XFER_MODE;
+ mode.mode = JTAG_XFER_HW_MODE;
+ ioctl(jtag_fd, JTAG_SIOCMODE, &mode);
+
+JTAG_IOCBITBANG
+~~~~~~~~~~~~~~~
+JTAG Bitbang low level operation.
+
+Example:
+::
+
+ struct tck_bitbang bitbang
+ bitbang.tms = 1;
+ bitbang.tdi = 0;
+ ioctl(jtag_fd, JTAG_IOCBITBANG, &bitbang);
+ tdo = bitbang.tdo;
+
+JTAG_SIOCTRST
+~~~~~~~~~~~~~~~
+Set TRST pin for JTAG.
+
+Example:
+::
+
+ unsigned int active = 1;
+ ioctl(jtag_fd, JTAG_SIOCTRST, active);
+
+THANKS TO
+---------
+Contributors to Linux-JTAG discussions include (in alphabetical order,
+by last name):
+
+- Omar Castro
+- Ernesto Corona
+- Steven Filary
+- Vadim Pasternak
+- Jiri Pirko
+- Oleksandr Shamray
+- Billy Tsai