linux_dsm_epyc7002/Documentation/driver-api/switchtec.rst

========================
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-byte 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 4 bytes will be interpreted as the Command ID 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 4-byte Command Return Value 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.

The following IOCTLs are also supported by the device:

* SWITCHTEC_IOCTL_FLASH_INFO - Retrieve firmware length and number
  of partitions in the device.

* SWITCHTEC_IOCTL_FLASH_PART_INFO - Retrieve address and lengeth for
  any specified partition in flash.

* SWITCHTEC_IOCTL_EVENT_SUMMARY - Read a structure of bitmaps
  indicating all uncleared events.

* SWITCHTEC_IOCTL_EVENT_CTL - Get the current count, clear and set flags
  for any event. This ioctl takes in a switchtec_ioctl_event_ctl struct
  with the event_id, index and flags set (index being the partition or PFF
  number for non-global events). It returns whether the event has
  occurred, the number of times and any event specific data. The flags
  can be used to clear the count or enable and disable actions to
  happen when the event occurs.
  By using the SWITCHTEC_IOCTL_EVENT_FLAG_EN_POLL flag,
  you can set an event to trigger a poll command to return with
  POLLPRI. In this way, userspace can wait for events to occur.

* SWITCHTEC_IOCTL_PFF_TO_PORT and SWITCHTEC_IOCTL_PORT_TO_PFF convert
  between PCI Function Framework number (used by the event system)
  and Switchtec Logic Port ID and Partition number (which is more
  user friendly).


Non-Transparent Bridge (NTB) Driver
===================================

An NTB hardware driver is provided for the Switchtec hardware in
ntb_hw_switchtec. Currently, it only supports switches configured with
exactly 2 NT partitions and zero or more non-NT partitions. It also requires
the following configuration settings:

* Both NT partitions must be able to access each other's GAS spaces.
  Thus, the bits in the GAS Access Vector under Management Settings
  must be set to support this.
* Kernel configuration MUST include support for NTB (CONFIG_NTB needs
  to be set)

NT EP BAR 2 will be dynamically configured as a Direct Window, and
the configuration file does not need to configure it explicitly.

Please refer to Documentation/driver-api/ntb.rst in Linux source tree for an overall
understanding of the Linux NTB stack. ntb_hw_switchtec works as an NTB
Hardware Driver in this stack.