switchtec: Add user interface documentation

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Linux Switchtec Support

========================

Microsemi's "Switchtec" line of PCI switch devices is already

supported by the kernel with standard PCI switch drivers. However, the

Switchtec device advertises a special management endpoint which

enables some additional functionality. This includes:

* Packet and Byte Counters

* Firmware Upgrades

* Event and Error logs

* Querying port link status

* Custom user firmware commands

The switchtec kernel module implements this functionality.

Interface

=========

The primary means of communicating with the Switchtec management firmware is

through the Memory-mapped Remote Procedure Call (MRPC) interface.

Commands are submitted to the interface with a 4-byte command

identifier and up to 1KB of command specific data. The firmware will

respond with a 4 bytes return code and up to 1KB of command specific

data. The interface only processes a single command at a time.

Userspace Interface

===================

The MRPC interface will be exposed to userspace through a simple char

device: /dev/switchtec#, one for each management endpoint in the system.

The char device has the following semantics:

* A write must consist of at least 4 bytes and no more than 1028 bytes.

  The first four bytes will be interpreted as the command to run and

  the remainder will be used as the input data. A write will send the

  command to the firmware to begin processing.

* Each write must be followed by exactly one read. Any double write will

  produce an error and any read that doesn't follow a write will

  produce an error.

* A read will block until the firmware completes the command and return

  the four bytes of status plus up to 1024 bytes of output data. (The

  length will be specified by the size parameter of the read call --

  reading less than 4 bytes will produce an error.

* The poll call will also be supported for userspace applications that

  need to do other things while waiting for the command to complete.

M:	Logan Gunthorpe <logang@deltatee.com>

L:	linux-pci@vger.kernel.org

S:	Maintained

F:	Documentation/switchtec.txt

F:	drivers/pci/switch/switchtec*

PCI DRIVER FOR NVIDIA TEGRA