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+----Introduction----
+
+Usually I2C adapters are implemented in a kernel driver.  It is also possible to
+implement an adapter in userspace, through the /dev/i2c-pseudo-controller
+interface.  Load module i2c-pseudo for this.
+
+Use cases for this module include:
+
+[A] Using local I2C device drivers, particularly i2c-dev, with I2C busses on
+remote systems.  For example, interacting with a Device Under Test (DUT)
+connected to a Linux host through a debug interface, or interacting with a
+remote host over a network.
+
+[B] Support I2C device driver tests that are too complex for the i2c-stub
+module.  For example, when simulating an I2C device where its driver might
+issue a sequence of reads and writes without interruption, and the value a
+certain address must change during the sequence.
+
+Any possible use case could of course be implemented as a kernel driver.
+However, it can be much faster and easier to implement such things in userspace,
+thanks to the far greater code reuse possibilities (libraries), the plethora of
+programming language options for rapid iteration, and not needing to understand
+how to implement Linux kernel drivers.
+
+This is not intended to replace kernel drivers for actual I2C busses on the
+local host machine.
+
+
+
+----Details----
+
+Each time /dev/i2c-pseudo-controller is opened, and the correct initialization
+command is written to it (ADAPTER_START), a new I2C adapter is created.  The
+adapter will live until its file descriptor is closed.  Multiple pseudo adapters
+can co-exist simultaneously, controlled by the same or different userspace
+processes.  When an I2C device driver sends an I2C message to a pseudo adapter,
+the message becomes readable from its file descriptor.  If a reply is written
+before the adapter timeout expires, that reply will be sent back to the I2C
+device driver.
+
+Reads and writes are buffered inside i2c-pseudo such that userspace controllers
+may split them up into arbitrarily small chunks.  Multiple commands, or portions
+of multiple commands, may be read or written together.
+
+Blocking I/O is the default.  Non-blocking I/O is supported as well, enabled by
+O_NONBLOCK.  Polling is supported, with or without non-blocking I/O.  A special
+command (ADAPTER_SHUTDOWN) is available to unblock any pollers or blocked
+reads or writes, as a convenience for a multi-threaded or multi-process program
+that wants to exit.
+
+It is safe to access a single controller fd from multiple threads or processes
+concurrently, though it is up to the controller to ensure proper ordering, and
+to ensure that writes for different commands do not get interleaved.  However,
+it is recommended (not required) that controller implementations have only one
+reader thread and one writer thread, which may or may not be the same thread.
+Avoiding multiple readers and multiple writers greatly simplifies controller
+implementation, and there is likely no performance benefit to be gained from
+concurrent reads or concurrent writes due to how i2c-pseudo serializes them
+internally.  After all, on a real I2C bus only one I2C message can be active at
+a time.
+
+Commands are newline-terminated, both those read from the controller device, and
+those written to it.
+
+
+
+----Read Commands----
+
+The commands that may be read from a pseudo controller device are:
+
+
+Read Command: I2C_ADAPTER_NUM <num>
+Example: "I2C_ADAPTER_NUM 5\n"
+
+Details: This is read in response to the GET_ADAPTER_NUM command being written.
+The number is the I2C adapter number in decimal.  This is can only occur after
+ADAPTER_START, because before that the number is not known and cannot be
+predicted reliably.
+
+
+Read Command: I2C_PSEUDO_ID <num>
+Example: "I2C_PSEUDO_ID 98\n"
+
+Details: This is read in response to the GET_PSEUDO_ID command being written.
+The number is the pseudo ID in decimal.
+
+
+Read Command: I2C_BEGIN_XFER
+Example: "I2C_BEGIN_XFER\n"
+
+Details: This indicates the start of an I2C transaction request, in other words
+the start of the I2C messages from a single invocation of the I2C adapter's
+master_xfer() callback.  This should only ever be read after ADAPTER_START.
+
+
+Read Command: I2C_XFER_REQ <xfer_id> <msg_id> <addr> <flags> <data_len> [<write_byte>, ...]
+Example: "I2C_XFER_REQ 3 0 0x0070 0x0000 2 AB:9F\n"
+Example: "I2C_XFER_REQ 3 1 0x0070 0x0001 4\n"
+
+Details: This is a single I2C message that a device driver requested be sent on
+the bus, in other words a single struct i2c_msg from master_xfer() msgs arg.
+
+The xfer_id is a number representing the whole I2C transaction, thus all
+I2C_XFER_REQ between a I2C_BEGIN_XFER + I2C_COMMIT_XFER pair share an xfer_id.
+The purpose is to ensure replies from the userspace controller are always
+properly matched to the intended master_xfer() request.  The first transaction
+has xfer_id 0, and it increases by 1 with each transaction, however it will
+eventually wrap back to 0 if enough transactions happen during the lifetime of a
+pseudo adapter.  It is guaranteed to have a large enough maximum value such that
+there can never be multiple outstanding transactions with the same ID, due to an
+internal limit in i2c-pseudo that will block master_xfer() calls when the
+controller is falling behind in its replies.
+
+The msg_id is a number representing the index of the I2C message within its
+transaction, in other words the index in master_xfer() *msgs array arg.  This
+starts at 0 after each I2C_BEGIN_XFER.  This is guaranteed to not wrap.
+
+The addr is the hexadecimal I2C address for this I2C message.
+
+The flags are the same bitmask flags used in struct i2c_msg, in hexadecimal
+form.  Of particular importance to any pseudo controller is the read bit, which
+is guaranteed to be 0x1 per Linux I2C documentation.
+
+The data_len is the decimal number of either how many bytes to write that will
+follow, or how many bytes to read and reply with if this is a read request.
+
+If this is a read, data_len will be the final field in this command.  If this is
+a write, data_len will be followed by the given number of colon-separated
+hexadecimal byte values, in the format shown in the example above.
+
+
+Read Command: I2C_COMMIT_XFER
+Example: "I2C_COMMIT_XFER\n"
+
+Details: This indicates the end of an I2C transacton request, in other words the
+end of the I2C messages from a single invocation of the I2C adapter's
+master_xfer() callback.  This should be read exactly once after each
+I2C_BEGIN_XFER, with a varying number of I2C_XFER_REQ between them.
+
+
+
+----Write Commands----
+
+The commands that may be written to a pseudo controller device are:
+
+
+Write Command: SET_ADAPTER_NAME_SUFFIX <suffix>
+Example: "SET_ADAPTER_NAME_SUFFIX My Adapter\n"
+
+Details: Sets a suffix to append to the auto-generated I2C adapter name.  Only
+valid before ADAPTER_START.  A space or other separator character will be placed
+between the auto-generated name and the suffix, so there is no need to include a
+leading separator in the suffix.  If the resulting name is too long for the I2C
+adapter name field, it will be quietly truncated.
+
+
+Write Command: SET_ADAPTER_TIMEOUT_MS <ms>
+Example: "SET_ADAPTER_TIMEOUT_MS 2000\n"
+
+Details: Sets the timeout in milliseconds for each I2C transaction, in other
+words for each master_xfer() reply.  Only valid before ADAPTER_START.  The I2C
+subsystem will automatically time out transactions based on this setting.  Set
+to 0 to use the I2C subsystem default timeout.  The default timeout for new
+pseudo adapters where this command has not been used is configurable at
+i2c-pseudo module load time, and itself has a default independent from the I2C
+subsystem default.  (Though if the i2c-pseudo module level default is set to 0,
+that has the same meaning as here.)
+
+
+Write Command: ADAPTER_START
+Example: "ADAPTER_START\n"
+
+Details: Tells i2c-pseudo the actually create the I2C adapter.  Only valid once
+per open controller fd.
+
+
+Write Command: GET_ADAPTER_NUM
+Example: "GET_ADAPTER_NUM\n"
+
+Details: Asks i2c-pseudo for the number assigned to this I2C adapter by the I2C
+subsystem.  Only valid after ADAPTER_START, because before that the number
+is not known and cannot be predicted reliably.
+
+
+Write Command: GET_PSEUDO_ID
+Example: "GET_PSEUDO_ID\n"
+
+Details: Asks i2c-pseudo for the pseudo ID of this I2C adapter.  The pseudo ID
+will not be reused for the lifetime of the i2c-pseudo module, unless an internal
+counter wraps.  I2C clients can use this to track specific instances of pseudo
+adapters, even when adapter numbers have been reused.
+
+
+Write Command: I2C_XFER_REPLY <xfer_id> <msg_id> <addr> <flags> <errno> [<read_byte>, ...]
+Example: "I2C_XFER_REPLY 3 0 0x0070 0x0000 0\n"
+Example: "I2C_XFER_REPLY 3 1 0x0070 0x0001 0 0B 29 02 D9\n"
+
+Details: This is how a pseudo controller can reply to I2C_XFER_REQ.  Only valid
+after I2C_XFER_REQ.  A pseudo controller should write one of these for each
+I2C_XFER_REQ it reads, including for failures, so that I2C device drivers need
+not wait for the adapter timeout upon failure (if failure is known sooner).
+
+The fields in common with I2C_XFER_REQ have their same meanings, and their
+values are expected to exactly match what was read in the I2C_XFER_REQ command
+that this is in reply to.
+
+The errno field is how the pseudo controller indicates success or failure for
+this I2C message.  A 0 value indicates success.  A non-zero value indicates a
+failure.  Pseudo controllers are encouraged to use errno values to encode some
+meaning in a failure response, but that is not a requirement, and the I2C
+adapter interface does not provide a way to pass per-message errno values to a
+device driver anyways.
+
+Pseudo controllers are encouraged to reply in the same order as messages were
+received, however i2c-pseudo will properly match up out-of-order replies with
+their original requests.
+
+
+Write Command: ADAPTER_SHUTDOWN
+Example: "ADAPTER_SHUTDOWN\n"
+
+Details: This tells i2c-pseudo that the pseudo controller wants to shutdown and
+intends to close the controller device fd soon.  Use of this is OPTIONAL, it is
+perfectly valid to close the controller device fd without ever using this
+command.
+
+This commands unblocks any blocked controller I/O (reads, writes, or polls), and
+that is its main reason for existing.
+
+Any I2C transactions attempted by a device driver after this command will fail,
+and will not be passed on to the userspace controller.
+
+
+
+----Example userspace controller code----
+
+In C, a simple exchange between i2c-pseudo and userspace might look like the
+example below.  Note that for brevity this lacks any error checking and
+handling, which a real pseudo controller implementation should have.
+
+int fd;
+char buf[1<<12];
+
+fd = open("/dev/i2c-pseudo-controller", O_RDWR);
+/* Create the I2C adapter. */
+dprintf(fd, "ADAPTER_START\n");
+
+/*
+ * Pretend this I2C adapter number is 5, and the first I2C xfer sent to it was
+ * from this command (using its i2c-dev interface):
+ * $ i2cset -y 5 0x70 0xC2
+ *
+ * Then this read would place the following into *buf:
+ * "I2C_BEGIN_XFER\n"
+ * "I2C_XFER_REQ 0 0 0x0070 0x0000 1 C2\n"
+ * "I2C_COMMIT_XFER\n"
+ */
+read(fd, buf, sizeof(buf));
+
+/* This reply would allow the i2cset command above to exit successfully. */
+dprintf(fd, "I2C_XFER_REPLY 0 0 0x0070 0x0000 0\n");
+
+/*
+ * Now pretend the next I2C xfer sent to this adapter was from:
+ * $ i2cget -y 5 0x70 0xAB
+ *
+ * Then this read would place the following into *buf:
+ * "I2C_BEGIN_XFER\n"
+ * "I2C_XFER_REQ 1 0 0x0070 0x0000 1 AB\n"
+ * "I2C_XFER_REQ 1 1 0x0070 0x0001 1\n'"
+ * "I2C_COMMIT_XFER\n"
+ */
+read(fd, buf, sizeof(buf));
+
+/*
+ * These replies would allow the i2cget command above to print the following to
+ * stdout and exit successfully:
+ * 0x0b
+ *
+ * Note that it is also valid to write these together in one write().
+ */
+dprintf(fd, "I2C_XFER_REPLY 3 0 0x0070 0x0000 0\n"
+dprintf(fd, "I2C_XFER_REPLY 3 1 0x0070 0x0001 0 0B\n");
+
+/* Destroy the I2C adapter. */
+close(fd);